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FEATURES
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LTC3788 2-Phase, Dual Output Synchronous Boost Controller DESCRIPTION
The LTC(R)3788 is a high performance 2-phase dual synchronous boost converter controller that drives all N-channel power MOSFETs. Synchronous rectification increases efficiency, reduces power losses and eases thermal requirements, allowing the LTC3788 to be used in high power boost applications. A constant-frequency current mode architecture allows a phase-lockable frequency of up to 850kHz. OPTI-LOOP(R) compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The LTC3788 features a precision 1.2V reference and dual power good output indicators. A 4.5V to 38V input supply range encompasses a wide range of system architectures and battery chemistries. Independent SS pins for each controller ramp the output voltages during start-up. The PLLIN/MODE pin selects among Burst Mode operation, pulse-skipping mode or continuous inductor current mode at light loads. For a leaded 28-lead SSOP package with a fixed current limit and one PGOOD output, without phase modulation or a clock output, see the LTC3788-1 data sheet.
L, LT, LTC, LTM, Linear Technology, Burst Mode, OPTI-LOOP PolyPhase and the Linear logo , are registered trademarks and No RSENSE and ThinSOT are trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U. S. Patents, including 5408150, 5481178, 5705919, 5929620, 6144194, 6177787, 6580258.
Synchronous Operation for Highest Efficiency and Reduced Heat Dissipation Wide Input Range: 4.5V to 38V (40V Abs Max) and Operates Down to 2.5V After Start-Up Output Voltages Up to 60V 1% 1.2V Reference Voltage RSENSE or Inductor DCR Current Sensing 100% Duty Cycle Capability for Synchronous MOSFET Low Quiescent Current: 125A Phase-Lockable Frequency (75kHz to 850kHz) Programmable Fixed Frequency (50kHz to 900kHz) Selectable Current Limit Adjustable Output Voltage Soft-Start Power Good Output Voltage Monitors Low Shutdown Current IQ: < 8A Internal LDO Powers Gate Drive from VBIAS or EXTVCC Thermally Enhanced Low Profile 32-Pin 5mm x 5mm QFN Package
APPLICATIONS
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Industrial Automotive Medical Military
TYPICAL APPLICATION
3m 4.7F TG1 VBIAS INTVCC VOUT 12V AT 5A 1.25H 0.1F SW1 BG1 SENSE1
+
VIN 4.5V TO 12V START-UP VOLTAGE OPERATES THROUGH TRANSIENTS DOWN TO 2.5V VIN
Efficiency and Power Loss vs Output Current
100 10000 90
4.7F TG2
4m
220F
BOOST1
BOOST2 SW2 LTC3788 BG2 SENSE2+ SENSE2- PGND
3.3H 0.1F
VOUT 24V AT 3A EFFICIENCY (%)
80 70 60 50 40 30
1000 POWER LOSS (mW)
100
SENSE1- RUN1 110k
10
RUN2 VFB1 VFB2 PGOOD1 EXTVCC FREQ PLLIN/MODE ITH1 SS1 SGND SS2 ITH2 15nF 12.1k 2.7k 100pF 0.1F 220pF 0.1F 15nF 8.66k
232k
VIN = 12V 1 VOUT = 24V 10 Burst Mode OPERATION FIGURE 9 CIRCUIT 0 0.1 0.00001 0.0001 0.001 0.01 0.1 1 10 OUTPUT CURRENT (A) 20 220F
3788 TA01b
220F
12.1k
3788 TA01a
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LTC3788 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW SENSE1+ PGOOD1
VBIAS......................................................... -0.3V to 40V BOOST1, BOOST2 ...................................... -0.3V to 76V SW1, SW2 ................................................. -0.3V to 70V RUN1, RUN2 ................................................ -0.3V to 8V Maximum Current Sourced into Pin from Source > 8V..............................................100A PGOOD1, PGOOD2, PLLIN/MODE ............... -0.3V to 6V INTVCC, (BOOST1-SW1, BOOST2-SW2) ...... -0.3V to 6V EXTVCC ......................................................... -0.3V to 6V SENSE1+, SENSE1-, SENSE2+, SENSE2-.................................... -0.3V to 40V SENSE1+ - SENSE1-, SENSE2+ - SENSE2- ................................. -0.3V to 0.3V ILIM, SS1, SS2, ITH1, ITH2, FREQ, PHASMD, VFB1, VFB2 .......................... -0.3V to INTVCC Operating Junction Temperature Range ... -40C to 125C Storage Temperature Range................... -65C to 125C
VFB1
ITH1
SW1 SW2
ILIM
SS1
32 31 30 29 28 27 26 25 SENSE1- 1 FREQ 2 PHASMD 3 CLKOUT 4 PLLIN/MODE 5 SGND 6 RUN1 7 RUN2 8 9 10 11 12 13 14 15 16 SENSE2- SENSE2+ VFB2 ITH2 SS2 PGOOD2 TG2 33 GND 24 BOOST1 23 BG1 22 VBIAS 21 PGND 20 EXTVCC 19 INTVCC 18 BG2 17 BOOST2
UH PACKAGE 32-LEAD (5mm 5mm) PLASTIC QFN TJMAX = 125C, JA = 34C/W EXPOSED PAD (PIN 33) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3788EUH#PBF LTC3788IUH#PBF TAPE AND REEL LTC3788EUH#TRPBF LTC3788IUH#TRPBF PART MARKING* 3788 3788 PACKAGE DESCRIPTION 32-Lead (5mm x 5mm) Plastic QFN 32-Lead (5mm x 5mm) Plastic QFN TEMPERATURE RANGE -40C to 125C -40C to 125C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
SYMBOL VBIAS VFB1,2 IFB1,2 VREFLNREG PARAMETER Chip Bias Voltage Operating Range Regulated Feedback Voltage Feedback Current Reference Line Voltage Regulation Main Control Loop
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C, VBIAS = 12V, unless otherwise noted (Note 2).
CONDITIONS MIN 4.5 ITH = 1.2V (Note 4) (Note 4) VBIAS = 6V to 38V
l
TYP
TG1
MAX 38
UNITS V V nA %/V
1.188
1.200 5 0.002
1.212 50 0.02
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LTC3788 ELECTRICAL CHARACTERISTICS
SYMBOL VLOADREG PARAMETER Output Voltage Load Regulation
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C, VBIAS = 12V, unless otherwise noted (Note 2).
CONDITIONS (Note 4) Measured in Servo Loop; ITH Voltage = 1.2V to 0.7V Measured in Servo Loop; ITH Voltage = 1.2V to 2V
l l
MIN
TYP 0.01 -0.01 2 0.9 1.2 125 200 8
MAX 0.1 -0.1
UNITS % % mmho mA mA
gm1,2 IQ
Error Amplifier Transconductance Input DC Supply Current
ITH = 1.2V (Note 5)
Pulse-Skipping or Forced Continuous Mode RUN1 = 5V and RUN2 = 0V or RUN1 = 0V (One Channel On) and RUN2 = 5V; VFB1(2) = 1.25V (No Load) Pulse-Skipping or Forced Continuous Mode RUN1,2 = 5V; VFB1,2 = 1.25V (No Load) (Both Channels On) Sleep Mode (One Channel On) Sleep Mode (Both Channels On) Shutdown UVLO VRUN1,2 VRUNHYS IRUN1,2 IRUN1,2 ISS1,2 VSENSE(MAX) INTVCC Undervoltage Lockout Thresholds RUN Pin On Threshold RUN Pin Hysteresis RUN Pin Hysteresis Current RUN Pin Current Soft-Start Charge Current Maximum Current Sense Threshold VRUN > 1.28V VRUN < 1.28V VSS = GND VFB = 1.1V, ILIM = INTVCC VFB = 1.1V, ILIM = Float VFB = 1.1V, ILIM = GND VSENSE(CM) ISENSE1,2+ ISENSE1,2- t r(TG1,2) t f(TG1,2) t r(BG1,2) t f(BG1,2) RUP(TG1,2) RDN(TG1,2) RUP(TG1,2) RDN(TG1,2) t D(TG/BG) t D(BG/TG) SENSE Pins Common Mode Range (BOOST Converter Input Supply Voltage VIN) SENSE+ Pin Current SENSE- Pin Current Top Gate Rise Time Top Gate Fall Time Bottom Gate Rise Time Bottom Gate Fall Time Top Gate Pull-Up Resistance Top Gate Pull-Down Resistance Bottom Gate Pull-Up Resistance Bottom Gate Pull-Down Resistance Top Gate Off to Bottom Gate On Switch-On Delay Time Bottom Gate Off to Top Gate On Switch-On Delay Time CLOAD = 3300pF (Each Driver) CLOAD = 3300pF (Each Driver) VFB = 1.1V, ILIM = Float VFB = 1.1V, ILIM = Float CLOAD = 3300pF (Note 6) CLOAD = 3300pF (Note 6) CLOAD = 3300pF (Note 6) CLOAD = 3300pF (Note 6)
l l l
RUN1 = 5V and RUN2 = 0V or RUN1 = 0V and RUN2 = 5V; VFB1(2) = 1.25V (No Load) RUN1,2 = 5V; VFB1,2 = 1.25V (No Load) RUN1,2 = 0V VINTVCC Ramping Up VINTVCC Ramping Down VRUN Rising
l l l
190 300 20 4.3 1.38
A A A V V V mV A A
4.1 3.6 1.18 3.8 1.28 100 4.5 0.5 7 90 68 42 2.5 200 20 20 20 20 1.5 1.5 1.5 1.5 70 70 10 100 75 50
13 110 82 56 38 300 1
A mV mV mV V A A ns ns ns ns ns ns
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LTC3788 ELECTRICAL CHARACTERISTICS
SYMBOL DFMAX(BG1,2) tON(MIN) VINTVCCVIN VLDOVIN VINTVCCEXT VLDOEXT VEXTVCC VLDOHYS fPROG PARAMETER Maximum BG Duty Factor Minimum BG On-Time Internal VCC Voltage INTVCC Load Regulation Internal VCC Voltage INTVCC Load Regulation EXTVCC Switchover Voltage EXTVCC Hysteresis Programmable Frequency RFREQ = 25k RFREQ = 60k RFREQ = 100k fLOW fHIGH fSYNC VPGL IPGOOD VPG Lowest Fixed Frequency Highest Fixed Frequency Synchronizable Frequency PGOOD Voltage Low PGOOD Leakage Current PGOOD Trip Level VFREQ = 0V VFREQ = INTVCC PLLIN/MODE = External Clock IPGOOD = 2mA VPGOOD = 5V VFB with Respect to Set Regulated Voltage VFB Ramping Negative Hysteresis VFB Ramping Positive Hysteresis tPGOOD(DELAY) IBOOST1,2 PGOOD Delay BOOST Charge Pump Available Output Current PGOOD Going High to Low VSW1,2 = 12V; VBOOST1,2 - VSW1,2 = 4.5V; FREQ = 0V, Forced Continuous or Pulse-Skipping Mode BOOST1 and BOOST2 Charge Pump 55 A 8 -12 -10 2.5 10 2.5 25 12 -8 % % % % s
l
The l denotes the specifications which apply over the full operating junction temperature range, otherwise specifications are at TA = 25C, VBIAS = 12V, unless otherwise noted (Note 2).
