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LTC3537 2.2 MHz, 600mA Synchronous Step-Up DC/DC Converter and 100mA LDO FEATURES
High Efficiency Step-Up DC/DC Converter and LDO Step-Up n V : 0.68V to 5V, V IN OUT: 1.5V to 5.25V IOUT: 100mA at 3.3V, VIN >0.8V n 2.2MHz Fixed Frequency Operation n Synchronous Rectifier with Output Disconnect n Burst Mode Operation (Pin Selectable) Linear LDO Regulator n V : 1.8V to 5.5V, V IN OUT: 0.6V to 5V IOUT: 100mA n 100mV Dropout Voltage at 50mA n 24dB Ripple Rejection at f SW Combined n Power Good Indicators n Low-Battery Comparator n 30A I Q n Low Profile 3mm x 3mm x 0.75mm Package
n
DESCRIPTION
The LTC(R)3537 combines a high efficiency, 2.2MHz step-up DC/DC converter with an idependent 100mA low dropout regulator (LDO). The step-up converter starts from an input voltage as low as 0.68V and contains an internal 0.4 switch and a 0.6 synchronous rectifier that disconnects from the output when disabled in shutdown. A switching frequency of 2.2MHz minimizes solution footprint by allowing the use of tiny, low profile inductors and ceramic capacitors. The current mode PWM design is internally compensated, reducing external parts count. Fixed frequency switching is maintained until a light load current is sensed, at which point Burst Mode(R) operation is engaged to maximize efficiency. For low noise operation, Burst Mode Operation can be disabled. Anti-ring circuitry reduces EMI by damping the inductor in discontinuous mode. Additional features include a low shutdown current of under 1A and thermal overload protection. The integrated LDO regulator provides a very low noise, programmable low dropout supply.
L, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIONS
n n n n
Wireless Microphones Portable Medical instruments Noise Cancelling/Portable Headsets RF and Audio Power
TYPICAL APPLICATION
Efficiency and Power Loss vs Load Current
2.2H R6 665k ALKALINE 0.8V TO 1.6V + R5 1.0M 1F OFF ON PWM BURST SW VOUTB 3.3V 4.7F VOLDO 3V 1F R1 1M R3 511k
3537 TA01a
100 90
1000
VINB
VOUTB R2 1.74M
80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 0.1
EFFICIENCY
100 POWER LOSS (mW)
LBI LTC3537 VINL LBO FBB PGDB PGDL VOLDO ENLDO ENBST MODE FBL SGND PGND
10
R4 2.05M
POWER LOSS
1
0.1 VIN, MODE = 1.8V 1 100 10 LOAD CURRENT (mA)
0.01 1000
3537 TA01b
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LTC3537 ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW VOUTB 12 VINL 17 11 VOLDO 10 FBL 9 FBB 5 PGDB 6 ENBST 7 PGDL 8 ENLDO PGND LBO SW
VINB and VINL Voltage................................... -0.3V to 6V SW DC Voltage............................................. -0.3V to 6V SW Pulsed (<100ns) Voltage ....................... -0.3V to 7V FBB, FBL, PGDB, PGDL Voltage ................... -0.3V to 6V MODE, ENBST, ENLDO Voltage ................... -0.3V to 6V LBI and LBO Voltage .................................... -0.3V to 6V VOUTB, VOLDO ............................................... -0.3V to 6V Operating Temperature (Notes 2, 5) ......... -40C to 85C Junction Temperature ........................................... 125C Storage Temperature Range................... -65C to 125C
16 15 14 13 MODE 1 LBI 2 SGND 3 VINB 4
UD PACKAGE 16-LEAD (3mm 3mm) PLASTIC QFN TJMAX = 125C, JA = 68C/W (Note 6) EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC3537EUD#PBF TAPE AND REEL LTC3537EUD#TRPBF PART MARKING LDBD PACKAGE DESCRIPTION 16-Lead (3mm x 3mm) Plastic QFN TEMPERATURE RANGE -40C to 85C Consult LTC Marketing for parts specified with wider operating temperature ranges. 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 VINMIN VOUTB VFBB IFBB IQSHDN IQACTIVE IQBURST INLEAK IPLEAK PARAMETER Minimum Start-Up Voltage Output Voltage Range Feedback Voltage Feedback Input Current Quiescent Current - Shutdown Quiescent Current - Active Quiescent Current - Burst CONDITIONS ILOAD = 1mA Boost Converter
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VINB = 1.2V, VOUTB = 3.3V, unless otherwise noted.
MIN TYP 0.68
l l
MAX 0.8 5.25 1.240 50 1 500
UNITS V V V nA A A A
1.5 1.179 1.21 1 0.02 300 15 0.1 0.1
VENBST = VENLDO = 0V, Not Including SW Leakage, VOUTB = 0V Measured on VOUTB, Nonswitching, MODE = 1.2V, VENLDO = 0V Measured on VOUTB, FBB >1.24V, MODE = 1.2V, VENLDO = 0V
NMOS Switch Leakage Current VSW = 5V PMOS Switch Leakage Current VSW = 5V, VOUTB = 0V
5 10
A A
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LTC3537 ELECTRICAL CHARACTERISTICS
SYMBOL RNMOS PARAMETER NMOS Switch On Resistance CONDITIONS VOUTB = 1.8V VOUTB = 3.3V VOUTB = 5V VOUTB = 1.8V VOUTB = 3.3V VOUTB = 5V (Note 4) (Note 3) VFBB = 1.15V VFBB = 1.3V
l l l l
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VINB = 1.2V, VOUTB = 3.3V, unless otherwise noted.
