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FEATURES
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LT1681 Dual Transistor Synchronous Forward Controller DESCRIPTIO
The LT (R)1681 controller simplifies the design of high power synchronous dual transistor forward DC/DC converters. The part employs fixed frequency current mode control and supports both isolated and nonisolated topologies. The IC drives external N-channel power MOSFETs and operates with input voltages up to 72V. The LT1681's operating frequency is programmable and can be synchronized up to 350kHz. Switch phase is also controlled during synchronized operation to accommodate multiple converter systems. Internal logic guarantees 50% maximum duty cycle operation to prevent transformer saturation. The LT1681 incorporates a soft-start feature that provides a controlled increase in supplied current during start-up and after an undervoltage lockout or overvoltage/overcurrent event. The part is available in a 20-lead wide SO package to support high voltage pin-to-pin clearance.
, LTC and LT are registered trademarks of Linear Technology Corporation.
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High Voltage: Operation Up to 72V Synchronizable Operating Frequency and Output Switch Phase for Multiple Controller Systems Fixed Frequency Operation to 350kHz Adaptive and Adjustable Blanking Synchronous Rectifier Driver Local 1% Voltage Reference Undervoltage Lockout Protection with Hysteresis Input Overvoltage Protection Programmable Start Inhibit Transformer Primary Saturation Protection Optocoupler Feedback Support Soft-Start Control
APPLICATIO S
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Isolated Telecommunication Systems Personal Computers and Peripherals Lead Acid Battery Backup Systems Automotive and Heavy Equipment
TYPICAL APPLICATIO
L1 4.7H VIN+
36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
L2 4.1H 6 Q1 MURS120T3 C2 22F 100V C3 1.5F 100V VIN- C4 1.5F 100V MURS120T3 7 5 8 4 0.025 1/2W 9 T1 Q5
*
+
* *
Q3
*
C2:SANYO 100MV22AX C3, C4: VITRAMON VJ1825Y155MXB C5: 4X KEMET T510X337KO10AS L1: COILCRAFT DO1608C-472 L2: PANASONIC ETQP6F4R1LF4 Q1,Q3:100V SILICONIX SUD40N10-25 Q5,Q6: SILICONIX Si4450 T1:COILTRONICS VP5-1200 Q10: ON SEMI MMBT3906LTI 73.2k 1% 270k 0.25W 20k CMPZ5248B 15V
ZVN3310F
BAT54
1OV BIAS MMBD914LT1 0.1F 100V
10k 330pF 5V OUT 3.3 0.047F 8 3 51
1OV BIAS
BAS21 14 20 17 19 18 16 11 12 15 VCC VBST BLKSENS TG BSTREF BG SENSE TMAX PGND 13 SG LT1681 OVLO 9 1 THERM SYNC SGND SS VC VFB SHDN 5VREF FSET 2 6 5 52.3k 1F 150pF 3 7 4 8 10 100 4.7nF 0.01F
CMPZ5248B 18V
0.1F 68F 20V
10k 1nF 24k
56k 1.24k 1%
3.01k 1% 1k 1%
+
Q10
LTC1693-2 6 VCC1 VCC2 5 IN2 OUT2 7 1 IN1 OUT1 2 4 GND2 GND1
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VOUT = 5V IOUT = 7A VOUT+
1 2 3
10 0.25W
1nF 100V 10 1nF 11 100V 10 0.25W 12
MBR0540T1 Q6
+
C5 330F 10V
4.7 VOUT-
100 FZT690 4.7F 16V CMPZ5242B 12V
2k 0.22F 50V
1681 TA01
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LT1681
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PACKAGE/ORDER INFORMATION
TOP VIEW SHDN 1 OVLO 2 THERM 3 SGND 4 5VREF 5 FSET 6 SYNC 7 SS 8 VFB 9 VC 10 SW PACKAGE 20-LEAD PLASTIC SO TJMAX = 125C, JA = 85C/ W 20 VBST 19 TG 18 BSTREF 17 BLKSENS 16 BG 15 PWRGND 14 VCC 13 SG 12 IMAX 11 SENSE
Supply Voltages Power Supply (VCC) ............................. - 0.3V to 20V Topside Supply (VBST) ................... VBSTREF - 0.3V to VBSTREF + 20V (VBST(MAX) = 90V) Topside Reference Pin (VBSTREF) .......... - 0.6V to 75V Input Voltages SHDN Pin .................................. - 0.3V to VCC + 0.3V All Other Inputs ..................... - 0.3V to 5VREF + 0.3V Maximum Currents 5VREF Pin ........................................ - 85mA to 10mA FSET Pin ............................................. - 2mA to 5mA All Other Inputs .................................. - 2mA to 2mA Operating Ambient Temperature Range LT1681E (Note 4) .............................. - 40C to 85C LT1681I ............................................. - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LT1681ESW LT1681ISW
Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL VCC ICC PARAMETER Operating Supply Voltage Range DC Active Supply Current DC Active UVL Supply Current DC Standby Supply Current IBST VSHDN ISS VSS VCCUVLO DC Active Supply Current DC Standby Supply Current Shutdown Rising Threshold Shutdown Threshold Hysteresis Soft-Start Charge Current Soft-Start Reset Threshold Undervoltage Lockout Threshold Undervoltage Lockout Hysteresis VBSTUVLO Boost Undervoltage Lockout (VBST-BSTREF) Boost UVLO Hysteresis Falling Edge Rising Edge Falling Edge Rising Edge
q q q q q q
CONDITIONS
q
MIN 9
TYP 12 17
MAX 18 22 25 1200 8.5 1.35 200 -6 8.60 8.95 7.1 7.5
UNITS V mA mA A A mA A V mV A mV V V V V V V
Supply and Protection (Note 2)
q
VSHDN > 1.35V, VCC = 8V VSHDN < 0.3V TG Logic High (Note 2) VSHDN < 0.3V
q
800 0.5 5 0.1 1.15 100 -14 8.0 8.3 0.25 5.7 6.5 0.3 1.25 150 - 10 225 8.40 8.75 0.35 6.4 7.0 0.6
q
q q
VSS = 2V
q
2
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LT1681
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL V5VREF I5VREFSC R5VREF Error Amp VFB IFB AV IVC VVC GBW AV ISENSE VSENSE tD IBLKSENS tMIN IMAX Sense IIMAX VIMAX tP VTHERM/ VOVLO tP fOSC IFSET VSYNC fSYNC tH, L Input Bias Current IMAX Threshold (Rising Edge) IMAX Threshold Hysteresis IMAX Output Switch Disable Delay Threshold (Rising Edge) Threshold Hysteresis Fault Delay to Output Disable Oscillator Frequency, Free Run Frequency Programming Error, Free Run FSET Input Bias Current SYNC Logic High Input Threshold SYNC Logic Low Input Threshold SYNC Frequency Maximum SYNC Pulse Width (Logic High or Logic Low) fOSC = Oscillator Free-Run Frequency 50mV Overdrive Measured at FSET Pin fOSC 500kHz (Note 3) FSET Charging, VFSET = 2V Positive-Going Edge Negative-Going Edge