CONDITIONS (Note 7) 6V < VBIAS < 38V, VEXTVCC = 0V ICC = 0mA to 50mA, VEXTVCC = 0V VEXTVCC = 6V ICC = 0mA to 40mA, VEXTVCC = 6V EXTVCC Ramping Positive 4.5 5.2 5.2 MIN TYP 96 110 5.4 0.5 5.4 0.5 4.8 250 105 335 320 485 75 0.2 400 760 350 535 380 585 850 0.4 1 465 5.6 2 5.6 2 5 MAX UNITS % ns V % V % V mV kHz kHz kHz kHz kHz kHz V A
INTVCC Linear Regulator
Oscillator and Phase-Locked Loop
PGOOD1 and PGOOD2 Outputs
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: The LTC3788E is guaranteed to meet specifications from 0C to 85C. Specifications over the -40C to 125C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3788I is guaranteed over the full -40C to 125C operating junction temperature range. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the formula: TJ = TA + (PD * 34C/W)
Note 4: The LTC3788 is tested in a feedback loop that servos VFB to the output of the error amplifier while maintaining ITH at the midpoint of the current limit range. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 6: Rise and fall times are measured using 10% and 90% levels. Delay times are measured using 50% levels. Note 7: See Minimum On-Time Considerations in the Applications Information section.
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LTC3788 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency and Power Loss vs Output Current
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 VIN = 12V VOUT = 24V FIGURE 9 CIRCUIT 0.1 1 OUTPUT CURRENT (A) BURST EFFICIENCY PULSE-SKIPPING EFFICIENCY CCM EFFICIENCY 10
3788 G01
Efficiency and Power Loss vs Output Current
10000 100 90 1000 POWER LOSS (mW) EFFICIENCY (%) 80 70 60 50 40 30 VIN = 12V 1 VOUT = 24V 10 Burst Mode OPERATION FIGURE 9 CIRCUIT 0.1 0 0.1 1 10 0.00001 0.0001 0.001 0.01 OUTPUT CURRENT (A) 20
3788 G02
10000
1000 POWER LOSS (mW)
100
100
10
10
1
0.1
BURST LOSS PULSE-SKIPPING LOSS CCM LOSS
BURST EFFICIENCY BURST LOSS
Efficiency vs Input Voltage
100 99 98 EFFICIENCY (%) 97 96 95 94 93 92 91 90 0 5 15 10 INPUT VOLTAGE (V) 20 25
3788 G03
Load Step Forced Continuous Mode
LOAD STEP 2A/DIV
ILOAD = 2A FIGURE 9 CIRCUIT
VOUT = 12V VOUT = 24V INDUCTOR CURRENT 5A/DIV VOUT 500mV/DIV
200s/DIV VIN = 12V VOUT = 24V FIGURE 9 CIRCUIT
3788 G04
Load Step Burst Mode Operation
LOAD STEP 2A/DIV INDUCTOR CURRENT 5A/DIV LOAD STEP 2A/DIV INDUCTOR CURRENT 5A/DIV
Load Step Pulse-Skipping Mode
VOUT 500mV/DIV
VOUT 500mV/DIV
200s/DIV VIN = 12V VOUT = 24V FIGURE 9 CIRCUIT
3788 G05
200s/DIV VIN = 12V VOUT = 24V FIGURE 9 CIRCUIT
3788 G06
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LTC3788 TYPICAL PERFORMANCE CHARACTERISTICS
Inductor Current at Light Load Soft Start-Up
FORCED CONTINUOUS MODE
Burst Mode OPERATION 5A/DIV PULSE-SKIPPING MODE
VOUT 5V/DIV
0V
5s/DIV VIN = 12V VOUT = 24V ILOAD = 200A FIGURE 9 CIRCUIT
3788 G07
VIN = 12V 20ms/DIV VOUT = 24V FIGURE 9 CIRCUIT
3788 G08
Regulated Feedback Voltage vs Temperature
1.212 REGULATED FEEDBACK VOLTAGE (V) 1.209 1.206 1.203 1.200 1.197 1.194 1.191 1.188 -45 -20 5 80 55 30 TEMPERATURE (C) 105 130
3788 G09
Soft-Start Pull-Up Current vs Temperature
11.0
SOFT-START CURRENT (A)
10.5
10.0
9.5
9.0 -45 -20
5
55 80 30 TEMPERATURE (C)
105
130
3788 G10
Shutdown Current vs Temperature
11.0 10.5 10.0 SHUTDOWN CURRENT (A) 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 -45 -20 80 5 55 30 TEMPERATURE (C) 105 130
3788 G11
Shutdown Current vs Input Voltage
20
VIN = 12V SHUTDOWN CURRENT (A)
15
10
5
0 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40
3788 G12
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LTC3788 TYPICAL PERFORMANCE CHARACTERISTICS
Quiescent Current vs Temperature
170 VIN = 12V VFB = 1.25V 160 RUN2 = GND QUIESCENT CURRENT (A) 150 140 130 120 110 100 -45 -20 RUN PIN VOLTAGE (V) 1.40 1.35 1.30 1.25 1.20 1.15 1.10 -45 -20 RUN RISING
Shutdown (RUN) Threshold vs Temperature
RUN FALLING
5
80 55 30 TEMPERATURE (C)
105
130
3788 G13
5
80 55 30 TEMPERATURE (C)
105
130
3788 G14
Undervoltage Lockout Threshold vs Temperature
4.4 4.3 4.2 INTVCC VOLTAGE (V) 4.1 4.0 3.9 3.8 3.7 3.6 3.5 3.4 -45 -20 5 55 80 30 TEMPERATURE (C) 105 130
3788 G15
INTVCC Line Regulation
5.5 5.4
INTVCC RISING INTVCC VOLTAGE (V)
5.3 5.2 5.1 5.0 4.9 4.8 4.7 4.6 4.5 0 5 10 15 20 25 30 INPUT VOLTAGE (V) 35 40
3788 G16
INTVCC FALLING
INTVCC vs INTVCC Load Current
5.50 5.45 5.40 INTVCC VOLTAGE (V) 5.35 5.30 5.25 5.20 5.15 5.10 5.05 5.00 0 20 40 60 80 100 120 140 160 180 200 INTVCC LOAD CURRENT (mA)
3788 G17
EXTVCC Switchover and INTVCC Voltages vs Temperature
6.0 5.8 EXTVCC AND INTVCC VOLTAGE (V)
VIN = 12V
EXTVCC = 0V
5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2 4.0 -45 -20
INTVCC
EXTVCC RISING EXTVCC FALLING
EXTVCC = 6V
55 30 5 80 TEMPERATURE (C)
105
130
3788 G18
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LTC3788 TYPICAL PERFORMANCE CHARACTERISTICS
Oscillator Frequency vs Temperature
600 FREQ = INTVCC OSCILLATOR FREQUENCY (kHz) 550 FREQUENCY (kHz) 500 450 400 FREQ = GND 350 300 -45 360 358 356 354 352 350 348 346 344 342 340 -20 55 30 5 80 TEMPERATURE (C) 105 130
3788 G19
Oscillator Frequency vs Input Voltage
FREQ = GND
5
10
20 25 30 15 INPUT VOLTAGE (V)
35
40
3788 G20
Maximum Current Sense Threshold vs ITH Voltage
MAXIMUM CURRENT SENSE VOLTAGE (mV) 120 100 80 60 40 20 0 -20 -40 -60 0 0.2 0.4 0.6 0.8 1.0 ITH VOLTAGE (V) 1.2 1.4
3788 G21
SENSE Pin Input Current vs Temperature
260 VSENSE = 12V 240 ILIM = FLOAT 220 SENSE+ PIN 200 180 160 140 120 100 80 60 40 20 SENSE - PIN 0 55 30 -45 -20 5 80 TEMPERATURE (C)
PULSE SKIPPING MODE Burst Mode OPERATION SENSE CURRENT (A)
ILIM = GND ILIM = FLOAT ILIM = INTVCC FORCED CONTINUOUS MODE
105
130
3788 G22
SENSE Pin Input Current vs ITH Voltage
260 240 220 200 180 160 140 120 100 80 60 40 20 0 0 VSENSE = 12V SENSE+ PIN ILIM = INTVCC ILIM = FLOAT ILIM = GND SENSE CURRENT (A) 260 240 220 200 180 160 140 120 100 80 60 40 20 0
SENSE Pin Input Current vs VSENSE Voltage
SENSE+ PIN ILIM = INTVCC ILIM = FLOAT ILIM = GND
SENSE CURRENT (A)
SENSE - PIN 0.5
ILIM = INTVCC ILIM = FLOAT ILIM = GND 2 1.5 1 ITH VOLTAGE (V) 2.5 3
3788 G23
SENSE - PIN 2.5
ILIM = INTVCC ILIM = FLOAT ILIM = GND
7.5 12.5 17.5 22.5 27.5 32.5 37.5 VSENSE COMMON MODE VOLTAGE (V)
3788 G24
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LTC3788 TYPICAL PERFORMANCE CHARACTERISTICS
Maximum Current Sense Threshold vs Duty Cycle
MAXIMUM CURRENT SENSE VOLTAGE (mV) 120 100 80 60 40 20 0 0 10 20 30 40 50 60 70 80 90 100 DUTY CYCLE (%)
3788 G25
Charge Pump Charging Current vs Operating Frequency
80 CHARGE PUMP CHARGING CURRENT (A) VSW = 12V 70 VBOOST - VSW = 4.5V 60 T = 25C 50 40 30 20 10 0 50 150 250 350 450 550 650 750 OPERATING FREQUENCY (kHz)
3788 G26
ILIM = INTVCC
T = -45C
ILIM = FLOAT
ILIM = GND
T = 130C
PIN FUNCTIONS
SENSE1-, SENSE2- (Pin 1, Pin 9): Negative Current Sense Comparator Input. The (-) input to the current comparator is normally connected to the negative terminal of a current sense resistor connected in series with the inductor. The common mode voltage range on these pins is 2.5V to 38V (abs max). FREQ (Pin 2): The frequency control pin for the internal VCO. Connecting the pin to GND forces the VCO to a fixed low frequency of 350kHz. Connecting the pin to INTVCC forces the VCO to a fixed high frequency of 535kHz. The frequency can be programmed from 50kHz to 900kHz by connecting a resistor from the FREQ pin to GND. The resistor and an internal 20A source current create a voltage used by the internal oscillator to set the frequency. Alternatively, this pin can be driven with a DC voltage to vary the frequency of the internal oscillator. PHASMD (Pin 3): This pin can be floated, tied to SGND, or tied to INTVCC to program the phase relationship between the rising edges of BG1 and BG2, as well as the phase relationship between BG1 and CLKOUT. CLKOUT (Pin 4): A Digital Output Used for Daisychaining Multiple LTC3788 ICs in Multiphase Systems. The PHASMD pin voltage controls the relationship between BG1, BG2 and CLKOUT. This pin swings between SGND and INTVCC . PLLIN/MODE (Pin 5): Forced Continuous Mode, Burst Mode or Pulse-Skipping Mode Selection Pin and External Synchronization Input to Phase Detector Pin. Pulling this pin to ground selects Burst Mode operation. Tying this pin to INTVCC forces continuous inductor current operation. Tying this pin to a voltage greater than 1.2V and less than INTVCC -1.3V selects pulse-skipping operation. A clock on the pin will force the controller into pulse-skipping mode of operation and synchronize the internal oscillator. SGND (Pin 6): Signal Ground. All small-signal components and compensation components should connect to this ground, which in turn connects to PGND at a single point. RUN1, RUN2 (Pin 7, Pin 8): Run Control Input. An external resistor divider connects to VIN and sets the thresholds for converter operation with a threshold of 1.28V. Once running, a 4.5A current is sourced from the RUN pin allowing the user to program hysteresis using the resistor values.