MIN TYP 0.8 0.4 0.3 1 0.6 0.4 600 750 40 87 2 0.8 0.3 VENBST = 5.5V 0.8 0.3 VMODE = 5.5V Falling Threshold VLBI = 1V ILBO = 5mA VLBO = 5.5V IPGDB = 5mA VPGDB = 5.5V VFBB Rising 530 1.5 0.5 553 35 10 200 0.01 200 0.01 94 6 1 1 50 575 1.5 92 0 2.2 2.4 MAX UNITS mA ns % % MHz V V A V V A ms mV mV nA mV A mV A % VOUTB %
RPMOS
PMOS Switch On Resistance
ILIM tLIMDELAY
NMOS Current Limit Current Limit Delay Time to Output Max Duty Cycle Min Duty Cycle
fSW VENBSTH VENBSTL IENBSTIN VMODEH VMODEL IMODEIN tSS VFBLBI ILBIIN VLBOLOW ILBOLEAK VPGDBLOW IPGDBLEAK
Switching Frequency ENBST Input High Voltage ENBST Input Low Voltage ENBST Input Current MODE Input High Voltage MODE Input Low Voltage MODE Input Current Soft-Start Time LBI Feedback Voltage LBI Hysteresis Voltage LBI Input Current LBO Voltage Low LBO Leakage Current PGDB Voltage Low PGDB Leakage Current PGDB Trip Point Voltage PGDB Hysteresis
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VINL = 3.3V, VOLDO = 3V, unless otherwise noted.
SYMBOL VINL VOLDO IOUTMAX VFBL PARAMETER Input Voltage Range Output Voltage Range Max Output Current Feedback Voltage Line Regulation Load Regulation VDROPOUT Dropout Voltage VINL = 1.8V to 5.5V ILOAD = 10mA to 90mA IO = 50mA ILOAD = 100mA
l l
CONDITIONS
MIN 1.8 VFBL 100 590
TYP
MAX 5.5 5
UNITS V V mA mV % % mV
LDO Regulator
600 0.1 0.4 100
610
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LTC3537 ELECTRICAL CHARACTERISTICS
SYMBOL PSRR ISHORT VENLDOH VENLDOL IENLDO VPGDLLOW IPGDLLEAK PARAMETER Ripple Rejection Short Circuit Current Limit ENLDO Input High Voltage ENLDO Input Low Voltage ENLDO Input Current PGDL Voltage Low PGDL Leakage Current PGDL Trip Point PGDL Hysteresis 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 LTC3537 is guaranteed to meet performance specifications from 0C to 85C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 3: Specification is guaranteed by design and not 100% tested in production. VENLDO = 5.5V IPGDL = 5mA VPGDL = 5.5V VFBL Rising 1.5 200 0.01 96 3 1
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VINL = 3.3V, VOLDO = 3V, unless otherwise noted.
CONDITIONS f = 2.2MHz at ILOAD = 100mA (Note 3) VOLDO = 0V
l
MIN 110 0.8
TYP 24 150
MAX
UNITS dB mA V
0.3
V A mV A % VOLDO %
Note 4: Current measurements are made when the output is not switching. Note 5: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may result in device degradation or failure. Note 6: Failure to solder the exposed backside of the package to the PC board ground plane will result in a thermal resistance much higher than 68C/W.
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LTC3537 TYPICAL PERFORMANCE CHARACTERISTICS
Efficiency vs Load Current and VINB for VOUTB = 1.8V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 10 100 POWER LOSS (mW) EFFICIENCY (%) 1000 100 90 80 70 60 50 40 30 20 10 0 0.01 10 100 POWER LOSS (mW)
TA = 25C unless otherwise noted.
Efficiency vs Load Current and VINB for VOUTB = 3.3V
1000 100 90 80 70 IIN (A) 60 50 40 30 20
No-Load Input Current vs VINB
VOUTB = 3.3V
1 VINB = 1V VINB = 1.2V VINB = 1.5V 0.1 PLOSS AT VINB = 1V PLOSS AT VINB = 1.2V PLOSS AT VINB = 1.5V 0.01 1 100 1000 0.1 10 LOAD CURRENT (mA)
3537 G01
VINB = 1.2V VINB = 1.8V 1 VINB = 2.4V VINB = 2.8V PLOSS AT VINB = 1.2V 0.1 PLOSS AT VINB = 1.8V PLOSS AT VINB = 2.4V PLOSS AT VINB = 2.8V 0.01 0.1 10 1 100 1000 LOAD CURRENT (mA)
3537 G02
10 0.5
1
1.5
2 2.5 VINB (V)
3
3.5
3537 G03
Efficiency vs Load Current and VINB for VOUTB = 5V
100 90 80 EFFICIENCY (%) 70 60 50 40 30 20 10 0 0.01 10 100 LOAD CURRENT (mA) POWER LOSS (mW) 1000 1000 900 800 700 600 500 400 300 200 100
Maximum Output Current vs VINB
1000 VOUTB = 5V
Minimum Load Resistance During Start-Up vs VINB
VOUTB = 3.3V LOAD () VOUTB = 2.5V VOUTB = 1.8V 1 2 4 4.5
3537 G05