q q q q
PARAMETER 5V Reference Voltage Short-Circuit Current Output Impedance Error Amplifier Reference Voltage Feedback Input Current Error Amplifier Voltage Gain Error Amplifier Current Limit Zero Current Output Voltage Maximum Output Voltage Gain Bandwidth Product Amplifier DC Gain Input Bias Current Current Limit Threshold Current Sense to Switch Delay
CONDITIONS 0 (I5VREF - IVC) < 20mA
q
MIN 4.85 4.80 20
TYP 5 45 1
MAX 5.10 5.15
UNITS V V mA
5V External Reference
Source, IVC = 0 0 (I5VREF - IVC) < 20mA Measured at Feedback Pin
q
q
1.242 1.225
1.250 -50 72
1.258 1.265
V V nA dB mA mA V V MHz V/V A
VFB = VREF Source Sink
q q
10 0.5
25 1 1.4 3.2
(Note 3)
1 12 - 275
Current Sense and Blanking
Measured at SENSE Pin
q
135 130 4.5
150 175
165 170 5.5
mV mV ns V A ns A
VBLKSENS Blanking Input Threshold Blanking Input Bias Current Switch Minimum On Time VBLKSENS = VBG, Measured at BG Output
q
5 -2 250 - 250
Measured at IMAX Input Measured at IMAX Input Measured at BG Output
q
320
360 140 130
400
mV mV ns
THERM and OVLO Fault Detectors
q q
1.2 20
1.25 40 650
1.3 60
V mV ns
Oscillator and Synchronization Decoder 700 -10 50 0.8 fOSC/2 1.4 1.4 2 350 1/fOSC 5 kHz % nA V V kHz s
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LT1681
ELECTRICAL CHARACTERISTICS
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VTS = 0V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL VTG tTGr/f VBG tBGr/f VSG tSGr/f tSG-BG tTG-BG PARAMETER TG On Voltage TG Off Voltage TG Rise/Fall Times BG On Voltage BG Off Voltage BG Rise/Fall Times SG On Voltage SG Off Voltage SG Rise/Fall Times SG to BG Enable Lag Time TG to BG Enable Lag Time 10% to 90%/90% to 10% 4V On/Off Thresholds 4V On/Off Thresholds
q
CONDITIONS
q q
MIN 11
TYP 11.5 0.1 35 11.5 0.1 35 11.5 0.1 35 150 100
MAX
UNITS V V ns V V ns V V ns ns ns
Output Drivers 0.5
10% to 90%/90% to 10%
q q
11
0.5
10% to 90%/90% to 10%
q q
11
0.5 300
80
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: Supply current specification does not include external FET gate charge currents. Actual supply currents will be higher and vary with operating frequency, operating voltages and the type of external switch elements used. See Applications Information.
Note 3: Guaranteed but not tested. Note 4: The LT1681E is guaranteed to meet performance specifications from 0C to 70C. Specifications over the - 40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. For guaranteed performance to specifications over the -40C to 85C range, the LT1681I is available.
TYPICAL PERFOR A CE CHARACTERISTICS
ICC Supply Current vs Temperature
20 VCC = 12V
ICC SUPPLY CURRENT (mA)
900
ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (nA)
19
18
17
16
15 -55 -40
40 80 TEMPERATURE (C)
0
4
UW
1681 G01
ICC Supply Current vs SHDN Pin Voltage
1100 TA = 25C
ICC Supply Current vs VCC Supply Voltage
18 TA = 25C
17
700
16
500 125 0 100 200 300 400 SHDN PIN VOLTAGE (mV) 500
1681 G02
15
9
10
12 14 16 SUPPLY VOLTAGE (V)
18
1681 G03
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LT1681 TYPICAL PERFOR A CE CHARACTERISTICS
IBST Boost Supply Current vs Temperature
5.2
IBST BOOST SUPPLY CURRENT (mA)
5.1
40
UVLO ICC SUPPLY CURRENT (mA)
ICC SUPPLY CURRENT (A)
5.0
4.9
4.8 -55 -40
40 80 TEMPERATURE (C)
0
5VREF Voltage vs Temperature
5VREF SHORT-CIRCUIT CURRENT LIMIT (mA)
5.10 60
5.05
ERROR AMP REFERENCE (V)
5VREF VOLTAGE (V)
5.00
4.95
4.90 -55 -40
40 80 TEMPERATURE (C)
0
VC Pin Short-Circuit Current Limit vs Temperature
VC PIN SHORT-CIRCUIT CURRENT LIMIT (mA)
25 12
SOFT-START OUTPUT CURRENT (A)
11
20
SOFT-START OUTPUT CURRENT (A)
15
10 -55 -40
40 80 TEMPERATURE (C)
0
UW
1681 G04 1681 G07 1681 G10
ICC Supply Current vs SHDN Pin Voltage
60 TA = 25C 1
UVLO ICC Supply Current vs Temperature
0.8
20
0.6
125
0 0 0.2 0.4 0.6 0.8 1.0 SHDN PIN CURRENT (V) 1.2
1681 G05
0.5 -55 -40
0 40 80 TEMPERATURE (C)
125
1681 G06
5VREF Short-Circuit Current Limit vs Temperature
1.260
Error Amp Reference vs Temperature
1.255
50
1.250
40
1.245
125
30 -55 -40
40 80 TEMPERATURE (C)
0
125
1681 G08
1.240 -55 -40
0 40 80 TEMPERATURE (C)
125
1681 G09
Soft-Start Output Current vs Temperature
60
Soft-Start Output Current vs Soft-Start Pin Voltage
TA = 25C
VSS = 2V
40
10
20
9
125
8 -55 -40
0
40 80 TEMPERATURE (C)
0
125
1681 G11
0
100 200 300 400 SOFT-START PIN VOLTAGE (mV)
500
1681 G12
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LT1681 TYPICAL PERFOR A CE CHARACTERISTICS
Soft-Start Output Current vs Soft-Start Pin Voltage
60 CURRENT SENSE AMP BANDWIDTH (MHz) 5
1681 G13
SOFT-START OUTPUT CURRENT (A)
40
20
0 0 1 2 3 4 SOFT-START PIN VOLTAGE (V)
PI FU CTIO S
SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding positive-going threshold of 1.25V enables the LT1681. 150mV of input hysteresis resists mode switching instability. The SHDN pin can be controlled by either a logic-level input or with an analog signal. This shutdown feature is typically used for input supply undervoltage protection. A resistor divider from the converter input supply to the SHDN pin monitors that supply for control of system power-up sequencing, etc. All internal functions are disabled during shutdown. OVLO (Pin 2): Overvoltage Shutdown Sense. Typically connected to input supply through a resistor divider. If pin voltage exceeds 1.25V, the LT1681 switching function is disabled to protect boosted circuitry from exceeding absolute maximum voltage. 40mV of input hysteresis resists mode switching instability. Exceeding the OVLO threshold also triggers soft-start reset, resulting in a graceful recovery from an input transient event. THERM (Pin 3): System Thermal Shutdown. Auxiliary shutdown pin that is typically used for system thermal protection. If pin voltage exceeds 1.25V, the LT1681 switching function is disabled. 