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LTC3788 PIN FUNCTIONS
PGOOD2 (Pin 14): Power Good Indicator for Channel 2. Open-drain logic output that is pulled to ground when the output voltage is more than 10% away from the regulated output voltage. To avoid false trips the output voltage must be outside the range for 25s before this output is activated. INTVCC (Pin 19): Output of Internal 5.4V LDO. Power supply for control circuits and gate drives. Decouple this pin to GND with a minimum 4.7F low ESR tantalum or ceramic capacitor. EXTVCC (Pin 20): External Power Input. When this pin is higher than 4.8V an internal switch bypasses the internal regulator and supply power to INTVCC directly from EXTVCC. PGND (Pin 21): Driver Power Ground. Connects to the sources of bottom (main) N-channel MOSFETs and the (-) terminal(s) of CIN and COUT. VBIAS (Pin 22): Main Supply Pin. It is normally tied to the input supply VIN or to the output of the boost converter. A bypass capacitor should be tied between this pin and the signal ground pin. The operating voltage range on this pin is 4.5V to 38V (40V abs max). BG1, BG2 (Pin 23, Pin 18): Bottom Gate. Connect to the gate of the main NMOS. BOOST1, BOOST2 (Pin 24, Pin 17): Floating power supply for the synchronous NMOS. Bypass to SW with a capacitor and supply with a Schottky diode connected to INTVCC. TG1, TG2 (Pin 25, Pin 16): Top Gate. Connect to the gate of the synchronous NMOS. SW1, SW2 (Pin 26, Pin 15): Switch Node. Connect to the source of the synchronous NMOS, the drain of the main NMOS and the inductor. PGOOD1 (Pin 27): Power Good Indicator for Channel 1. Open-drain logic output that is pulled to ground when the output voltage is more than 10% away from the regulated output voltage. To avoid false trips the output voltage must be outside the range for 25s before this output is activated. ILIM (Pin 28): Current Comparator Sense Voltage Range Input. This pin is used to set the peak current sense voltage in the current comparator. Connect this pin to SGND, open and INTVCC to set the peak current sense voltage to 50mV, 75mV, and 100mV, respectively. SS1, SS2 (Pin 29, Pin 13): Output Soft-Start Input. A capacitor to ground at this pin sets the ramp rate of the output voltage during start-up. ITH1, ITH2 (Pin 30, Pin 12): Current Control Threshold and Error Amplifier Compensation Point. The voltage on this pin sets the current trip threshold. VFB1, VFB2 (Pin 31, Pin 11): Error Amplifier Feedback Input. This pin receives the remotely sensed feedback voltage from an external resistive divider connected across the output. SENSE1+, SENSE2+ (Pin 32, Pin 10): Positive Current Sense Comparator Input. The (+) input to the current comparator is normally connected to the positive terminal of a current sense resistor. The current sense resistor is normally placed at the input of the boost controller in series with the inductor. This pin also supplies power to the current comparator. GND (Exposed Pad Pin 33): Ground. The exposed pad must be soldered to the circuit board for rated thermal performance.
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LTC3788 BLOCK DIAGRAM
INTVCC PHASMD CLKOUT DUPLICATE FOR SECOND CONTROLLER CHANNEL S R Q TG SHDN SWITCHING LOGIC AND CHARGE PUMP CLK2 VCO CLK1 0.425V + - PFD - + 2.8V 0.7V PLLIN/ MODE SYNC DET 100k ILIM CURRENT LIMIT VBIAS OV EXTVCC 5.4V LDO EN + 4.8V - INTVCC SGND 5.4V LDO EN SHDN 0.5A/ 4.5A 10A 11V SENS LO SS 1.32V VFB + 1.08V - + - SLOPE COMP + SENS LO - 2.5V VFB + EA - - + - 1.32V ITH CC 1.2V SS SENSE+ RSENSE VIN CIN -+ +- 2mV + - SENSE - SLEEP SW INTVCC BG PGND VOUT COUT CB BOOST DB
20A FREQ
L
PGOOD
CC2
RC
-
+ 3.8V SHDN RUN
CSS
3788 BD
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LTC3788 OPERATION
Main Control Loop The LTC3788 uses a constant-frequency, current mode step-up architecture with the two controller channels operating 180 or 240 degrees out-of-phase (depending on the PHASMD pin connection). During normal operation, each external bottom MOSFET is turned on when the clock for that channel sets the RS latch, and is turned off when the main current comparator, ICMP, resets the RS latch. The peak inductor current at which ICMP trips and resets the latch is controlled by the voltage on the ITH pin, which is the output of the error amplifier EA. The error amplifier compares the output voltage feedback signal at the VFB pin, (which is generated with an external resistor divider connected across the output voltage, VOUT, to ground) to the internal 1.200V reference voltage. When the load current increases, it causes a slight decrease in VFB relative to the reference, which causes the EA to increase the ITH voltage until the average inductor current matches the new load current. After the bottom MOSFET is turned off each cycle, the top MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current comparator IR, or the beginning of the next clock cycle. INTVCC /EXTVCC Power Power for the top and bottom MOSFET drivers and most other internal circuitry is derived from the INTVCC pin. When the EXTVCC pin is left open or tied to a voltage less than 4.8V, the VBIAS LDO (low dropout linear regulator) supplies 5.4V from VBIAS to INTVCC. If EXTVCC is taken above 4.8V, the VBIAS LDO is turned off and an EXTVCC LDO is turned on. Once enabled, the EXTVCC LDO supplies
(Refer to Block Diagram)
5.4V from EXTVCC to INTVCC. Using the EXTVCC pin allows the INTVCC power to be derived from a high efficiency external source such as one of the LTC3788 switching regulator outputs. Shutdown and Start-Up (RUN1, RUN2 and SS1, SS2 Pins) The two channels of the LTC3788 can be independently shut down using the RUN1 and RUN2 pins. Pulling either of these pins below 1.28V shuts down the main control loop for that controller. Pulling both pins below 0.7V disables both controllers and most internal circuits, including the INTVCC LDOs. In this state, the LTC3788 draws only 8A of quiescent current. The RUN pin may be externally pulled up or driven directly by logic. When driving the RUN pin with a low impedance source, do not exceed the absolute maximum rating of 8V. The RUN pin has an internal 11V voltage clamp that allows the RUN pin to be connected through a resistor to a higher voltage (for example, VIN), as long as the maximum current into the RUN pin does not exceed 100A. The start-up of each controller's output voltage VOUT is controlled by the voltage on the SS pin for that channel. When the voltage on the SS pin is less than the 1.2V internal reference, the LTC3788 regulates the VFB voltage to the SS pin voltage instead of the 1.2V reference. This allows the SS pin to be used to program a soft-start by connecting an external capacitor from the SS pin to SGND. An internal 10A pull-up current charges this capacitor creating a voltage ramp on the SS pin. As the SS voltage rises linearly from 0V to 1.2V (and beyond up to INTVCC), the output voltage VOUT rises smoothly to its final value.