VINB = 1.2V VINB = 2.4V 1 VINB = 3.6V VINB = 4.2V PLOSS AT VINB = 1.2V 0.1 PLOSS AT VINB = 2.4V PLOSS AT VINB = 3.6V PLOSS AT VINB = 4.2V 0.01 0.1 10 1 100 1000 LOAD CURRENT (mA)
3537 G04
100
0 0.5
1.5
2.5 3 VINB (V)
3.5
10 0.8 0.9
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 VINB (V)
3537 G06
Start-Up Delay Time vs VINB
60 50 LOAD CURRENT (mA) 40 DELAY (s) 30 20 10 0 35
Burst Mode Threshold Current vs VINB
VOUTB = 1.8V = 4.7F C 30 OUT L = 2.2H LOAD CURRENT (mA) 25 20 15 10 5 0 0.8 LEAVE BURST 45
Burst Mode Threshold Current vs VINB
VOUTB = 2.5V 40 COUT = 4.7F L = 2.2H 35 30 25 20 15 10 ENTER BURST LEAVE BURST
ENTER BURST 0.9 1 1.1 1.2 VINB (V) 1.3 1.4 1.5
3537 G08
5 0 0.8 1 1.2 1.4 1.6 VINB (V) 1.8 2
3537 G09
1
1.5
2
2.5
3 VINB (V)
3.5
4
4.5
5
3537 G07
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LTC3537 TYPICAL PERFORMANCE CHARACTERISTICS
Burst Mode Threshold Current vs VINB
60 120 100 LOAD CURRENT (mA) 80 60 LEAVE BURST 40 20 ENTER BURST 2.3
3537 G10
TA = 25C unless otherwise noted. Oscillator Frequency Change vs VOUTB
1 NORMALIZED TO 3.3V 0 FREQUENCY CHANGE (%) -1 -2 -3 -4 -5 -6 1.5
Burst Mode Threshold Current vs VINB
VOUTB = 5V COUT = 4.7F L = 2.2H
VOUTB = 3.3V COUT = 4.7F 50 L = 2.2H LOAD CURRENT (mA) 40 LEAVE BURST 30 20 10 0 0.8
ENTER BURST
1.3 VINB (V)
1.8
0 0.8
1.3
1.8
2.3 2.8 VINB (V)
3.3
3.8
4.3
3537 G11
2
2.5
3 3.5 VOUTB (V)
4
4.5
5
3537 G12
RDS(ON) vs VOUTB
1.0 0.9 1
Oscillator Frequency Change vs Temperature
30 NORMALIZED TO 25C 0 RDS(ON) CHANGE (%) 20 10 0 -10 -20
RDS(ON) Change vs Temperature
NORMALIZED TO 25C
0.8 RDS(ON) () 0.7 0.6 0.5 0.4 0.3 0.2 1.5 2 2.5 3 3.5 VOUTB (V) 4 4.5 5
3537 G13
FREQUENCY CHANGE (%)
PMOS
-1
-2
PMOS
NMOS
-3
NMOS -20 0 20 40 TEMPERATURE (C) 60 80
3537 G15
-4 -40
-20
0 20 40 TEMPERATURE (C)
60
80
3537 G14
-30 -40
Voltage Feedback Change vs Temperature
0.05 0.00 VOLTAGE CHANGE (%) VFBB AND VFBL -0.10 VINB (V) -0.15 -0.20 -0.25 -0.30 -40 0.70 0.65 0.60 0.55 NORMALIZED TO 20C 0.80 0.75
Start-Up Voltage vs Temperature
60
Burst Mode Quiescent Current vs VOUTB
50
IQ (A)
40
30
20 ENLDO = HIGH 2.3 2.8 3.8 VOUTB (V) 3.3 4.3 4.8
3537 G18
-20
0 20 40 TEMPERATURE (C)
60
80
3537 G16
0.50 -40
-20
0 20 40 TEMPERATURE (C)
60
80
3537 G17
10 1.8
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LTC3537 TYPICAL PERFORMANCE CHARACTERISTICS
Fixed Frequency Switching Waveform and VOUTB Ripple Burst Mode Waveforms TA = 25C unless otherwise noted.
VOUTB and IINB During Soft-Start
ENBST SW 2V/DIV VOUTB 20mV/DIV
VOUTB 2V/DIV VOUTB 20mV/ DIV IL 10mA/DIV IVINB 200mA/ DIV VINB = 2.4V VOUTB = 3.3V COUTB = 4.7F 10s/DIV
3537 G20
VINB = 2.4V VOUTB = 3.3V COUTB = 4.7F
200ns/DIV
3537 G19
VINB = 1.2V VOUTB = 3.3V COUTB = 4.7F ILOAD = 10mA
100s/DIV
3537 G21
Load Current Step Response (from Burst Mode Operation)
Load Current Step Response (Fixed Frequency)
Load Current Step Response (Fixed Frequency)
VOUTB 100mV/ DIV
VOUTB 100mV/ DIV
VOUTB 100mV/ DIV
ILOAD 100mA/ DIV
ILOAD 100mA/ DIV
ILOAD 100mA/ DIV
VINB = 2.4V VOUTB = 3.3V COUT = 4.7F
100s/DIV
3537 G22
VINB = 2.4V VOUTB = 3.3V COUT = 4.7F
100s/DIV
3537 G23
VINB = 3.6V VOUTB = 5V COUTB = 4.7F
100s/DIV
3537 G24
Load Current Step Response (from Burst Mode Operation)
140 120 DROPOUT VOLTAGE (mV) VOUTB 100mV/ DIV
LDO Dropout Voltage vs Load Current
60 50 ATTENUATIOIN (dB) 40 30 20 10
LDO Input Ripple Rejection vs Frequency
VINL = 3.3V VOLDO = 3V CLOAD = 4.7F ILOAD = 50mA
100 80 60 40 20 0
ILOAD 100mA/ DIV
VINB = 3.6V VOUTB = 5V COUTB = 4.7F
100s/DIV
3537 G25
0
10 20 30 40 50 60 70 80 90 100 LOAD CURRENT (mA)
3537 G26
0 0.01
0.1
1 10 FREQUENCY (kHz)
100
3537 G29
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LTC3537 TYPICAL PERFORMANCE CHARACTERISTICS
LDO Current Limit vs Temperature
7 NORMALIZED TO 25C 6 5 LOAD CURRENT (%) 4 3 2 1 0 -1 -2 -40 -20 0 20 40 TEMPERATURE (C) 60 80
3537 G30
TA = 25C unless otherwise noted.