40mV of input hysteresis resists mode switching instability. Exceeding the THERM threshold also triggers soft-start reset, resulting in a graceful recovery. SGND (Pin 4): Signal Ground Reference. Careful board layout techniques must be used to prevent corruption of the signal ground reference. High current switching paths must be oriented on the converter ground plane such that currents to/from the switches do not affect the integrity of the LT1681 signal ground reference. 5VREF (Pin 5): 5V Local Reference. Allows connection of external loads up to 20mA DC. Typically bypassed with 1F ceramic capacitor to SGND. Reference output is current limit protected to a typical value of 45mA. If the load on the 5V reference exceeds the current limit value, LT1681 switching function is disabled and the soft-start function is reset. FSET (Pin 6): Oscillator Timing Pin. Connect a resistor (RFSET) from the 5VREF pin to this pin and a capacitor (CFSET) from this pin to ground. The LT1681 oscillator operates by monitoring the voltage on CFSET as it is charged via RFSET. When the voltage on the FSET pin reaches 2.5V, the oscillator rapidly discharges the capacitor with an average current of 0.8mA. Once the
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Current Sense Amplifier Bandwidth vs Temperature
TA = 25C
8 7 6 5 4 3 2 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
1681 G14
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LT1681
PI FU CTIO S
voltage on the pin is reduced to 1.5V, the pin becomes high impedance and the charging cycle repeats. The oscillator operates at twice the switching frequency of the controller. Oscillator frequency fOSC can be approximated by the relation:
fOSC
-1 R 2 -6 + C FSET + 8 * 10- 4 + 0.5 * 10 FSET RFSET 3 -1
SYNC (Pin 7): Oscillator Synchronization Input Pin with TTL-Level Compatible Input. The SYNC input signal (at the desired synchronized operating frequency) controls both the internal oscillator (running at twice the SYNC frequency) and the output switch phase. If the synchronization function is not desired, this pin may be shorted to ground. The LT1681 internal oscillator drives a toggle flip-flop that assures 50% duty cycle operation during oscillator freerun. The oscillator, therefore, runs at twice the operating frequency of the converter. The SYNC input decoder incorporates a frequency doubling circuit for oscillator synchronization, resetting the internal oscillator on both the rising and falling edges of the input signal. The SYNC input decoder also differentiates transition phase and forces the toggle flip-flop to phase-lock with the SYNC input. A transition to logic high on the SYNC input signal corresponds to the initiation of a new switching cycle (primary switches turning on pending current control) and a transition to logic low forces a primary switch off state. As such, the maximum operating duty cycle is equal to the duty cycle of the SYNC signal. The SYNC input can therefore be used to reduce the maximum duty cycle of the converter by reducing the duty cycle of the SYNC input. SS (Pin 8): Soft-Start. Connect a capacitor (CSS) from this pin to ground. The output voltage of the LT1681 error amplifier corresponds to the peak current sense amplifier output detected before resetting the switch outputs. The soft-start circuit forces the error amplifier output to a zero sense
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current for start-up. A 10A current is forced from this pin onto an external capacitor. As the SS pin voltage ramps up, so does the LT1681 internally sensed current limit. This effectively forces the internal current limit to ramp from zero, allowing overall converter current to slowly increase until normal output regulation is achieved. This function reduces output overshoot on converter start-up. The soft-start function incorporates a 1VBE "dead zone" such that a zero current condition is maintained on the V C pin until the SS pin rises to 1VBE above ground. The SS pin voltage is reset to start-up condition during shutdown, undervoltage lockout and overvoltage or overcurrent events, yielding a graceful converter output recovery from these events. VFB (Pin 9): Error Amplifier Inverting Input. Typically connected to a resistor divider from the output and compensation components to the VC pin. The VFB pin is the converter output voltage feedback node. Input bias current of ~50nA forces the pin high in the event of an open-feedback path condition. The error amplifier is internally referenced to 1.25V. Values for the VOUT to VFB feedback resistor (RFB1) and the VFB to ground resistor (RFB2) can be calculated to program converter output voltage (VOUT) via the following relation: VOUT = 1.25 * (RFB1 + RFB2)/RFB2 VC (Pin 10): Error Amplifier Output. The LT1681 error amplifier is a low impedance output inverting gain stage. The amplifier has ample current source capability to allow easy integration of isolation optocouplers that require bias currents up to 10mA. External DC loading of the VC pin reduces the external current sourcing capacity of the 5VREF pin by the same amount as the load on the VC pin. The error amplifier is typically configured using a feedback RC network to realize an integrator circuit. This circuit creates the dominant pole for the converter regulation feedback loop. Integrator characteristics are dominated by the value of the capacitor connected from the VC pin to the VFB pin and the feedback resistor connected to the VFB pin. Specific integrator characteristics can be configured to optimize transient response.