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LTC3788 OPERATION
Light Load Current Operation--Burst Mode Operation, Pulse-Skipping or Continuous Conduction (PLLIN/MODE Pin) The LTC3788 can be enabled to enter high efficiency Burst Mode operation, constant-frequency pulse-skipping mode or forced continuous conduction mode at low load currents. To select Burst Mode operation, tie the PLLIN/ MODE pin to a ground (e.g., SGND). To select forced continuous operation, tie the PLLIN/MODE pin to INTVCC. To select pulse-skipping mode, tie the PLLIN/MODE pin to a DC voltage greater than 1.2V and less than INTVCC - 1.3V. When a controller is enabled for Burst Mode operation, the minimum peak current in the inductor is set to approximately 30% of the maximum sense voltage even though the voltage on the ITH pin indicates a lower value. If the average inductor current is higher than the load current, the error amplifier EA will decrease the voltage on the ITH pin. When the ITH voltage drops below 0.425V, the internal sleep signal goes high (enabling sleep mode) and both external MOSFETs are turned off. In sleep mode, much of the internal circuitry is turned off, reducing the quiescent current that the LTC3788 draws. If one channel is shut down and the other channel is in sleep mode, the LTC3788 draws only 125A of quiescent current. If both channels are in sleep mode, the LTC3788 draws only 200A of quiescent current. In sleep mode, the load current is supplied by the output capacitor. As the output voltage decreases, the EA's output begins to rise. When the output voltage drops enough, the ITH pin is reconnected to the output of the EA, the sleep signal goes low, and the controller resumes normal operation by turning on the bottom external MOSFET on the next cycle of the internal oscillator. When a controller is enabled for Burst Mode operation, the inductor current is not allowed to reverse. The reverse current comparator (IR) turns off the top external MOSFET just before the inductor current reaches zero, preventing it from reversing and going negative. Thus, the controller operates in discontinuous current operation. In forced continuous operation or when clocked by an external clock source to use the phase-locked loop (see the Frequency Selection and Phase-Locked Loop section), the inductor current is allowed to reverse at light loads or under large transient conditions. The peak inductor current is determined by the voltage on the ITH pin, just as in normal operation. In this mode, the efficiency at light loads is lower than in Burst Mode operation. However, continuous operation has the advantages of lower output voltage ripple and less interference to audio circuitry, as it maintains constant-frequency operation independent of load current. When the PLLIN/MODE pin is connected for pulse-skipping mode, the LTC3788 operates in PWM pulse-skipping mode at light loads. In this mode, constant-frequency operation is maintained down to approximately 1% of designed maximum output current. At very light loads, the current comparator ICMP may remain tripped for several cycles and force the external bottom MOSFET to stay off for the same number of cycles (i.e., skipping pulses). The inductor current is not allowed to reverse (discontinuous operation). This mode, like forced continuous operation, exhibits low output ripple as well as low audio noise and reduced RF interference as compared to Burst Mode operation. It provides higher low current efficiency than forced continuous mode, but not nearly as high as Burst Mode operation.
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LTC3788 OPERATION
Frequency Selection and Phase-Locked Loop (FREQ and PLLIN/MODE Pins) The selection of switching frequency is a trade-off between efficiency and component size. Low frequency operation increases efficiency by reducing MOSFET switching losses, but requires larger inductance and/or capacitance to maintain low output ripple voltage. The switching frequency of the LTC3788's controllers can be selected using the FREQ pin. If the PLLIN/MODE pin is not being driven by an external clock source, the FREQ pin can be tied to SGND, tied to INTVCC , or programmed through an external resistor. Tying FREQ to SGND selects 350kHz while tying FREQ to INTVCC selects 535kHz. Placing a resistor between FREQ and SGND allows the frequency to be programmed between 50kHz and 900kHz, as shown in Figure 6. A phase-locked loop (PLL) is available on the LTC3788 to synchronize the internal oscillator to an external clock source that is connected to the PLLIN/MODE pin. The LTC3788's phase detector adjusts the voltage (through an internal lowpass filter) of the VCO input to align the turn-on of the first controller's external bottom MOSFET to the rising edge of the synchronizing signal. Thus, the turn-on of the second controller's external bottom MOSFET is 180 or 240 degrees out-of-phase to the rising edge of the external clock source. The VCO input voltage is prebiased to the operating frequency set by the FREQ pin before the external clock is applied. If prebiased near the external clock frequency, the PLL loop only needs to make slight changes to the VCO input in order to synchronize the rising edge of the external clock's to the rising edge of BG1. The ability to prebias the loop filter allows the PLL to lock-in rapidly without deviating far from the desired frequency. The typical capture range of the LTC3788's PLL is from approximately 55kHz to 1MHz, and is guaranteed to lock to an external clock source whose frequency is between 75kHz and 850kHz. The typical input clock thresholds on the PLLIN/MODE pin are 1.6V (rising) and 1.2V (falling). PolyPhase Applications (CLKOUT and PHASMD Pins) The LTC3788 features two pins (CLKOUT and PHASMD) that allow other controller ICs to be daisychained with the LTC3788 in PolyPhase(R) applications. The clock output signal on the CLKOUT pin can be used to synchronize additional power stages in a multiphase power supply solution feeding a single, high current output or multiple separate outputs. The PHASMD pin is used to adjust the phase of the CLKOUT signal as well as the relative phases between the two internal controllers, as summarized in Table 1. The phases are calculated relative to the zero degrees phase being defined as the rising edge of the bottom gate driver output of controller 1 (BG1).
Table 1.
VPHASMD GND Floating INTVCC CONTROLLER 2 PHASE (C) 180 180 240 CLKOUT PHASE (C) 60 90 120
CLKOUT is disabled when one of the channels is in sleep mode and another channel is either in shutdown or in sleep mode.
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LTC3788 OPERATION
Operation When VIN > VOUT When VIN rises above the regulated VOUT voltage, the boost controller can behave differently depending on the mode, inductor current and VIN voltage. In forced continuous mode, the loop works to keep the top MOSFET on continuously once VIN rises above VOUT. The internal charge pump delivers current to the boost capacitor to maintain a sufficiently high TG voltage. In pulse-skipping mode, if VIN is between 100% and 110% of the regulated VOUT voltage, TG turns on if the inductor current rises above a certain threshold and turns off if the inductor current falls below this threshold. This threshold current is set to approximately 6%, 4% or 3% of the maximum ILIM current when the ILIM pin is grounded, floating or tied to INTVCC, respectively. If the controller is programmed to Burst Mode operation under this same VIN window, then TG remains off regardless of the inductor current. If VIN rises above 110% of the regulated VOUT voltage in any mode, the controller turns on TG regardless of the inductor current. In Burst Mode operation, however, the internal charge pump turns off if the entire chip is asleep (the other channel is asleep or shut down). With the charge pump off, there would be nothing to prevent the boost capacitor from discharging, resulting in an insufficient TG voltage needed to keep the top MOSFET completely on. To prevent excessive power dissipation across the body diode of the top MOSFET in this situation, the chip can be switched over to forced continuous mode to enable the charge pump, or a Schottky diode can also be placed in parallel to the top MOSFET. Power Good The PGOOD1, 2 pin is connected to an open-drain of an internal N-channel MOSFET. The MOSFET turns on and pulls the PGOOD1, 2 pin low when the corresponding VFB1, 2 pin voltage is not within 10% of the 1.2V reference voltage. The PGOOD1, 2 pin is also pulled low when the corresponding RUN1, 2 pin is low (shut down). When the VFB1, 2 pin voltage is within the 10% requirement, the MOSFET is turned off and the pin is allowed to be pulled up by an external resistor to a source of up to 6V. Operation at Low SENSE Pin Common Voltage The current comparator in the LTC3788 is powered directly from the SENSE + pin. This enables the common mode voltage of SENSE + and SENSE - pins to operate at as low as 2.5V, which is below the UVLO threshold. The figure on the first page shows a typical application when the controller's VBIAS is powered from VOUT while VIN supply can go as low as 2.5V. If the voltage on SENSE + drops below 2.5V, the SS pin will be held low. When the SENSE voltage returns to the normal operating range, the SS pin will be released, initiating a new soft-start cycle. BOOST Supply Refresh and Internal Charge Pump Each top MOSFET driver is biased from the floating bootstrap capacitor CB, which normally recharges during each cycle through an external diode when the bottom MOSFET turns on. There are two considerations to keep the BOOST supply at the required bias level. During start-up, if the bottom MOSFET is not turned on within 100s after UVLO goes low, the bottom MOSFET will be forced to turn on for ~400ns. This forced refresh generates enough BOOST-SW voltage to allow the top MOSFET ready to be fully enhanced instead of waiting for the initial few cycles to charge up. There is also an internal charge pump that keeps the required bias on BOOST. The charge pump always operates in both forced continuous mode and pulse-skipping mode. In Burst Mode operation, the charge pump is turned off during sleep and enabled when the chip wakes up. The internal charge pump can normally supply a charging current of 55A.