LDO Load Current Step Response
VOLDO 100mV/ DIV
ILOAD 100mA/ DIV
VINL = 3.3V VOLDO = 3V COUT = 1F
100s/DIV
3537 G31
LDO Load Current Step Response
LDO Load Current Step Response
VOLDO 100mV/ DIV
VOLDO 100mV/ DIV
ILOAD 100mA/ DIV
ILOAD 100mA/ DIV
VINL = 5V VOLDO = 3V COUT = 1F
100s/DIV
3537 G32
VINL = 5V VOLDO = 1.8V COUT = 1F
100s/DIV
3537 G33
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LTC3537 PIN FUNCTIONS
MODE (Pin 1): Logic Controlled Input for the Auto-Burst Mode Feature. MODE = High: PWM operation with Burst Mode Operation MODE = Low: PWM operation only LBI (Pin 2): Low-Battery Comparator Non-Inverting Input. (Comparator enabled with ENBST or ENLDO) SGND (Pin 3): Signal Ground. Provide a short direct PCB path between GND and the (-) side of the input and output capacitors. VINB (Pin 4): Input Supply for the Step-Up Converter. Connect a minimum of 1F ceramic decoupling capacitor from this pin to ground. PGDB (Pin 5): Power Good Indicator for the Boost Converter. This is an open-drain output that sinks current when VOUTB is greater than 94% of the programmed voltage. ENBST (Pin 6): Logic controlled shutdown input for the boost converter. ENBST = High: Normal operation ENBST = Low: Shutdown PGDL (Pin 7): Power Good Indicator for the LDO Regulator. This is an open-drain output that sinks current when VOLDO is greater than 96% of the programmed voltage. ENLDO (Pin 8): Logic Controlled Shutdown Input for the LDO Regulator. ENLDO = High: Normal operation ENLDO = Low: Shutdown FBB (Pin 9): Feedback Input to the gm Error Amplifier of the Boost Converter. Connect resistor divider tap to this pin. The output voltage can be adjusted from 1.5V to 5.25V by: VOUTB = 1.2V * [1 + (R2/R1)] FBL (Pin 10): Feedback Input to the gm Error Amplifier of the LDO. Connect resistor divider tap to this pin. The output voltage can be adjusted from 0.6V (typical) to 5V by: VOLDO = 0.6V * [1 + (R4/R3)] VOLDO (Pin 11): LDO Regulator Output. PCB trace from VOLDO to the output filter capacitor (1F minimum) should be as short and as wide as possible. VINL (Pin 12): Input Supply for the LDO Regulator. VOUTB (Pin 13): Output Voltage Sense Input and Drain of the Internal Synchronous Rectifier. PCB trace length from VOUTB to the output filter capacitor (4.7F minimum) should be as short and wide as possible. SW (Pin 14): Switch Pin. Connect the inductor between SW and VINB. Keep these PCB trace lengths as short and wide as possible to reduce EMI. If the inductor current falls to zero or ENBST is low, an internal anti-ringing switch is connected from SW to VINB to minimize EMI. PGND (Pin 15): Power Ground. Provide a short direct PCB path between GND and the (-) side of the input and output capacitors. LBO (Pin 16): Low-Battery Comparator Output. (OpenDrain) Exposed Pad (Pin 17): Power Ground. The Exposed Pad must be soldered to the PCB.
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LTC3537 BLOCK DIAGRAM
VOUT SW VINB
VBEST
VBEST
WELL SWITCH
VOUTB GATE DRIVERS AND ANTI-CROSS CONDUCTION SHUTDOWN ENBST SLOPE COMPENSATION 1.2V VREF SHUTDOWN R2
-
+
FBB R1
ENLDO VREF UVLO VREF
STARTUP 2.2MHz OSC THERMAL SHUTDOWN
LOGIC
MODE CONTROL CLAMP WELL SWITCH VINL
1.13V
PGDB
FBB
0.55V 0.55V LBI
PGDL FBL
LBO
MODE
SGND
PGND
VIN
R6
R5
10
-
+
+
-
+ -
- + + + - -
VOLDO R4 GATE DRIVER R3 0.6V
FBL
3537 BD
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LTC3537 OPERATION
The LTC3537 is a 2.2MHz synchronous step-up (boost) converter and LDO regulator housed in a 16-lead 3mm x 3mm QFN package. Included with the ability to start up and operate from inputs less than 0.7V, the LTC3537 features fixed frequency, current mode PWM control for exceptional line and load regulation. The current mode architecture with adaptive slope compensation provides excellent transient load response, requiring minimal output filtering. Internal soft-start and loop compensation simplifies the design process while minimizing the number of external components. With its low RDS(ON) and low gate charge internal N-channel MOSFET switch and P-channel MOSFET synchronous rectifier, the LTC3537 achieves high efficiency over a wide range of load currents. Automatic Burst Mode operation maintains high efficiency at very light loads, reducing the quiescent current to just 30A. Operation can be best understood by referring to the Block Diagram. LOW VOLTAGE START-UP The LTC3537 step-up converter includes an independent start-up oscillator designed to operate at an input voltage of 0.68V (typical). Soft-start and inrush current limiting are provided during start-up, as well as normal mode. When either VINB or VOUTB exceeds 1.4V typical, the IC enters normal operating mode. When the output voltage exceeds the input by 0.24V, the IC powers itself from VOUTB instead of VINB. At this point the internal circuitry has no dependency on the VINB input voltage, eliminating the requirement for a large input capacitor. The input voltage can drop as low as 0.5V after start-up is achieved. The limiting factor for the application becomes the availability of the power source to supply sufficient energy to the output at low voltages, and maximum duty cycle, which is clamped at 92% typical. Note that at low input voltages, small voltage drops due to series resistance become critical, and greatly limit the power delivery capability of the converter. LOW NOISE FIXED FREQUENCY OPERATION Soft-Start The LTC3537 