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LT1681
PI FU CTIO S
The error amplifier can also be configured as a transimpedance amplifier for use in secondary-side controller applications. (See Applications Information section for configuration and compensation details) SENSE (Pin 11): Current Sense Amplifier (CSA) Noninverting Input. Current is monitored via a ground referenced current sense resistor, typically in series with the source of the bottom-side switch FET. Internal limit circuitry provides for a maximum peak value of 150mV across the sense resistor during normal operation. IMAX (Pin 12): Primary Current Runaway Protection. The IMAX pin is used to detect primary-side switch currents and shuts down the primary switches if a current runaway condition is detected. The IMAX function is not disabled during the current sense blanking interval. The pin is typically connected to the primary bottom-side switch source and monitors switch current via a ground-referenced current sense resistor. If the pin voltage exceeds 360mV, LT1681 switching function is disabled in 130ns. Exceeding the IMAX threshold also triggers a soft-start reset, resulting in a graceful recovery from a current runaway event. For single-sense resistor systems, this pin can be shorted to SENSE for protection during the blanking interval or shorted to SGND if not used. SG (Pin 13): Synchronous Switch Output Driver. This pin can be connected directly to the gate of the synchronous switch if small FETs are used (CGATE < 5000pF), however, the use of a gate drive buffer is recommended for peak efficiencies. The SG pin output is synchronized and out-of-phase with the BG output. The control timing of the SG output causes its transition to "lead" the primary switch path during turnon by 150ns. VCC (Pin 14): IC Local Power Supply Input. Bypass with a capacitor at least 10 times greater than C5VREF to PGND. The LT1681 incorporates undervoltage lockout that disables switching functions if VCC is below 8.4V. The LT1681 supports operational VCC power supply voltages from 9V to 18V (20V absolute maximum). PWRGND (Pin 15): Output Driver Ground Reference. Connect through low impedance trace to VIN decoupling capacitor. BG (Pin 16): Bottom-Side Primary Switch/Forward Switch Output Driver. Pin can be connected directly to gate of primary bottom-side and forward switches if small FETs are used (CGATE total < 5000pF), however, the use of a gate drive buffer is recommended for peak efficiencies. The BG output is enabled at the start of each oscillator cycle in phase with the TG pin but is timed to "lag" the TG output during turn-on and "lead" the TG output during turn-off. These delays force the concentration of transitional losses onto the bottom-side primary switch. BLKSENS (Pin 17): Blanking Sense Input. The current sense function (via SENSE pin) is disabled while the BLKSENS pin is below 5V. BLKSENS is typically connected to the gate of the bottom-side primary switch MOSFET. BSTREF (Pin 18): VBST Supply Reference. Typically connects to source of topside external power FET switch. TG (Pin 19): Topside (Boosted) Primary Output Driver. Pin can be connected directly to gate of primary topside switch if small FETs are used (CGATE < 5000pF), however, the use of a gate drive buffer is recommended for peak efficiencies. VBST (Pin 20): Topside Primary Driver Bootstrapped Supply. This "boosted" supply rail is referenced to the BSTREF pin. Supply voltage is maintained by a bootstrap capacitor tied from the VBST pin to the boosted supply reference (BSTREF) pin. The charge on the capacitor is refreshed each switch cycle through a Schottky diode connected from the VCC supply (cathode) to the VBST pin (anode). The bootstrap capacitor (CBOOST) must be at least 100 times greater than the total load capacitance on the TG pin. A capacitor in the range of 0.1F to 1F is generally adequate for most applications. The bootstrap diode must have a reversebreakdown voltage greater than the converter VIN. The LT1681 supports operational VBST supply voltages up to 90V (absolute maximum) referenced to ground. Undervoltage lockout disables the topside switch until VBST-BSTREF > 7.0V for start-up protection of the topside switch.
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20 VBST 19 TG 18 BSTREF NOL LOGIC
FSET 6 f = x2 T Q S R Q
PHASE DETECT S
13 SG
VC 10
SENSE 11 5VREF
IMAX 12
VFB 9 ERROR AMP 350mV
VCC 14 1.25V
SHDN 1
+
UVL (<8V)
-
x4 ILIM 10A
1.25V
REFERENCE GENERATOR 1.25V
THERM 3
5VREF 5
1.25V
+ - +
225mV
1681 BD
OVLO 2
SGND 4
-
+
-
+
x12
+ - -
15 PWRGND
17 BLKSENS
S Q R
8 SS
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LT1681
BLOCK DIAGRA
SYNC 7
16 BG
- + + -
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LT1681
APPLICATIO S I FOR ATIO
Overview
The LT1681 is a high voltage, high current synchronous regulator controller, optimized for use with dual transistor forward topologies. The IC uses a constant frequency, current mode architecture with internal logic that prevents operation over 50% duty cycle. A unique synchronization scheme allows the system clock to be synchronized up to an operational frequency of 350kHz, along with phase control for easy integration of multicontroller systems. A local precision 5V supply is available for external support circuitry and can be loaded up to 20mA. Internal fault detection circuitry disables switching when a variety of system faults are detected such as: input supply overvoltage or undervoltage faults, excessive system temperature, transformer primary-side saturation and local supply overcurrent conditions. The LT1681 has a current limit soft-start feature that gradually increases the current drive capability of a converter system to yield a smooth start-up with minimal overshoot. The soft-start circuitry is also used for smooth recoveries from system fault conditions. External FET switches are employed for the switch elements, and hearty switch drivers allow implementation of high current designs. An adaptive blanking scheme built into the LT1681 allows for correct current-sense blanking regardless of switch size and even while using external switch drive buffers. The LT1681 employs a voltage output error amplifier, providing superior integrator linearity and allowing easy high bandwidth integration of optocoupler feedback for fully isolated solutions. Theory of Operation (See Block Diagram) The LT1681 senses the output voltage of its associated converter via the VFB pin. The difference between the voltage on this pin and an internal 1.25V reference is amplified to generate an error voltage on the VC pin, which is used as a threshold for the current sense comparator. The current sense comparator gets its information from the SENSE pin, which monitors the voltage drop across an external current sense resistor. When the detected switch current increases to the level corresponding to the error
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voltage on the VC pin, the switches are disabled until the next switch cycle. During normal operation, the LT1681 internal oscillator runs at twice the switching frequency. The oscillator output toggles a T flip-flop, generating a 50% duty cycle pulse that is used internally as the system clock for the IC. When the output of this flip-flop transitions high, the primary switches are enabled. The primary-side switches stay enabled until the transformer primary current, sensed via the SENSE pin, connected to a ground-referenced resistor in series with the bottom-side switch FET, is sufficient to trip the current sense comparator and, in turn, reset the RS latch. When the RS latch resets, the primary switches are disabled and the synchronous switch is enabled. The adaptive blanking circuit senses the bottomside gate voltage via the BLKSENS pin and prevents current sensing until the FET is fully enabled, preventing false triggering due to a turn-on transition glitch. If the current comparator threshold is not obtained when the flip-flop output transitions low, the RS latch is bypassed and the primary switches are disabled until the next flipflop output transition, forcing a maximum switch duty cycle less than 50%. System Fault Detection--The General Fault Condition (GFC) The LT1681 contains circuitry for detecting internal and system faults. Detection of a fault triggers a "general fault condition" or GFC. When a GFC is detected, the LT1681 disables switching and discharges the soft-start capacitor. When the GFC subsides, the LT1681 initiates a startup cycle via the soft-start circuitry to assure a graceful recovery. Recovery from a GFC is gated by the soft-start capacitor discharge. The capacitor must be discharged to a threshold of 225mV before the GFC can be concluded. As the zero output current threshold of the SS pin is typically a transistor VBE, or 0.7V, latching the GFC until a 225mV threshold is achieved assures a zero output current state is obtained in the event of a short-duration fault. A GFC is also triggered during a system state change event, such as entering shutdown mode, to prevent any mode transition abnormalities.