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LTC3788 APPLICATIONS INFORMATION
The Typical Application on the first page is a basic LTC3788 application circuit. LTC3788 can be configured to use either inductor DCR (DC resistance) sensing or a discrete sense resistor (RSENSE) for current sensing. The choice between the two current sensing schemes is largely a design trade-off between cost, power consumption and accuracy. DCR sensing is becoming popular because it does not require current sensing resistors and is more power-efficient, especially in high current applications. However, current sensing resistors provide the most accurate current limits for the controller. Other external component selection is driven by the load requirement, and begins with the selection of RSENSE (if RSENSE is used) and inductor value. Next, the power MOSFETs are selected. Finally, input and output capacitors are selected. SENSE+ and SENSE- Pins The SENSE + and SENSE - pins are the inputs to the current comparators. The common mode input voltage range of the current comparators is 2.5V to 38V. The current sense resistor is normally placed at the input of the boost controller in series with the inductor. The SENSE+ pin also provides power to the current comparator. It draws ~200A during normal operation. There is a small base current of less than 1A that flows into the SENSE - pin. The high impedance SENSE - input to the current comparators allow accurate DCR sensing. Filter components mutual to the sense lines should be placed close to the LTC3788, and the sense lines should run close together to a Kelvin connection underneath the current sense element (shown in Figure 1). Sensing current elsewhere can effectively add parasitic inductance and capacitance to the current sense element, degrading the information at the sense terminals and making the programmed current limit unpredictable. If DCR sensing is used (Figure 2b), sense resistor R1 should be placed close to the switching node, to prevent noise from coupling into sensitive small-signal nodes.
TO SENSE FILTER, NEXT TO THE CONTROLLER
VIN INDUCTOR OR RSENSE
3788 F01
Figure 1. Sense Lines Placement with Inductor or Sense Resistor
VBIAS SENSE+ (OPTIONAL) SENSE- INTVCC LTC3788 BOOST TG SW BG SGND
3788 F02a
VIN
VBIAS SENSE+ C1 SENSE- INTVCC LTC3788 BOOST TG R1 L R2 DCR
VIN
INDUCTOR
VOUT
SW BG SGND
3788 F02b
VOUT
PLACE C1 NEAR SENSE PINS
(R1||R2) * C1 =
L DCR
RSENSE(EQ) = DCR *
R2 R1 + R2
(2a) Using a Resistor to Sense Current
(2b) Using the Inductor DCR to Sense Current
Figure 2. Two Different Methods of Sensing Current
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LTC3788 APPLICATIONS INFORMATION
Sense Resistor Current Sensing A typical sensing circuit using a discrete resistor is shown in Figure 2a. RSENSE is chosen based on the required output current. The current comparator has a maximum threshold VSENSE(MAX). When the ILIM pin is grounded, floating or tied to INTVCC, the maximum threshold is set to 50mV, 75mV or 100mV, respectively. The current comparator threshold sets the peak of the inductor current, yielding a maximum average output current, IMAX, equal to the peak value less half the peak-to-peak ripple current, IL. To calculate the sense resistor value, use the equation: R SENSE = VSENSE(MAX ) IMAX + IL 2 using a good RLC meter, but the DCR tolerance is not always the same and varies with temperature. Consult the manufacturer's data sheets for detailed information. Using the inductor ripple current value from the inductor value calculation section, the target sense resistor value is: VSENSE(MAX ) R SENSE(EQUIV ) = I IMAX + L 2 To ensure that the application will deliver full load current over the full operating temperature range, choose the minimum value for the maximum current sense threshold (VSENSE(MAX)). Next, determine the DCR of the inductor. Where provided, use the manufacturer's maximum value, usually given at 20C. Increase this value to account for the temperature coefficient of resistance, which is approximately 0.4%/C. A conservative value for the maximum inductor temperature (TL(MAX)) is 100C. To scale the maximum inductor DCR to the desired sense resistor value, use the divider ratio: R SENSE(EQUIV ) RD = DCRMAX at TL(MAX ) C1 is usually selected to be in the range of 0.1F to 0.47F . This forces R1|| R2 to around 2k, reducing error that might have been caused by the SENSE + pin's 1A current. The equivalent resistance R1|| R2 is scaled to the room temperature inductance and maximum DCR: L R1|| R2 = (DCR at 20 C) * C1 The sense resistor values are: R1 * RD R1|| R2 R1 = ; R2 = RD 1 - RD The maximum power loss in R1 is related to duty cycle, and will occur in continuous mode at VIN = 1/2 VOUT: (V - VIN ) * VIN PLOSS R1 = OUT R1
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When using the controller in low VIN and very high voltage output applications, the maximum output current level will be reduced due to the internal compensation required to meet stability criterion for boost regulators operating at greater than 50% duty factor. A curve is provided in the Typical Performance Characteristics section to estimate this reduction in peak output current level depending upon the operating duty factor. Inductor DCR Sensing For applications requiring the highest possible efficiency at high load currents, the LTC3788 is capable of sensing the voltage drop across the inductor DCR, as shown in Figure 2b. The DCR of the inductor can be less than 1m for high current inductors. In a high current application requiring such an inductor, conduction loss through a sense resistor could reduce the efficiency by a few percent compared to DCR sensing. If the external R1||R2 * C1 time constant is chosen to be exactly equal to the L/DCR time constant, the voltage drop across the external capacitor is equal to the drop across the inductor DCR multiplied by R2/(R1 + R2). R2 scales the voltage across the sense terminals for applications where the DCR is greater than the target sense resistor value. To properly dimension the external filter components, the DCR of the inductor must be known. It can be measured
17
LTC3788 APPLICATIONS INFORMATION
Ensure that R1 has a power rating higher than this value. If high efficiency is necessary at light loads, consider this power loss when deciding whether to use DCR sensing or sense resistors. Light load power loss can be modestly higher with a DCR network than with a sense resistor, due to the extra switching losses incurred through R1. However, DCR sensing eliminates a sense resistor, reduces conduction losses and provides higher efficiency at heavy loads. Peak efficiency is about the same with either method. Inductor Value Calculation The operating frequency and inductor selection are interrelated in that higher operating frequencies allow the use of smaller inductor and capacitor values. Why would anyone ever choose to operate at lower frequencies with larger components? The answer is efficiency. A higher frequency generally results in lower efficiency because of MOSFET gate charge and switching losses. In addition to this basic trade-off, the effect of inductor value on ripple current and low current operation must also be considered. The inductor value has a direct effect on ripple current. The inductor ripple current IL decreases with higher inductance or frequency and increases with higher VIN: V V IL = IN 1 - IN f *L VOUT Accepting larger values of IL allows the use of low inductances, but results in higher output voltage ripple and greater core losses. A reasonable starting point for setting ripple current is IL = 0.3(IMAX). The maximum IL occurs at VIN = 1/2 VOUT. The inductor value also has secondary effects. The transition to Burst Mode operation begins when the average inductor current required results in a peak current below 10% of the current limit determined by RSENSE. Lower inductor values (higher IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to decrease. Inductor Core Selection Once the value for L is known, the type of inductor must be selected. High efficiency converters generally cannot afford the core loss found in low cost powdered iron cores, forcing the use of more expensive ferrite or molypermalloy cores. Actual core loss is independent of core size for a fixed inductor value, but it is very dependent on inductance selected. As inductance increases, core losses go down. Unfortunately, because increased inductance requires more turns of wire, copper losses will increase. Ferrite core inductors have very low core loss and are preferred at high switching frequencies, so design goals can concentrate on copper loss and preventing saturation. Ferrite core material saturates "hard," which means that inductance collapses abruptly when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and consequent output voltage ripple. Do not allow the core to saturate! Power MOSFET Selection Two external power MOSFETs must be selected for each controller in the LTC3788: one N-channel MOSFET for the bottom (main) switch, and one N-channel MOSFET for the top (synchronous) switch. The peak-to-peak gate drive levels are set by the INTVCC voltage. This voltage is typically 5.4V during start-up (see EXTVCC pin connection). Consequently, logic-level threshold MOSFETs must be used in most applications. The only exception is if low input voltage is expected (VIN < 5V); then, sub-logic level threshold MOSFETs (VGS(TH) < 3V) should be used. Pay close attention to the BVDSS specification for the MOSFETs as well; many of the logic level MOSFETs are limited to 30V or less. Selection criteria for the power MOSFETs include the on-resistance RDS(ON), Miller capacitance CMILLER, input voltage and maximum output current. Miller capacitance, CMILLER, can be approximated from the gate charge curve usually provided on the MOSFET manufacturer's data sheet. CMILLER is equal to the increase in gate charge along the horizontal axis while the curve is approximately flat divided by the specified change in VDS. This result is then multiplied by the ratio of the application applied VDS to the gate charge curve specified VDS. When the IC is
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LTC3788 APPLICATIONS INFORMATION
operating in continuous mode, the duty cycles for the top and bottom MOSFETs are given by: Main Switch Duty Cycle = VOUT - VIN VOUT VIN VOUT The term (1+ ) is generally given for a MOSFET in the form of a normalized RDS(ON) vs Temperature curve, but = 0.005/C can be used as an approximation for low voltage MOSFETs. CIN and COUT Selection The input ripple current in a boost converter is relatively low (compared with the output ripple current), because this current is continuous. The input capacitor CIN voltage rating should comfortably exceed the maximum input voltage. Although ceramic capacitors can be relatively tolerant of overvoltage conditions, aluminum electrolytic capacitors are not. Be sure to characterize the input voltage for any possible overvoltage transients that could apply excess stress to the input capacitors. The value of the CIN is a function of the source impedance, and in general, the higher the source impedance, the higher the required input capacitance. The required amount of input capacitance is also greatly affected by the duty cycle. High output current applications that also experience high duty cycles can place great demands on the input supply, both in terms of DC current and ripple current. In a boost converter, the output has a discontinuous current, so COUT must be capable of reducing the output voltage ripple. The effects of ESR (equivalent series resistance) and the bulk capacitance must be considered when choosing the right capacitor for a given output ripple voltage. The steady ripple voltage due to charging and discharging the bulk capacitance is given by: VRIPPLE = IOUT(MAX ) * ( VOUT - VIN(MIN) ) COUT * VOUT * f V
Synchronous S witch Duty Cycle =
The MOSFET power dissipations at maximum output current are given by: PMAIN = ( VOUT - VIN )VOUT V 2IN * IOUT(MAX )2 * (1 + ) IOUT(MAX ) VIN * RDR
* RDS(ON) + k * V 3OUT * * CMILLER * f PSYNC =
VIN * IOUT(MAX )2 * (1 + ) * RDS(ON) VOUT
where is the temperature dependency of RDS(ON) and RDR (approximately 1) is the effective driver resistance at the MOSFET's Miller threshold voltage. The constant k, which accounts for the loss caused by reverse recovery current, is inversely proportional to the gate drive current and has an empirical value of 1.7. Both MOSFETs have I2R losses while the bottom N-channel equation includes an additional term for transition losses, which are highest at low input voltages. For high VIN the high current efficiency generally improves with larger MOSFETs, while for low VIN the transition losses rapidly increase to the point that the use of a higher RDS(ON) device with lower CMILLER actually provides higher efficiency. The synchronous MOSFET losses are greatest at high input voltage when the bottom switch duty factor is low or during overvoltage when the synchronous switch is on close to 100% of the period.