contains internal circuitry to provide softstart operation. The soft-start circuitry slowly ramps the peak inductor current from zero to its peak value of 750mA (typical) in approximately 0.5ms, allowing start-up into heavy loads. The soft-start circuitry is reset in the event of a shutdown command or a thermal shutdown. Oscillator An internal oscillator sets the switching frequency to 2.2MHz. Shutdown Shutdown of the boost converter is accomplished by pulling ENBST below 0.3V and enabled by pulling ENBST above 0.8V. Note that ENBST can be driven above VINB or VOUTB, as long as it is limited to less than the absolute maximum rating. Boost Error Amplifier The non-inverting input of the transconductance error amplifier is internally connected to the 1.2V reference and the inverting input is connected to FBB. Clamps limit the minimum and maximum error amp output voltage for improved large-signal transient response. Power converter control loop compensation is provided internally. An external resistive voltage divider from VOUTB to ground programs the output voltage via FBB from 1.5V to 5.25V. R2 VOUTB = 1.2V 1+ R1 Boost Current Sensing Lossless current sensing converts the peak current signal of the N-channel MOSFET switch into a voltage that is summed with the internal slope compensation. The summed signal is compared to the error amplifier output to provide a peak current control command for the PWM.
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LTC3537 OPERATION
Boost Current Limit The current limit comparator shuts off the N-channel MOSFET switch once its threshold is reached. The current limit comparator delay to output is typically 40ns. Peak switch current is limited to approximately 750mA, independent of input or output voltage, unless VOUTB falls below 0.8V, in which case the current limit is cut in half. Boost Zero Current Comparator The zero current comparator monitors the inductor current to the output and shuts off the synchronous rectifier when this current reduces to approximately 30mA. This prevents the inductor current from reversing in polarity, improving efficiency at light loads. Boost Synchronous Rectifier To control inrush current and to prevent the inductor current from running away when VOUTB is close to VINB, the P-channel MOSFET synchronous rectifier is only enabled when VOUTB > (VINB + 0.24V). Boost Anti-Ringing Control The anti-ringing control connects a resistor across the inductor to prevent high frequency ringing on the SW pin during discontinuous current mode operation. Although the ringing of the resonant circuit formed by L and CSW (capacitance on SW pin) is low energy, it can cause EMI radiation. Boost Output Disconnect The LTC3537 is designed to allow true output disconnect by eliminating body diode conduction of the internal Pchannel MOSFET synchronous rectifier. This allows VOUTB to go to zero volts during shutdown, drawing no current from the input source. It also allows inrush current limiting at turn-on, minimizing surge currents seen by the input supply. Note that to obtain the advantages of output disconnect, there cannot be an external Schottky diode connected between the SW pin and VOUTB. The output disconnect feature also allows VOUTB to be pulled high, above the nominal regulation voltage, without any reverse current into the power source connected to VINB. Thermal Overload Protection If the die temperature exceeds 160C typical, the LTC3537 boost converter will shut down. All switches will be off and the soft-start capacitor will be discharged. The boost converter will be enabled when the die temperature drops by approximately 15C. BOOST BURST MODE OPERATION When enabled (MODE pin high), the LTC3537 will automatically enter Burst Mode operation at light load current and return to fixed frequency PWM mode when the load increases. Refer to the Typical Performance Characteristics to see the Burst Mode Threshold Current vs VINB. The load current at which Burst Mode operation is entered can be changed by adjusting the inductor value. Raising the inductor value will lower the load current at which Burst Mode operation is entered. In Burst Mode operation, the LTC3537 still switches at a fixed frequency of 2.2MHz, using the same error amplifier and loop compensation for peak current mode control. This control method eliminates any output transient when switching between modes. In Burst Mode operation, energy is delivered to the output until it reaches the nominal voltage regulation value, then the LTC3537 transitions to sleep mode where the outputs are off and the LTC3537 consumes only 30A of quiescent current from VOUTB including the current required to keep the LDO enabled. When the output voltage droops slightly, switching resumes. This maximizes efficiency at very light loads by minimizing switching and quiescent losses. Burst Mode output voltage ripple, which is typically 1% peak-to-peak, can be reduced by using more output capacitance (10F or greater), or with a small capacitor (10pF to 50pF) connected between VOUTB and FBB. As the load current increases, the LTC3537 will automatically leave Burst Mode operation. Note that larger output capacitor values may cause this transition to occur at lighter loads. Once the LTC3537 has left Burst Mode operation and returned to normal operation, it will remain there until the output load is reduced below the burst threshold.