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LT1681
APPLICATIO S I FOR ATIO
Events that trigger a GFC are: a) Exceeding the current limit of the 5VREF pin
b) Detecting an undervoltage condition on VCC c) Detecting an undervoltage condition on 5VREF d) Pulling the SHDN pin below the shutdown threshold e) Exceeding the IMAX pin threshold f) Exceeding the 1.25V fault detector threshold on either the OVLO or THERM pins The OVLO and THERM pins are used to directly trigger a GFC. If either of these pins are not used, they can be disabled by connecting the pin to SGND. The intention of the OLVO pin is to allow monitoring of the input supply to protect from an overvoltage condition. Monitoring of system temperature (THERM) is possible through use of a resistor divider using a thermistor as a resistor divider component. The 5VREF pin can provide the precision supply required for these applications. When these fault detection circuits are disabled during shutdown or VCC pin UVLO conditions, a reduction in OVLO and THERM pin input impedance to ground will occur. To prevent excessive pin input currents, low impedance pull-up devices must not be used on these pins. Undervoltage Lockout The LT1681 maintains a low current operational mode when an undervoltage condition is detected on the VCC supply pin, or when VCC is below the undervoltage lockout (UVLO) threshold. During a UVLO condition on the VCC pin, the LT1681 disables all internal functions with the exception of the shutdown and UVLO circuitry. The external 5VREF supply is also disabled during this condition. Disabling of all switching control circuity reduces the LT1681 supply current to < 1mA, simplifying integration of trickle charging in systems that employ output feedback supply generation. The function of the high side switch output (TG) is also gated by UVLO circuitry monitoring the bootstrap supply (VBST-BSTREF). Switching of the TG pin is disabled until the voltage across the bootstrap supply is greater than 7.4V. This helps prevent the possibility of forcing the high side switch into a linear operational region, potentially
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LT1681
1681 F01
Figure 1. Nonisolated Error Amp Configuration
Another common error amplifier configuration is for optocoupler use in fully isolated converters with secondary-side control (see Figure 2). In such a system, the dominant pole for the feedback loop is created at the secondary-side controller, so the error amplifier needs only to
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causing excessive power dissipation due to inadequate gate drive during start-up. Error Amplifier Configurations The converter output voltage information is fed back to the LT1681 onto the VFB pin where it is transformed into an output current control voltage by the error amplifier. The error amplifier is generally configured as an integrator and is used to create the dominant pole for the main converter feedback loop. The LT1681 error amplifier is a true high gain voltage amplifier. The amplifier noninverting input is internally referenced to 1.25V; the inverting input is the VFB pin and the output is the VC pin. Because both low frequency gain and integrator frequency characteristics can be controlled with external components, this amplifier allows far greater flexibility and precision compared with use of a transconductance error amplifier. In a nonisolated converter configuration where a resistor divider is used to program the desired output voltage, the error amplifier can be configured as a simple active integrator, forming the system dominant pole (see Figure 1). Placing a capacitor CERR from the VFB pin to the VC pin will set the single-pole crossover frequency at (2RFBCERR)-1. Additional poles and zeros can be added by increasing the complexity of the RC network.
VOUT RFB 9 CERR VFB 1.25V
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LT1681
APPLICATIO S I FOR ATIO
translate the optocoupler information. The bandwidths of the optocoupler and amplifier should be as high as possible to simplify system compensation. This high bandwidth operation is accomplished by using the error amplifier as a transimpedance amplifier, with the optocoupler transistor emitter providing feedback information directly into the VFB pin. A resistor from VFB to ground provides the DC bias condition for the optocoupler. Connecting the optocoupler transistor collector to the local 5VREF supply reduces Miller capacitance effects and maximizes the bandwidth of the optocoupler. Higher optocoupler current also means higher bandwidth, and the 5VREF supply can provide collector currents up to 10mA.
VOUT SENSE 5 5VREF 5V 9 VFB
OSCILLATOR FREQUENCY (kHz)
10
VC
1.25V LT1681
1681 F01
Figure 2. Optocoupler High BW Configuration
Oscillator Frequency Programming and Synchronization The LT1681 internal oscillator runs at twice the system switching frequency. The oscillator output toggles a T flipflop, generating a 50% duty cycle pulse that is used internally as the system clock for the IC. Free-run frequency for the internal oscillator is programmed via an RC timing network connected to the FSET pin. A pull-up resistor RFSET, connected from the 5VREF pin to FSET, provides current to charge a timing capacitor CFSET connected from the FSET pin to ground. The oscillator operates by allowing RFSET to charge CFSET up to 2.5V at which point RFSET is pulled back toward ground by a 2.5k resistor internal to the LT1681. When the voltage across CFSET is pulled down to 1.5V, the FSET pin becomes high impedance, once again allowing RFSET to charge CFSET.
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Figure 3 is a plot of oscillator frequency vs CFSET and RFSET. Typical values for 300kHz operation (150kHz system frequency) are CFSET = 150pF and RFSET = 51k.
600 550 500 450 400 350 300 250 200 150 100 20 30 40 50 60 70 80 TIMING RESISTOR (k) 90 100 330pF 200pF 150pF 100pF
1681 F03
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Figure 3. Oscillator Frequency vs Timing Components
Due the relatively fast fall time of the oscillator waveform, the FSET pin is held at its 1.5V threshold by an internal lowimpedance clamp to reduce undershoot error. If this pin is externally forced low for any reason, external current limiting is required to prevent damage to the LT1681. Continuous source current from the FSET pin should not exceed 1mA. Putting a 2k resistor in series with any low impedance pull-down device will assure proper function and protect the IC from damage. Oscillator Synchronization Synchronization of the LT1681 system clock is accomplished by driving a TTL level logic pulse train at the desired system switching frequency into the SYNC pin. In order to assure proper synchronization, each phase of the synchronization signal must be less then an oscillator free-run cycle. The SYNC input pulse controls the phasing as well as the frequency of controller switching. The SYNC circuit functions by forcing the phase of the oscillator output flip-flop to match the phase of the SYNC pulse and prematurely ending the oscillator charge cycle on each transition edge. At the SYNC low-to-high transition, the LT1681 starts a switch-on cycle and the minimum switch-off period is forced during the SYNC logic low period. Because the SYNC logic low period corresponds directly
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LT1681
APPLICATIO S I FOR ATIO
to the minimum off time, the converter maximum duty cycle can be forced using the SYNC input. For example, a 30% duty cycle SYNC pulse forces 30% maximum duty cycle operation for the converter. Because the logic low pulse width exceeds the logic high pulse width in < 50% duty cycle operation, the oscillator free-run cycle time must be programmed to exceed the logic low duration.