where COUT is the output filter capacitor. The steady ripple due to the voltage drop across the ESR is given by: VESR = IL(MAX) * ESR
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LTC3788 APPLICATIONS INFORMATION
The LTC3788 can also be configured as a 2-phase single output converter where the outputs of the two channels are connected together and both channels have the same duty cycle. With 2-phase operation, the two channels of the dual switching regulator are operated 180 degrees outof-phase. This effectively interleaves the output capacitor current pulses, greatly reducing the output capacitor ripple current. As a result, the ESR requirement of the capacitor can be relaxed. Because the ripple current in the output capacitor is a square wave, the ripple current requirements for the output capacitor depend on the duty cycle, the number of phases and the maximum output current. Figure 3 illustrates the normalized output capacitor ripple current as a function of duty cycle in a 2-phase configuration. To choose a ripple current rating for the output capacitor, first establish the duty cycle range based on the output voltage and range of input voltage. Referring to Figure 3, choose the worst-case high normalized ripple current as a percentage of the maximum load current. Multiple capacitors placed in parallel may be needed to meet the ESR and RMS current handling requirements. Dry tantalum, special polymer, aluminum electrolytic and ceramic capacitors are all available in surface mount packages. Ceramic capacitors have excellent low ESR characteristics but can have a high voltage coefficient. Capacitors are now available with low ESR and high ripple current ratings (i.e., OS-CON and POSCAP).
3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00 0.75 0.50 0.25 0 0.1
IORIPPLE /IOUT
1-PHASE
2-PHASE
0.2
0.3 0.4 0.5 0.6 0.7 0.8 DUTY CYCLE OR (1-VIN /VOUT)
0.9
3788 F03
Figure 3. Normalized Output Capacitor Ripple Current (RMS) for a Boost Converter
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LTC3788 APPLICATIONS INFORMATION
Setting Output Voltage The LTC3788 output voltages are each set by an external feedback resistor divider carefully placed across the output, as shown in Figure 4. The regulated output voltage is determined by: R VOUT = 1 . 2V 1 + B R
A
INTVCC Regulators The LTC3788 features two separate internal P-channel low dropout linear regulators (LDO) that supply power at the INTVCC pin from either the VBIAS supply pin or the EXTVCC pin depending on the connection of the EXTVCC pin. INTVCC powers the gate drivers and much of the LTC3788's internal circuitry. The VBIAS LDO and the EXTVCC LDO regulate INTVCC to 5.4V. Each of these can supply a peak current of 50mA and must be bypassed to ground with a minimum of 4.7F ceramic capacitor. Good bypassing is needed to supply the high transient currents required by the MOSFET gate drivers and to prevent interaction between the channels. High input voltage applications in which large MOSFETs are being driven at high frequencies may cause the maximum junction temperature rating for the LTC3788 to be exceeded. The INTVCC current, which is dominated by the gate charge current, may be supplied by either the VBIAS LDO or the EXTVCC LDO. When the voltage on the EXTVCC pin is less than 4.8V, the VBIAS LDO is enabled. In this case, power dissipation for the IC is highest and is equal to VIN * IINTVCC. The gate charge current is dependent on operating frequency, as discussed in the Efficiency Considerations section. The junction temperature can be estimated by using the equations given in Note 3 of the Electrical Characteristics. For example, the LTC3788 INTVCC current is limited to less than 40mA from a 40V supply when not using the EXTVCC supply: TJ = 70C + (40mA)(40V)(34C/W) = 125C
VOUT RB CSS SGND
3788 F05
Great care should be taken to route the VFB line away from noise sources, such as the inductor or the SW line. Soft-Start (SS Pins) The start-up of each VOUT is controlled by the voltage on the respective SS pins. When the voltage on the SS pin is less than the internal 1.2V reference, the LTC3788 regulates the VFB pin voltage to the voltage on the SS pin instead of 1.2V. Soft-start is enabled by simply connecting a capacitor from the SS pin to ground, as shown in Figure 5. An internal 10A current source charges the capacitor, providing a linear ramping voltage at the SS pin. The LTC3788 will regulate the VFB pin (and hence, VOUT) according to the voltage on the SS pin, allowing VOUT to rise smoothly from VIN to its final regulated value. The total soft-start time will be approximately: t SS = C SS * 1 . 2V 10A
LTC3788 VFB
LTC3788 SS
RA
3788 F04
Figure 4. Setting Output Voltage
Figure 5. Using the SS Pin to Program Soft-Start
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LTC3788 APPLICATIONS INFORMATION
To prevent the maximum junction temperature from being exceeded, the input supply current must be checked while operating in continuous conduction mode (PLLIN/MODE = INTVCC) at maximum VIN. When the voltage applied to EXTVCC rises above 4.7V, the VIN LDO is turned off and the EXTVCC LDO is enabled. The EXTVCC LDO remains on as long as the voltage applied to EXTVCC remains above 4.55V. The EXTVCC LDO attempts to regulate the INTVCC voltage to 5.4V, so while EXTVCC is less than 5.4V, the LDO is in dropout and the INTVCC voltage is approximately equal to EXTVCC. When EXTVCC is greater than 5.4V, up to an absolute maximum of 6V, INTVCC is regulated to 5.4V. The following list summarizes possible connections for EXTVCC: Fault Conditions: Overtemperature Protection At higher temperatures, or in cases where the internal power dissipation causes excessive self heating on-chip (such as an INTVCC short to ground), the overtemperature shutdown circuitry will shut down the LTC3788. When the junction temperature exceeds approximately 170C, the overtemperature circuitry disables the INTVCC LDO, causing the INTVCC supply to collapse and effectively shut down the entire LTC3788 chip. Once the junction temperature drops back to approximately 155C, the INTVCC LDO turns back on. Long term overstress (TJ > 125C) should be avoided as it can degrade the performance or shorten the life of the part. Phase-Locked Loop and Frequency Synchronization The LTC3788 has an internal phase-locked loop (PLL) comprised of a phase frequency detector, a low pass filter and a voltage-controlled oscillator (VCO). This allows the turn-on of the top MOSFET of controller 1 to be locked to the rising edge of an external clock signal applied to the PLLIN/MODE pin. The turn-on of controller 2's top MOSFET is thus 180 degrees out-of-phase with the external clock. The phase detector is an edge-sensitive digital type that provides zero degrees phase shift between the external and internal oscillators. This type of phase detector does not exhibit false lock to harmonics of the external clock. If the external clock frequency is greater than the internal oscillator's frequency, fOSC, then current is sourced continuously from the phase detector output, pulling up the VCO input. When the external clock frequency is less than fOSC, current is sunk continuously, pulling down the VCO input. If the external and internal frequencies are the same but exhibit a phase difference, the current sources turn on for an amount of time corresponding to the phase difference. The voltage at the VCO input is adjusted until the phase and frequency of the internal and external oscillators are identical. At the stable operating point, the phase detector output is high impedance and the internal filter capacitor, CLP, holds the voltage at the VCO input.
EXTVCC Left Open (or Grounded). This will cause INTVCC to be powered from the internal 5.4V regulator resulting in an efficiency penalty at high input voltages. EXTVCC Connected to an External Supply. If an external supply is available in the 5.4V to 6V range, it may be used to power EXTVCC providing it is compatible with the MOSFET gate drive requirements. Ensure that EXTVCC < VBIAS.
Topside MOSFET Driver Supply (CB, DB) External bootstrap capacitors CB connected to the BOOST pins supply the gate drive voltages for the topside MOSFETs. Capacitor CB in the Block Diagram is charged though external diode DB from INTVCC when the SW pin is low. When one of the topside MOSFETs is to be turned on, the driver places the CB voltage across the gate-source of the desired MOSFET. This enhances the MOSFET and turns on the topside switch. The switch node voltage, SW, rises to VIN and the BOOST pin follows. With the topside MOSFET on, the boost voltage is above the input supply: VBOOST = VIN + VINTVCC. The value of the boost capacitor CB needs to be 100 times that of the total input capacitance of the topside MOSFET(s). The reverse breakdown of the external Schottky diode must be greater than VIN(MAX).
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LTC3788 APPLICATIONS INFORMATION
Typically, the external clock (on PLLIN/MODE pin) input high threshold is 1.6V, while the input low threshold is 1.2V. Note that the LTC3788 can only be synchronized to an external clock whose frequency is within range of the LTC3788's internal VCO, which is nominally 55kHz to 1MHz. This is guaranteed to be between 75kHz and 850kHz. Rapid phase locking can be achieved by using the FREQ pin to set a free-running frequency near the desired synchronization frequency. The VCO's input voltage is prebiased at a frequency corresponding to the frequency set by the FREQ pin. Once prebiased, the PLL only needs to adjust the frequency slightly to achieve phase lock and synchronization. Although it is not required that the free-running frequency be near external clock frequency, doing so will prevent the operating frequency from passing through a large range of frequencies as the PLL locks. Table 2 summarizes the different states in which the FREQ pin can be used.