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LTC3537 OPERATION
Burst Mode operation is inhibited during start-up and softstart and until VOUTB is at least 0.24V greater than VINB. The LTC3537 will operate at a continuous PWM frequency of 2.2MHz by connecting MODE to GND. At very light loads, the LTC3537 will exhibit pulse-skip operation. LDO REGULATOR OPERATION The LTC3537 includes an independent 100mA low dropout linear regulator (LDO). The VINL pin can be connected to an independent source or connected to the output of the boost regulator. An input capacitor on VINL is optional, but it will help to improve transient responses. The LDO will operate with a VINL down to 1.5V, but specifications are guaranteed with VINL from 1.8V to 5.5V. Shutdown Shutdown of the LDO is accomplished by pulling ENLDO below 0.3V and enabled by pulling ENLDO above 0.8V. Note that ENLDO can be driven above VINL or VOLDO, as long as it is limited to less than the absolute maximum rating. In the event that the LDO output voltage is held above the input voltage, the LDO goes in to shutdown until the output drops back below the input voltage. In shutdown the LDO will block reverse current from VOLDO to VINL. LDO Error Amplifier The non-inverting input of the transconductance error amplifier is internally connected to a 0.6V reference and the inverting input is connected to FBL. The control loop compensation is provided internally. An external resistive voltage divider from VOLDO to ground programs the output voltage via FBL from 0.6V to 5V. R4 VOLDO = 0.6V 1+ R3 LDO Current Sensing and Limiting Current is sensed across an internal resistor. The guaranteed minimum output current is 100mA. LOW-BATTERY INDICATOR The LTC3537 includes a low-battery comparator. The noninverting input of the comparator is internally connected to a 0.6V reference and the inverting input is connected to LBI. An external resistive voltage divider from VINL to ground programs the threshold voltage. When the voltage at LBI drops below 0.6V, the open-drain N-channel MOSFET will turn on. The N-channel MOSFET device is forced off when both the step-up converter and LDO are in shutdown. R6 VLBI = 0.6V 1+ R5 BOOST POWER-GOOD INDICATOR The LTC3537 includes a power-good comparator for the step-up converter. The non-inverting input of the comparator is internally connected to a 1.08V reference and the inverting input is connected to the FBB pin. The open-drain MOSFET on PGDB will turn on when the output voltage is typically within 6% of the programmed output voltage. Output sequencing can be achieved by connecting PGDB to the LDO enable pin (ENLDO). This would allow the user to keep the LDO off until the step-up converter is regulating. The N-channel MOSFET is forced on in shutdown. LDO POWER-GOOD INDICATOR The LTC3537 includes a power-good comparator for the LDO. The non-inverting input of the comparator is internally connected to a 540mV reference and the inverting input is connected to the FBL pin. The open-drain MOSFET on the PGDL pin will turn on when the output voltage is typically within 4% of the programmed output voltage. Output sequencing can be achieved by connecting PGDL to the boost enable pin (ENBST). This would allow the user to keep the step-up converter off until the LDO is regulating. The N-channel MOSFET is forced on in shutdown.
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13
LTC3537 APPLICATIONS INFORMATION
VINB > VOUTB OPERATION The LTC3537 step-up converter will maintain voltage regulation even when the input voltage is above the desired output voltage. Note that the efficiency is much lower in this mode, and the maximum output current capability will be less. Refer to the Typical Performance Characteristics. STEP-UP SHORT-CIRCUIT PROTECTION The LTC3537 output disconnect feature provides output short circuit protection. To reduce power dissipation under short-circuit conditions, the peak switch current limit is reduced to 400mA (typical). SCHOTTKY DIODE Although it is not required, adding a Schottky diode from SW to VOUTB will improve efficiency by about 4%. Note that this defeats the output disconnect and short-circuit protection features. PCB LAYOUT GUIDELINES The high speed operation of the LTC3537 demands careful attention to board layout. A careless layout will result in reduced performance. Figure 1 shows the recommended component placement. A large ground pin copper area will help to lower the die temperature. A multilayer board with a separate ground plane is ideal, but not absolutely necessary. COMPONENT SELECTION Inductor Selection The LTC3537 can utilize small surface mount chip inductors due to its fast 2.2MHz switching frequency. Inductor values between 1H and 4.7H are suitable for most applications. Larger values of inductance will allow slightly greater output current capability (and lower the Burst Mode threshold) by reducing the inductor ripple current. Increasing the inductance above 10H will increase size while providing little improvement in output current capability. The minimum inductance value is given by: L> where: Ripple = Allowable inductor current ripple (amps peak-peak) VINB(MIN) = Minimum converter input voltage VOUTB(MAX) = Maximum output voltage The inductor current ripple is typically set for 20% to 40% of the maximum inductor current. High frequency ferrite core inductor materials reduce frequency dependent power losses compared to cheaper powdered iron types, improving efficiency. The inductor should have low ESR (series resistance of the windings) to reduce the I2R power losses, and must be able to support the peak inductor current without saturating. Molded chokes and some chip inductors usually do not have enough core area to support the peak inductor currents of 750mA seen on the LTC3537. To minimize radiated noise, use a shielded inductor. See Table 1 for suggested components and suppliers.