2.5V FSET 1.5V SYNC SYSTEM CLOCK (INTERNAL)
1681 F04
Figure 4. Oscillator/SYNC Waveforms
It is also possible to run the LT1681 in a SYNC-only mode by disabling the oscillator completely. Connecting a resistor divider from the 5VREF pin to the FSET pin, forcing a voltage within the charge range of 1.5V to 2.5V, will allow the oscillator to follow the SYNC input exclusively with no provision for free-run. Setting values to force a voltage as close to 2V as possible is recommended.
5 5VREF 75k LT1681 6 FSET 50k 100pF
1681 F05
Figure 5. Oscillator Connection for Sync-Only Mode Operation
Shutdown The LT1681 SHDN pin will support TTL and CMOS logic signals and also analog inputs. The SHDN pin turn-on (rising) threshold is 1.25V with 150mV of hysteresis. A common use of the SHDN pin is for undervoltage detection on the input supply. Driving the SHDN pin with a resistor divider connected from the input supply to ground will prevent switching until the desired input supply voltage is achieved.
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The LT1681 enters an ultralow current shutdown mode when the SHDN pin is below 350mV. During this mode, total supply current drops to a typical value of less than 1A. When SHDN rises above 350mV, the IC will draw increasing amounts of supply current until just before the 1.25V turn-on threshold is achieved, when the typical supply current reaches 60A. The shutdown function can be disabled by connecting the SHDN pin to VCC. This pin is internally clamped to 2.5V through a 20k series input resistance and can therefore draw almost 1mA when tied directly to the VCC supply. This additional current can be minimized by making the connection through an external series resistor (100k is typically used). Soft-Start The LT1681 current control pin (VC) limits sensed current to zero at voltages less than 1.4V through full current limit at VC = 3.2V, yielding 1.8V over the full regulation range. The voltage on the VC pin is internally forced to be less than or equal to SS + 0.7V. As such, the SS pin has a "dead zone" between 0V and 0.7V, where a zero sensed current condition is maintained. At SS voltages above 0.7V, the sensed current limit threshold on pin VC may rise as needed up to the SS maintained current limit value. Once the SS pin rises to the VC pin maximum value less 0.7V, or 2.5V, the SS circuit has no effect. The SS pin sources a typical current of 10A. Placing a capacitor (CSS) from the SS pin to ground will cause the voltage on the SS pin to ramp up at a controlled rate, allowing a graceful increase of maximum converter output current during a start-up condition. The start-up delay time to full available current limit is: tSS = 2.5 * 105 * CSS (sec) The LT1681 internally pulls the SS pin below the zero current threshold during any fault condition to assure graceful recovery. The SS circuit also acts as a fault control latch to assure a full-range recovery from a short duration fault. Once a fault condition is detected, the LT1681 will suspend switching until the SS pin has discharged to approximately 225mV.
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LT1681
APPLICATIO S I FOR ATIO
Layout Considerations--Grounding
The LT1681 is typically used in high current converter designs that involve substantial switching transients. The switch drivers on the IC are designed to drive large capacitances and, as such, generate significant transient currents. Careful consideration must be made regarding input and local power-supply bypassing to avoid corrupting the ground references used by the error amplifier and current sense circuitry. Effective grounding of the two-transistor synchronous forward topology where the LT1681 is used is inherently difficult. The situation is complicated further by the number of bypass elements that must be considered. Typically, high current paths and transients from the input supply and any local drive supplies must be kept isolated from SGND, to which sensitive circuits such as the error amp reference and the current sense circuits, as well as the local 5VREF supply, are referred. By virtue of the topologies used in LT1681 applications, the large currents from the primary switches, as well as the switch drive transients,
LT1681
5VREF
SGND
Figure 6. High Current Transient Return Paths
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pass through the sense resistor to ground. This defines the ground connection of the sense resistor as the reference point for both SGND and PGND. In nonisolated applications where SGND is the output reference, we now have a condition where every bypass capacitor in the converter is referenced to the same point. Effective grounding can be achieved by considering the return current paths from the sense resistor to each respective bypass capacitor. Don't be tempted to run small traces to separate the grounds. A power ground plane is important as always in high-power converters, but bypass elements must be oriented such that transient currents in the return paths of VIN and VCC do not mix. Care must be taken to keep these transients away from the SGND reference. An effective approach is to use a 2-layer ground plane, reserving an entire layer for SGND. The 5VREF and non-isolated converter output bypasses can then be directly connected to the SGND plane.