Table 2.
FREQ PIN 0V INTVCC Resistor Any of the Above PLLIN/MODE PIN DC Voltage DC Voltage DC Voltage External Clock FREQUENCY 350kHz 535kHz 50kHz to 900kHz Phase Locked to External Clock
Minimum On-Time Considerations Minimum on-time, tON(MIN), is the smallest time duration that the LTC3788 is capable of turning on the bottom MOSFET. It is determined by internal timing delays and the gate charge required to turn on the top MOSFET. Low duty cycle applications may approach this minimum ontime limit. In forced continuous mode, if the duty cycle falls below what can be accommodated by the minimum on-time, the controller will begin to skip cycles but the output will continue to be regulated. More cycles will be skipped when VIN increases. Once VIN rises above VOUT, the loop works to keep the top MOSFET on continuously. The minimum on-time for the LTC3788 is approximately 110ns. Efficiency Considerations The percent efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the greatest improvement. Percent efficiency can be expressed as: %Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc., are the individual losses as a percentage of input power.
1000 900 800 FREQUENCY (kHz) 700 600 500 400 300 200 100 0 15 25 35 45 55 65 75 85 95 105 115 125 FREQ PIN RESISTOR (k)
3788 F06
Figure 6. Relationship Between Oscillator Frequency and Resistor Value at the FREQ Pin
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LTC3788 APPLICATIONS INFORMATION
Although all dissipative elements in the circuit produce losses, four main sources usually account for most of the losses in LTC3788 circuits: 1) IC VIN current, 2) INTVCC regulator current, 3) I2R losses, 4) Bottom MOSFET transition losses. 1. The VIN current is the DC supply current given in the Electrical Characteristics table, which excludes MOSFET driver and control currents. VIN current typically results in a small (<0.1%) loss. 2. INTVCC current is the sum of the MOSFET driver and control currents. The MOSFET driver current results from switching the gate capacitance of the power MOSFETs. Each time a MOSFET gate is switched from low to high to low again, a packet of charge, dQ, moves from INTVCC to ground. The resulting dQ/dt is a current out of INTVCC that is typically much larger than the control circuit current. In continuous mode, IGATECHG = f(QT + QB), where QT and QB are the gate charges of the topside and bottom side MOSFETs. 3. DC I2R losses. These arise from the resistances of the MOSFETs, sensing resistor, inductor and PC board traces and cause the efficiency to drop at high output currents. 4. Transition losses apply only to the bottom MOSFET(s), and become significant only when operating at low input voltages. Transition losses can be estimated from: VOUT 3 Transition Loss = (1 . 7) *C I f VIN O(MAX ) RSS Other hidden losses, such as copper trace and internal battery resistances, can account for an additional 5% to 10% efficiency degradation in portable systems. It is very important to include these system-level losses during the design phase. Checking Transient Response The regulator loop response can be checked by looking at the load current transient response. Switching regulators take several cycles to respond to a step in DC (resistive) load current. When a load step occurs, VOUT shifts by an amount equal to ILOAD (ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT generating the feedback error signal that forces the regulator to adapt to the current change and return VOUT to its steady-state value. During this recovery time VOUT can be monitored for excessive overshoot or ringing, which would indicate a stability problem. OPTI-LOOP compensation allows the transient response to be optimized over a wide range of output capacitance and ESR values. The availability of the ITH pin not only allows optimization of control loop behavior, but it also provides a DC coupled and AC filtered closed loop response test point. The DC step, rise time and settling at this test point truly reflects the closed loop response. Assuming a predominantly second order system, phase margin and/or damping factor can be estimated using the percentage of overshoot seen at this pin. The bandwidth can also be estimated by examining the rise time at the pin. The ITH external components shown in the Figure 9 circuit will provide an adequate starting point for most applications. The ITH series RC-CC filter sets the dominant pole-zero loop compensation. The values can be modified slightly (from 0.5 to 2 times their suggested values) to optimize transient response once the final PC layout is complete and the particular output capacitor type and value have been determined. The output capacitors must be selected because the various types and values determine the loop gain and phase. An output current pulse of 20% to 80% of full-load current having a rise time of 1s to 10s will produce output voltage and ITH pin waveforms that will give a sense of the overall loop stability without breaking the feedback loop. Placing a power MOSFET and load resistor directly across the output capacitor and driving the gate with an appropriate signal generator is a practical way to produce a realistic load step condition. The initial output voltage step resulting from the step change in output current may
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LTC3788 APPLICATIONS INFORMATION
not be within the bandwidth of the feedback loop, so this signal cannot be used to determine phase margin. This is why it is better to look at the ITH pin signal which is in the feedback loop and is the filtered and compensated control loop response. The gain of the loop will be increased by increasing RC and the bandwidth of the loop will be increased by decreasing CC. If RC is increased by the same factor that CC is decreased, the zero frequency will be kept the same, thereby keeping the phase shift the same in the most critical frequency range of the feedback loop. The output voltage settling behavior is related to the stability of the closed-loop system and will demonstrate the actual overall supply performance. A second, more severe transient is caused by switching in loads with large (>1F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can alter its delivery of current quickly enough to prevent this sudden step change in output voltage if the load switch resistance is low and it is driven quickly. If the ratio of CLOAD to COUT is greater than 1:50, the switch rise time should be controlled so that the load rise time is limited to approximately 25 * CLOAD. Thus, a 10F capacitor would require a 250s rise time, limiting the charging current to about 200mA. Design Example As a design example for one channel, assume VIN = 12V(nominal), VIN = 22V (max), VOUT = 24V, IMAX = 4A, VSENSE(MAX) = 75mV, and f = 350kHz. The inductance value is chosen first based on a 30% ripple current assumption. The highest value of ripple current occurs at the maximum input voltage. Tie the PLLLPF pin to GND, generating 350kHz operation. The minimum inductance for 30% ripple current is: IL = VIN VIN 1- V f *L OUT
A 6.8H inductor will produce a 31% ripple current. The peak inductor current will be the maximum DC value plus one half the ripple current, or 9.25A. The RSENSE resistor value can be calculated by using the maximum current sense voltage specification with some accommodation for tolerances:
R SENSE 75mV = 0 . 008 9 . 25A
Choosing 1% resistors: RA = 5k and RB = 95.3k yields an output voltage of 24.072V. The power dissipation on the top side MOSFET can be easily estimated. Choosing a Vishay Si7848BDP . MOSFET results in: RDS(ON) = 0.012, CMILLER = 150pF At maximum input voltage with T(estimated) = 50C:
PMAIN = (24V - 12V) 24V (12V)2 * (4A )2
* 1 + (0 . 005)(50 C - 25 C) * 0 . 008 + (1 . 7)(24V)3 4A (150pF)(350kHz) = 0 . 7 W 12V
COUT is chosen to filter the square current in the output. The maximum output current peak is: IOUT(PEAK ) = 24 31 % * 4 1+ = 9 . 3A 12 2
A low ESR (5m) capacitor is suggested. This capacitor will limit output voltage ripple to 46.5mV (assuming ESR dominate ripple).
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LTC3788 APPLICATIONS INFORMATION
PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the IC. These items are also illustrated graphically in the layout diagram of Figure 7. Figure 8 illustrates the current waveforms present in the various branches of the 2-phase synchronous regulators operating in the continuous mode. Check the following in your layout: 1. Put the bottom N-channel MOSFETs MBOT1 and MBOT2 and the top N-channel MOSFETs MTOP1 and MTOP2 in one compact area with COUT. 2. Are the signal and power grounds kept separate? The combined IC signal ground pin and the ground return of CINTVCC must return to the combined COUT (-) terminals. The path formed by the bottom N-channel MOSFET and the CIN capacitor should have short leads and PC trace lengths. The output capacitor (-) terminals should be connected as close as possible to the (-) terminals of the input capacitor by placing the capacitors next to each other. 3. Do the LTC3788 VFB pins' resistive dividers connect to the (+) terminals of COUT? The resistive divider must be connected between the (+) terminal of COUT and signal ground and placed close to the VFB pin. The feedback resistor connections should not be along the high current input feeds from the input capacitor(s). 4. Are the SENSE - and SENSE + leads routed together with minimum PC trace spacing? The filter capacitor between SENSE + and SENSE - should be as close as possible to the IC. Ensure accurate current sensing with Kelvin connections at the sense resistor. 5. Is the INTVCC decoupling capacitor connected close to the IC, between the INTVCC and the power ground pins? This capacitor carries the MOSFET drivers' current peaks. An additional 1F ceramic capacitor placed immediately next to the INTVCC and PGND pins can help improve noise performance substantially. 6. Keep the switching nodes (SW1, SW2), top gate nodes (TG1, TG2) and boost nodes (BOOST1, BOOST2) away from sensitive small-signal nodes, especially from the opposites channel's voltage and current sensing feedback pins. All of these nodes have very large and fast moving signals and, therefore, should be kept on the output side of the LTC3788 and occupy a minimal PC trace area. 7. Use a modified "star ground" technique: a low impedance, large copper area central grounding point on the same side of the PC board as the input and output capacitors with tie-ins for the bottom of the INTVCC decoupling capacitor, the bottom of the voltage feedback resistive divider and the SGND pin of the IC. PC Board Layout Debugging Start with one controller on at a time. It is helpful to use a DC-50MHz current probe to monitor the current in the inductor while testing the circuit. Monitor the output switching node (SW pin) to synchronize the oscilloscope to the internal oscillator and probe the actual output voltage. Check for proper performance over the operating voltage and current range expected in the application. The frequency of operation should be maintained over the input voltage range down to dropout and until the output load drops below the low current operation threshold-- typically 10% of the maximum designed current level in Burst Mode operation. The duty cycle percentage should be maintained from cycle to cycle in a well designed, low noise PCB implementation. Variation in the duty cycle at a subharmonic rate can suggest noise pickup at the current or voltage sensing inputs or inadequate loop compensation. Overcompensation of the loop can be used to tame a poor PC layout if regulator bandwidth optimization is not required. Only after each controller is checked for its individual performance should both controllers be turned on at the same time. A particularly difficult region of operation is when one controller channel is nearing its current comparator trip point while the other channel is turning on its bottom MOSFET. This occurs around the 50% duty cycle on either channel due to the phasing of the internal clocks and may cause minor duty cycle jitter.