3537 F01
VINB(MIN) * VOUTB(MAX) - VINB(MIN) Ripple* VOUTB(MAX)
(
)
LBO 16 MODE 1 LBI 2 SGND 3 15
SW 14
VOUTB 13 12 VINL 11 VOLDO 10 FBL 9 FBB
+
4 VINB 5 6 7 8 MULTIPLE VIAS TO INNER GROUND LAYERS
PGDB ENBST PGDL ENLDO
Figure 1
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14
LTC3537 APPLICATIONS INFORMATION
Table 1: Recommended Inductors
VENDOR Coilcraft (847) 639-6400 www.coilcraft.com PART/STYLE LPO4815 LPS4012, LPS4018 MSS5131 MSS4020 MOS6020 ME3220 DS1605, DO1608 SD10, SD12, SD14, SD18, SD20, SD52, SD3114, SD3118 MIP3226D4R7M, MIP3226D3R3M MIPF2520D4R7 MIPWT3226D3R0 LQH43C LQH32C (-53 series) 301015 CDRH5D18 CDRH2D14 CDRH3D16 CDRH3D11 CR43 CMD4D06-4R7MC CMD4D06-3R3MC NP03SB NR3015T NR3012T VLP VLF VLCF , D412C D518LC D52LC D62LCB WE-TPC Type S, M
ESR ()
wide voltage and temperature ranges. Y5V types should not be used. The internal loop compensation of the LTC3537 is designed to be stable with output capacitor values of 4.7F or greater on the boost regulator and 1F or greater on the LDO regulator (without the need for any external series resistor). Although ceramic capacitors are recommended, low ESR tantalum capacitors may be used as well. For the LDO, see Figures 2 and 3 for output capacitor value and ESR requirements.
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 1 10 CAPACITANCE (F) 100
3537 F02
Coiltronics www.cooperet.com FDK (408) 432-8331 www.fdk.com Murata (714) 852-2001 www.murata.com Sumida (847) 956-0666 www.sumida.com
REGION OF OPERATION
Taiyo-Yuden www.t-yuden.com TDK (847) 803-6100 www.component.tdk.com Toko (408) 432-8282 www.tokoam.com Wurth (201) 785-8800 www.we-online.com
Figure 2. LDO Regulator Output Capacitance vs ESR
5.0 MINIMUM OUTPUT CAPACITANCE (F) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 1 2 3 4 5 VINL/VOLDO 6 7
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Output and Input Capacitor Selection Low ESR (equivalent series resistance) capacitors should be used to minimize the output voltage ripple. Multilayer ceramic capacitors are an excellent choice as they have extremely low ESR and are available in small footprints. A 4.7F to 10F output capacitor is sufficient for most boost applications. Larger values up to 22F may be used to obtain extremely low output voltage ripple and improve transient response. X5R and X7R dielectric materials are preferred for their ability to maintain capacitance over
Figure 3. LDO Regulator Minimum Output Capacitance vs VINL/VOLDO
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15
LTC3537 APPLICATIONS INFORMATION
For the step-up converter, a tantalum capacitor may be used in demanding applications that have large load transients. Another method of improving the transient response is to add a small feedforward capacitor across the top resistor of the feedback divider (from VOUTB to FBB). A typical value of 22pF will generally suffice. Ceramic capacitors are also a good choice for input decoupling of the step-up converter and should be located as close as possible to the device. A 2.2F input capacitor is sufficient for most applications, although larger values may be used without limitations. The LDO regulator will have improved performance with an input capacitor, but it is not required. Table 2 shows a list of several ceramic capacitor manufacturers. Consult the manufacturers directly for detailed information on their selection of ceramic capacitors.
Table 2. Capacitor Vendor Information
SUPPLIER AVX Murata Taiyo-Yuden TDK Samsung PHONE (803) 448-9411 (714) 852-2001 (408) 573-4150 (847) 803-6100 (408) 544-5200 WEBSITE www.avxcorp.com www.murata.com www.t-yuden.com www.component.tdk.com www.sem.samsung.com
TYPICAL APPLICATIONS
1-Cell to 1.8V, 1.5V
2.2H R6 665k ALKALINE 0.8V TO 1.6V + R5 1M 1F OFF ON PVM BURST SW VOUTB 1.8V R2 499k 4.7F VOLDO 1.5V 1F R1 1M R3 1M
3537 TA02
VINB
VOUTB
LBI LTC3537 V INL LBO FBB PGDB PGDL VOLDO ENDLO ENBST MODE FBL SGND PGND
R4 1.5M
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16
LTC3537 TYPICAL APPLICATIONS
1-Cell to 3.3V, 2.8V
2.2H R6 665k ALKALINE 0.8V TO 1.6V + R5 1M 1F OFF ON PVM BURST SW VOUTB 3.3V R2 1.74M 4.7F VOLDO 2.8V 1F R1 1M R3 301k
3537 TA03
VINB
VOUTB
LBI LTC3537 V INL LBO FBB PGDB PGDL VOLDO ENDLO ENBST MODE FBL SGND PGND
R4 1.1M
2-Cell to Low Noise 3.3V
2.2H R6 2M SW
VINB
VOUTB R2 2M 4.7F VOLDO 3.3V 1F R1 1M R3 523k
3537 TA04
2-CELL ALKALINE 1.6V TO 3.2V
+
R5 1M 1F OFF ON PVM BURST
LBI LTC3537 VINL LBO FBB PGDB PGDL VOLDO ENDLO ENBST MODE FBL SGND PGND
R4 2.37M
2-Cell to 5V, 1.8V
2.2H R6 2M SW VOUTB 5V R2 1.91M 4.7F VOLDO 1.8V 1F R1 604k R3 1M
3537 TA05
VINB
VOUTB
2-CELL ALKALINE 1.6V TO 3.2V
+
R5 1M 1F OFF ON PVM BURST
LBI LTC3537 V INL LBO FBB PGDB PGDL VOLDO ENDLO ENBST MODE FBL SGND PGND
R4 2M
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17
LTC3537 TYPICAL APPLICATIONS
Li-Ion to 5V, 3.3V
2.2H R6 2M + Li-Ion 2.8V TO 4.2V R5 499k 1F OFF ON PVM BURST SW VOUTB 5V R2 1.91M 4.7F VOLDO 3.3V 1F R1 604k R3 523k
3537 TA06
VINB
VOUTB
LBI LTC3537 V INL LBO FBB PGDB PGDL VOLDO ENDLO ENBST MODE FBL SGND PGND
R4 2.37M
Single Cell or 5V Input to 3.3V
USB OR 0.8V TO 1.6V 5V ADAPTER ALKALINE + + 1F R5 1.02M OFF ON PWM BURST R6 510k
2.2H SW
10F VOUTB 3.3V/100mA R3 1.74M 10F R2 511k
VINB
LBI LTC3537 LBO FBB PGDB VINL PGDL ENLDO VOLDO ENBST MODE FBL SGND PGND
R1 487k
3537 TA07
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18
LTC3537 PACKAGE DESCRIPTION
UD Package 16-Lead Plastic QFN (3mm x 3mm)
(Reference LTC DWG # 05-08-1691)
0.70
0.05
3.50
0.05 2.10
1.45 0.05 0.05 (4 SIDES)
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 0.75 0.05 BOTTOM VIEW--EXPOSED PAD R = 0.115 TYP 15 16 0.40 1 1.45 0.10 (4-SIDES) 2 0.10 PIN 1 NOTCH R = 0.20 TYP OR 0.25 45 CHAMFER
3.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6)
(UD16) QFN 0904
0.200 REF 0.00 - 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WEED-2) 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 0.15mm 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.