VBST VIN VBST BSTREF VCC VCC PGND
1681 F06
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L1 4.7H L2 4.8H 10 MMBT3906LT1 3 T1 Q1 VOUT+ C4 1.5F 100V
VIN+
*
C2 1.5F 100V 10 0.25W 4.7 1nF 100V 1000pF 100V 10 0.25W Q14, Q15 FDS6680A x2 C3 1.5F 100V MURS120T3 MURS120T3 6 4 10 Q3 MMBT3906LT1 0.030 1/2W 2 Q5, Q6 FDS6680A x2 2.2nF 250V VOUT- S MBR0530 5 NC
*
12 11 10 9 8 7
+
C5 TO C8 330F 10V x4
*
*
TYPICAL APPLICATIO S U
4.22k 1% 3.01k 976 1k 0.1F
1681 F07
1
VIN-
ZVN3310F BAT54 FZT690B 4.7F 10k
100 0.25W
2k 0.22F MMBZ5240BLT1 10V
S 330pF
C1: MURATA ERIE GHM3045X7R222K-GC C2, C3, C4: VITRAMON VJ1825Y155MXB C5 TO C8: 330F 10V KEMET T510X337K010AS OR 330F 6.3V KEMET T520D337M006AS ISO1: FAIRCHILD MOC207 L1: COILCRAFT DO1608C-472 L2: PANASONIC ETQPAF4R8HFA L3: COILCRAFT DO1608C-105 Q1, Q3: SILICONIX Si4486EY Q5, Q6, Q14,Q15: FAIRCHILD FDS6680A T1: MIDCOM 31267R OR COILTRONICS CTX02-14675 (FUNCTIONAL INSULATION) OR MIDCOM 31322R (BASIC INSULATION) T2: MIDCOM 31264R (FUNCTIONAL INSULATION) OR MIDCOM 31323R (BASIC INSULATION) BAT54
1000pF L3 1mH 0.1F BAS21 19 3.3 18 16 11 12 15 1* 0.1F 3 4 1k 4700pF 3k 3300pF 10 82pF 4700pF 14 52.3k 1F 5 1k 8 2 S 0.1F BAS21 20 17 BAT54 T2 S BAS21 0.022F 3.3k
73.2k
267k 0.25W
20k
*6
220pF 10k 15 5
1 SYNC
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11
16 2
6 VDD ISNS ISNSGND FG CG VCOMP VFB OPTODRV LTC1698 OVPIN MARGIN VAUX PGND GND PWRGD ICOMP 3 4 10 8 9 7 13 1.24k 1%
14
MMBZ5245LT1 15V 5 5VREF 6 3 7 4 8 10
2
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13 SG LT1681SW OVLO 9 1 THERM SYNC SGND SS VC VFB SHDN 5VREF FSET
MMBZ5248B-7 18V
ISO1 5VREF MOC207 431 7 6
10k
56k
0.1F
68F 20V
+
MMBT3906LT1
24k 1.24k
1nF
LT1681
15
Figure 7. 36V-72V DC in to 5V/10A Isolated Synchronous Forward Converter
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L1 3.3H VSEC 10 MMBT3906LT1 1 T1 Q1 VOUT+ 4.7 1000pF 100V 5 2 10 Q3 4 MMBT3906LT1 0.025 1/2W 3 1000pF 100V 10 0.25W Q6, Q15, Q17 FDS6680A x3 C4 1.5F 100V L2 2.35H
LT1681
*
C3 1.5F 100V 7 MURS120T3 MURS120T3
*
*
TYPICAL APPLICATIO S
VOUT- S MBR0530 Q12 ZVN3310F BAT54 100 4.7F 16V Q13 FZT690B 2k 0.25W
0.22F 50V MMBZ5240BLT1 10V
10k 330pF
VIN- C1: MURATA ERIE GHM3045X7R222K-GC C2, C3, C4: VITRAMON VJ1825Y155MXB C5 TO C8: 330F 10V KEMET T510X337K010AS OR 330F 6.3V KEMET T520D337M006AS C26: AVX TPSE686M020R0150 ISO1: FAIRCHILD MOC207 L1: COILCRAFT DO1608C-332 L2: PULSE P1977 PLANAR INDUCTOR L3: COILCRAFT DO1608C-105 Q1, Q3: SILICONIX Si4486EY Q5, Q6, Q14,Q15,Q17: FAIRCHILD FDS6680A Q7: FAIRCHILD NDT410EL Q12: ZETEX ZVN3310F Q13: ZETEX FZT690 T1: PULSE P1976 PLANAR TRANSFORMER (FUNCTIONAL INSULATION) OR PULSE PA-0191 (BASIC INSULATION) T2: MIDCOM 31264R (FUNCTIONAL INSULATION) OR MIDCOM 31323R (BASIC INSULATION) S BAT54 1000pF L3 1mH 0.1F 100V BAT54 BAS21 T2 1* 3.3 0.1F 3 4 1k 4700pF 14 3300pF 10 82pF 4700pF 5 8 1k 2 S 0.1F 20 12 15 17 19 18 16 11 S BAS21 0.022F
73.2k 1%
1k 1 220pF 10k 15 5 SYNC OPTODRV LTC1698 12 11 16 2 6 VDD ISNS ISNSGND FG CG VCOMP VFB OVPIN MARGIN VAUX PGND GND PWRGD ICOMP 3 4 10 8 9 7 2.43k 1% 3.01k 1.78k 1% TRIM 1k 13 0.33F
267k 0.25W
20k
BAS21
*6
14
MMBZ5245LT1 15V 5 5VREF 52.3k 1% 6 8 10 3 7 4
2
MMBZ5248B-7 18V 3k
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13 SG LT1681SW OVLO 9 1 THERM SYNC SGND SS VC VFB SHDN 5VREF FSET
ISO1 5VREF MOC207 431 7 6
10k
56k
0.1F 1F
MMBT3906LT1
+
1.24k 1%
1698 F11
C26 68F 20V
24k 1.24k 1%
1nF
Figure 8. 36V-72V DC in to 3.3V/20A Isolated Synchronous Forward Converter
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10 0.25W
VIN+
C2 1.5F 100V
+
Q5, Q14 FDS6680A x2 C1 2200pF 250V
C5 TO C8 330F 10V x4
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L1 3.3H VSEC 10 MMBT3906LT1 1 T1 Q1 VOUT+ 4.7 1000pF 100V 5 2 10 Q3 4 MMBT3906LT1 0.025 1/2W 3 VOUT- S 1000pF 100V 10 0.25W Q6, Q15, Q17 FDS6680A x3 C4 1.5F 100V L2 2.35H
VIN+
*
C2 1.5F 100V 7 10 0.25W C3 1.5F 100V MURS120T3 MURS120T3
*
+
C5 TO C8 330F 10V x4
*
Q5, Q14 FDS6680A x2 C1 2200pF 250V
TYPICAL APPLICATIO S
MBR0530 2k 0.25W 100
5VREF 9V BAT54 62k 0.25W 10k MMBZ5248LT1 18V 330pF 4.7F 16V Q13 FZT690
1.5k 0.25W Q12 ZVN3310F
1.5k 0.25W
0.22F 50V MMBZ5240BLT1 10V
Q7 NDT410EL
47k
MMBT3904LT1
S
MMBD914LT1
C1: MURATA ERIE GHM3045X7R222K-GC C2, C3, C4: VITRAMON VJ1825Y155MXB C5 TO C8: 330F 10V KEMET T510X337K010AS OR 330F 6.3V KEMET T520D337M006AS C26: AVX TPSE686M020R0150 ISO1: FAIRCHILD MOC207 L1: COILCRAFT DO1608C-332 L2: PULSE P1977 PLANAR INDUCTOR L3: COILCRAFT DO1608C-105 Q1, Q3: SILICONIX Si4486EY Q5, Q6, Q14,Q15,Q17: FAIRCHILD FDS6680A Q7: FAIRCHILD NDT410EL Q12: ZETEX ZVN3310F Q13: ZETEX FZT690 T1: PULSE P1976 PLANAR TRANSFORMER (FUNCTIONAL INSULATION) OR PULSE PA-0191 (BASIC INSULATION) T2: MIDCOM 31264R (FUNCTIONAL INSULATION) OR MIDCOM 31323R (BASIC INSULATION) VOUT+ 1000pF
4.7F BAT54 L3 1mH 0.1F 100V BAS21 BAS21 20 3.3 3 0.1F 4 1k 4700pF 14 3300pF 10 82pF 4700pF 5 1k 8 2 S 0.1F 17 19 18 16 11 12 15 1* T2 S BAT54 BAS21
100 0.25W 9V 0.022F 1k 1 12 11 16 2 6 2.43k 1% 1 6 7 3.01k 1% 3 SENSE+
73.2k 1%
+
LT1006S8 10k 15 5 VDD ISNS ISNSGND FG CG VCOMP SYNC OPTODRV LTC1698 VFB OVPIN MARGIN VAUX PGND GND PWRGD ICOMP 3 4 10 8 9 7 13 1k 3.01k 1%