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LTC3788 APPLICATIONS INFORMATION
Reduce VIN from its nominal level to verify operation with high duty cycle. Check the operation of the undervoltage lockout circuit by further lowering VIN while monitoring the outputs to verify operation. Investigate whether any problems exist only at higher output currents or only at higher input voltages. If problems coincide with high input voltages and low output currents, look for capacitive coupling between the BOOST, SW, TG, and possibly BG connections and the sensitive voltage and current pins. The capacitor placed across the current sensing pins needs to be placed immediately adjacent to the pins of the IC. This capacitor helps to minimize the effects of differential noise injection due to high frequency capacitive coupling. An embarrassing problem, which can be missed in an otherwise properly working switching regulator results when the current sensing leads are hooked up backwards. The output voltage under this improper hook-up will still be maintained, but the advantages of current mode control will not be realized. Compensation of the voltage loop will be much more sensitive to component selection. This behavior can be investigated by temporarily shorting out the current sensing resistor--don't worry, the regulator will still maintain control of the output voltage.
SENSE1- SENSE1+ SS1
ILIM PGOOD1 SW1 TG1 LTC3788 BOOST1
VPULL-UP CB1 M1
L1
RSENSE1
ITH1
+
VOUT1
BG1
M2
fIN
VFB1 FREQ PHSMD CLKOUT PLLIN/MODE SGND RUN1 RUN2 VFB2
VBIAS GND PGND EXTVCC INTVCC
+
VIN
BG2 BOOST2 TG2 SW2
M4 L2
ITH2
RSENSE2
SS2 SENSE2+ SENSE2-
PGOOD2
VPULL-UP
Figure 7. Recommended Printed Circuit Layout Diagram
+
VOUT2
3788 F07
CB2
M3
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LTC3788 APPLICATIONS INFORMATION
RSENSE1
L1
VOUT1
SW1
COUT1
RL1
VIN RIN CIN
RSENSE2
L2
VOUT2
BOLD LINES INDICATE HIGH SWITCHING CURRENT. KEEP LINES TO A MINIMUM LENGTH.
SW2
COUT2
RL2
3788 F08
Figure 8. Branch Current Waveforms
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LTC3788 TYPICAL APPLICATIONS
SENSE1- RB1 232k RA1 12.1k VFB1 CITH1, 220pF CITH1, 15nF CSS1, 0.1F RITH1 8.66k ITH1 SENSE1+
100k PGOOD2 100k PGOOD1
INTVCC VOUT1 24V, 5A COUTB1 220F
LTC3788 TG1 SW1 BOOST1 BG1 CB1, 0.1F D1 VBIAS SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND BG2 EXTVCC RUN1 BOOST2 RUN2 FREQ CSS2, 0.1F SS2 SW2 TG2
L1 MTOP1 3.3H
COUTA1 22F 4 RSENSE1 4m
+
MBOT1 VIN 5V TO 24V CINB 220F
CINT 4.7F D2 CB1, 0.1F MBOT2
CINA 22F 4
+
L2 1.25H
RSENSE2 3m
CITH2, 15nF
RITH2 2.7k ITH2 MTOP2 VOUT 12V, 10A COUTB2 220F
CITHA2, 100pF RA2 12.1k RB2 110k VFB2 SENSE2+ SENSE2- COUTA2 22F 4
+
3788 F09
CINA, COUTA1, COUTA2: SANYO, 50CE220AX CINB, COUTB1, COUTB2: TDK C4532X5R1E226M L1: PULSE PA1494.362NL L2: PULSE PA1294.132NL MBOT1, MBOT2, MTOP1, MTOP2: RENESAS HAT2169H
Figure 9. High Efficiency Dual 12V/24V Boost Converter
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LTC3788 APPLICATIONS INFORMATION
RS1 53.6k, 1% RS2 26.1k, 1% RB1 232k 1% RA1 12.1k, 1% CITH1, 220pF CITH1, 15nF RITH1 8.87k, 1% ITH1 CSS1, 0.01F VBIAS SS1 INTVCC ILIM PHSMD CLKOUT PGND PLLIN/MODE SGND BG2 EXTVCC RUN1 BOOST2 RUN2 FREQ SS2 CITH2, 4.7nF RITH2 23.7k, 1% ITH2 CITH2A 220pF RA2 12.1k, 1% VFB2 RB2 475k 1% RS4 30.1k, 1% C4 0.1F C2 0.1F SENSE2+ COUTA2 22F 4 VOUT2 48V, 2A COUTB2 220F TG2 MTOP2 SW2 D4 C3 0.1F C1 0.1F
SENSE1- SENSE1+
100k PGOOD2 100k PGOOD1
INTVCC VOUT1 24V, 4A COUTB1 220F
LTC3788 VFB1 TG1 SW1 BOOST1 BG1 CB1, 0.1F D1 MBOT1 MTOP1
D3
L1 10.2H
COUTA1 6.8F 4
+
CINT 4.7F D2 CB1, 0.1F MBOT2A MBOT2B
CINA 22F 4
+
VIN 5V TO 24V CINB 220F
INTVCC
RFREQ 41.2k CSS2, 0.1F
L2 16H
+
SENSE2- RS3 42.2k, 1%
3788 F10
COUTA1: C4532x7R1H685K COUTB1: SANYO 63CE220KX CINA, COUTA2: TDK C4532X5R1E226M CINB, COUTB2: SANYO 50CE220AX L1: PULSE PA2050.103NL L2: PULSE PA2050.163NL
MBOT1, MTOP1: RENESAS RJK0305 MBOT2A, MBOT2B, MTOP2: RENESAS RJK0652 D3: DIODES INC B340B D4: DIODES INC B360A
Figure 10. High Efficiency Dual 24V/48V Boost Converter with Inductor DCR Current Sensing
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LTC3788 PACKAGE DESCRIPTION
UH Package 32-Lead Plastic QFN (5mm x 5mm)
(Reference LTC DWG # 05-08-1693 Rev D)
0.70 0.05
5.50 0.05 4.10 0.05 3.50 REF (4 SIDES) 3.45 0.05 3.45 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 5.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 0.75 0.05 R = 0.05 TYP 0.00 - 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP 31 32 0.40 1 2 3.45 0.10 0.10 PIN 1 NOTCH R = 0.30 TYP OR 0.35 45 CHAMFER
3.50 REF (4-SIDES)
3.45
0.10
(UH32) QFN 0406 REV D
0.200 REF NOTE: 1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE M0-220 VARIATION WHHD-(X) (TO BE APPROVED) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
0.25
0.05
0.50 BSC
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Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
31
LTC3788 RELATED PARTS
PART NUMBER LTC3788-1 LTC3862/LTC3862-1 LTC3813 LTC3814-5 LTC1871/LTC1871-1/ LTC1871-7 LT3757 LT3758 LT3782A LT3580 LTC3872 LTC3857/LTC3857-1 LTC3780 DESCRIPTION 2-Phase, Dual Output Synchronous Step-Up Controller Multiphase Current Mode Step-Up DC/DC Controller 100V Maximum VOUT Current Mode Synchronous Step-Up DC/DC Controller 60V Maximum VOUT Current Mode Synchronous Step-Up DC/DC Controller Wide Input Range, No RSENSE Low Quiescent Current Flyback, Boost and SEPIC Controller Boost, Flyback, SEPIC and Inverting Controller Boost, Flyback, SEPIC and Inverting Controller 2-Phase Step-Up DC/DC Controller Boost/Inverting DC/DC Converter with 2A Switch, Soft-Start and Synchronization No RSENSE Current Mode Boost DC/DC Controller Low IQ, Dual 2-Phase Synchronous Step-Down DC/DC Controller High Efficiency Synchronous 4-Switch Buck-Boost DC/DC Controller COMMENTS 4.5V VIN 38V, VOUT Up to 60V, 50kHz to 900kHz, SSOP-28 Package 4V VIN 36V, 5V or 10V Gate Drive, 75kHz to 500kHz No RSENSE, Large 1 Gate Driver, Adjustable Off-Time, SSOP-28 Package No RSENSE, Large 1 Gate Driver, Adjustable Off-Time, TSSOP-16 Package Adjustable Switching Frequency, 2.5V VIN 36V, Burst Mode Operation at Light Load, MSOP-10 Package 2.9V VIN 40V, 100kHz to 1MHz Programmable Operation Frequency, 3mm x 3mm DFN-10 and MSOP-10E Packages 5.5V VIN 100V, 100kHz to 1MHz Programmable Operation Frequency, 3mm x 3mm DFN-10 and MSOP-10E Packages 6V VIN 40V, Optional Synchronous Operation 2.5V VIN 32V, 200kHz to 2.5MHz, 3mm x 3mm DFN-8 and MSOP-8E Packages 550kHz Fixed Frequency, 2.75V VIN 9.8V, ThinSOT Package 4V VIN 38V, 0.8V VOUT 24V, 50A IQ 4V VIN 36V, 0.8V VOUT 30V, SSOP-24 and 5mm x 5mm QFN-32 Packages
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32 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1209 * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2009

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