19
LTC3537 RELATED PARTS
PART NUMBER LTC3401 LTC3402 LTC3421 LTC3422 DESCRIPTION 1A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter 2A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter 3A (ISW), 3MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect 1.5A (ISW), 3MHz Synchronous Step-Up DC/DC Converter with Output Disconnect COMMENTS 97% Efficiency, VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38A, ISD <1A, 10-Lead MS Package 97% Efficiency, VIN: 0.5V to 5V, VOUT(MAX) = 6V, IQ = 38A, ISD <1A, 10-Lead MS Package 95% Efficiency, VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 12A, ISD <1A, QFN24 Package 95% Efficiency, VIN: 0.5V to 4.5V, VOUT(MAX) = 5.25V, IQ = 25A, ISD <1A, 3mm x 3mm DFN Package 95% Efficiency, VIN: 0.5V to 5.5V, VOUT(MAX) = 5.5V, IQ = 38A, ISD <1A, 10-Lead MS Package 92% Efficiency, VIN: 1.6V to 4.3V, VOUT(MAX) = 5V, ISD <1A, SOT-23 Package 92% Efficiency, VIN: 1.8V to 5V, VOUT(MAX) = 5.25V, ISD <1A, 2mm x 2mm DFN Package 96% Efficiency, VIN: 0.5V to 4.4V, VOUT(MAX) = 5V, IQ = 20A/300A, ISD <1A, ThinSOT Package 93% Efficiency, VIN: 1.5V to 6V, VOUT(MAX) = 7.5V, IQ = 15A, ISD <1A, DFN12 Package 94% Efficiency, VOUT(MAX) = 6V, IQ = 12A, DFN12 Package
LTC3423/LTC3424 1A/2A (ISW), 3MHz, Synchronous Step-Up DC/DC Converters LTC3426 LTC3428 LTC3429 LTC3458 LTC3458L 2A (ISW), 1.2MHz, Step-Up DC/DC Converter 500mA (ISW), 1.25MHz/2.5MHz, Synchronous Step-Up DC/DC Converters with Output Disconnect 600mA (ISW), 500kHz, Synchronous Step-Up DC/DC Converter with Output Disconnect and Soft-Start 1.4A (ISW), 1.5MHz, Synchronous Step-Up DC/DC Converter/Output Disconnect/Burst Mode Operation 1.7A (ISW), 1.5MHz, Synchronous Step-Up DC/DC Converter with Output Disconnect, Automatic Burst Mode Operation 70mA (ISW), 10V Micropower Synchronous Boost Converter/Output Disconnect/Burst Mode Operation 400mA Synchronous Buck-Boost and 200mA Synchronous Buck Converter 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 400mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 600mA Micropower Synchronous Step-Up DC/DC Converter with Output Disconnect 1A, 1MHz, Synchronous Step-Up DC/DC Converters
LTC3459 LTC3522 LTC3525-3/ LTC3525-3.3/ LTC3525-5 LTC3525L-3 LTC3526/ LTC3526L LTC3528/ LTC3528B
VIN: 1.5V to 5.5V, VOUT(MAX) = 10V, IQ = 10A, ISD <1A, ThinSOT Package 95% Efficiency, VIN: 2.4V to 5.5V, VOUT: 5.25V to 0.6V, IQ = 25A, ISD < 1A, 3mm x 3mm DFN Package 95% Efficiency, VIN: 1V to 4.5V, VOUT(MAX) = 3.3V or 5V, IQ = 7A, ISD <1A, SC-70 Package 90% Efficiency, VIN: 0.7V to 4.5V, VOUT = 3V, IQ = 7A, ISD < 1A, SC70 Package 95% Efficiency, VIN: 0.75V to 5V, VOUT(MAX): 1.5V to 5.25V, IQ = 12A, ISD <1A, DFN6 Package 94% Efficiency, VIN: 0.7V to 5V, VOUT: 1.6V to 5.25V, IQ = 12A, ISD < 1A, 2mm x 3mm DFN Package, LTC3528B (PWM Mode Only)
ThinSOT is a trademark of Linear Technology Corporation.
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20 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0608 REV A * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007

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