267k 0.25W
20k
*6
220pF
14
MMBZ5245LT1 15V 5 6 3k 3 7 4 5VREF 52.3k 1% 8 10
VCC VBST BLKSENS TG BSTREF BG SENSE IMAX PGND 13 SG 2 LT1681SW OVLO 9 1 THERM SYNC SGND SS VC VFB 5VREF FSET SHDN
1.78k 1%
3.01k 1%
5 4
-
2 3.01k 1%
SENSE- TRIM 0.33F
10k
ISO1 5VREF MOC207 431 7 6
56k
0.1F 1F
MMBT3906LT1
+
1.24k 1%
3.01k 1%
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C26 68F 20V
24k 1.24k 1%
100 0.25W
1nF
Figure 9. 36V-72V DC in to 3.3V/20A Isolated Synchronous Forward Converter with Fast Start and Differential Sense
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LT1681
VIN-
17
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LT1681
TYPICAL APPLICATIO S
LT1681/LTC1698 36V-72V VIN to 5V/10A Module (See Figure 7 for Application Schematic) LT1681/LTC1698 Isolated 5V/10A Converter Efficiency vs Load Current
100 95 36V 90
EFFICIENCY (%)
LT1681/LTC1698 36V-72V VIN to 3.3V/20A Module (See Figure 9 for Application Schematic)
EFFICIENCY (%)
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48V 85 80 75 70 65 60 1 2 3 4 5 6 7 CURRENT (A) 8 9 10 72V
1681 TA04
LT1681/LTC1698 Isolated 3.3V/20A Converter Efficiency vs Load Current
100 95 36V 90 85 80 75 70 2 4 6 8 10 12 14 CURRENT (A) 16 18 20 48V 72V
1681 TA05
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LT1681
PACKAGE DESCRIPTION
SW Package 20-Lead Plastic Small Outline (Wide .300 Inch)
(Reference LTC DWG # 05-08-1620)
0.291 - 0.299** (7.391 - 7.595) 0.010 - 0.029 x 45 (0.254 - 0.737)
0.009 - 0.013 (0.229 - 0.330)
NOTE 1 0.016 - 0.050 (0.406 - 1.270)
NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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.
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0.496 - 0.512* (12.598 - 13.005) 20 19 18 17 16 15 14 13 12 11
NOTE 1
0.394 - 0.419 (10.007 - 10.643)
1 0.093 - 0.104 (2.362 - 2.642)
2
3
4
5
6
7
8
9
10 0.037 - 0.045 (0.940 - 1.143)
0 - 8 TYP
0.050 (1.270) BSC 0.014 - 0.019 (0.356 - 0.482) TYP
0.004 - 0.012 (0.102 - 0.305)
S20 (WIDE) 1098
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LT1681 RELATED PARTS
PART NUMBER LT1158 LT1160 LT1162 LT1336 LT1339 LTC(R)1530 LTC1622 LTC1628-PG LTC1628-SYNC LT1680 LTC1698 DESCRIPTION Half-Bridge N-Channel MOSFET Driver Half-Bridge N-Channel MOSFET Driver Dual Half-Bridge N-Channel MOSFET Driver Half-Bridge N-Channel MOSFET Driver High Power Synchronous DC/DC Controller High Power Step-Down Switching Regulator Controller 550kHz Step-Down Controller
TM
COMMENTS Current Limit Protection, 100% of Duty Cycle Up to 60V Input Supply, No Shoot-Through VIN to 60V, Good for Full-Bridge Applications Smooth Operation at High Duty Cycle (95% to 100%) 60V Dual N-Channel MOSFET Controller Excellent for 5V to 3.x Up to 50A 8-Pin MSOP; Synchronizable; Soft-Start; Current Mode 97% Efficiency; No Sense Resistor; 16-Pin SSOP Power Good Output; Minimum Input/Output Capacitors; 3.5V VIN 36V Synchronizable 150kHz to 300kHz, VIN to 36V High Side Current Sense, Up to 60V Input Use with the LT1681, Isolated Power Supplies, Contains Voltage Margining, Optocoupler Driver, Synchronization Circuit with the Primary Side Up to 42A Output; 0.925V VOUT 2V Up to 42A Output; VRM 8.4; 1.3V VOUT 3.5V Burst Mode(R) Operation; 16-Pin Narrow SSOP; 3.5V VIN 36V Mobile VID; 0.925V VOUT 2V; 3.5V VIN 36V Current Mode; 550kHz; Very Small Solution Size Up to 95% Efficiency, 550kHz, 2.65V VIN 8.5V, 0.8V VOUT VIN, Synchronizable to 750kHz GN16-Pin, 0.8V FB Reference Current Mode; 550kHz; Small 16-Pin SSOP, VIN < 9.8V 3.5V VIN 36V, Power Good Output, 300kHz Operation 50W to 2kW Power Supply Design, Adaptive Direct Sense ZVS Minimizes CIN and COUT, 4V VIN 36V, 300kHz G28 Package, VOUT = 0.6V to 1.75V 5-Bit Mobile VID, Active Voltage Positioning IMVP2, VIN to 36V VOUT = 0.6V to 1.75V, Active Voltage Positioning IMVP2, VIN to 36V
LTC1625/LTC1775 No RSENSE Current Mode Synchronous Step-Down Controller Dual, 2-Phase Synchronous Step-Down Controller Dual, 2-Phase Synchronous Step-Down Controller High Power DC/DC Current Mode Step-Up Controller Secondary Synchronous Rectifier Controller
LTC1709-7 LTC1709-8 LTC1735 LTC1736 LTC1772 LTC1773 LTC1778 LTC1874 LTC1876 LTC1922-1 LTC1929 LTC3714 LTC3716
High Efficiency, 2-Phase Synchronous Step-Down Controller with 5-Bit VID High Efficiency, 2-Phase Synchronous Step-Down Controller High Efficiency, Synchronous Step-Down Controller High Efficiency, Synchronous Step-Down Controller with 5-Bit VID ThinSOTTM Step-Down Controller Synchronous Step-Down Controller Wide Operating Range, No RSENSE Step-Down Controller Dual, Step-Down Controller 2-Phase, Dual Synchronous Step-Down Controller with Step-Up Regulator Synchronous Phase Modulated Full-Bridge Controller 2-Phase 42A Synchronous Controller Intel Compatible, Wide Operating Range, No RSENSE Step-Down Controller with Internal Op Amp High Efficiency, 2-Phase Synchronous Step-Down Controller with 5-Bit Mobile VID
No RSENSE and ThinSOT are trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technolgy Corporation.
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
LT/TP 0302 2K * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 2001

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