Datasheet

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LT1681

1681f

APPLICATIO S

DESCRIPTIO

FEATURES

TYPICAL APPLICATIO

Dual Transistor

Synchronous Forward Controller

The LT

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 con-

trolled during synchronized operation to accommodate mul-

tiple converter systems. Internal logic guarantees 50% maxi-

mum 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.

■

Isolated Telecommunication Systems

■

Personal Computers and Peripherals

■

Lead Acid Battery Backup Systems

■

Automotive and Heavy Equipment

■

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

, LTC and LT are registered trademarks of Linear Technology Corporation.

36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)

1µF 150pF

OVLO

SHDN

73.2k

20k

10k

270k

0.25W

68µF

20V

REF

SET

4.7nF

BAT54

ZVN3310F

PGND

374

THERM

LT1681

SYNC SGND

52.3k

100Ω

FZT690

4.7µF

16V

0.22µF

50V

CMPZ5242B

12V

3.3Ω

10k

0.01µF

0.1µF

SENSE

BSTREF

TGBLKSENS T

MAX

BAS21

0.1µF

100V

BST

1.5µF

100V

1.5µF

100V

4.7µH

0.025Ω

1/2W

1nF

100V

1nF

100V

MURS120T3

–

22µF

100V

OUT

–

330µF

10V

10Ω

0.25W

10Ω

0.25W

4.1µH

330pF

1.24k

•

CC1

IN2

IN1

GND2

CC2

OUT2

OUT1

GND1

LTC1693-2

1OV

BIAS

1OV

BIAS

MBR-

0540T1

MMBD914LT1

OUT

3.01k

51Ω

0.047µF

1nF

4.7Ω

1681 TA01

56k

1217

CMPZ5248B

18V

CMPZ-

5248B

15V

Q10

24k

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

OUT

= 5V

OUT

= 7A

Summary of content (20 pages)

PAGE 1
LT1681 Dual Transistor Synchronous Forward Controller DESCRIPTIO U FEATURES 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-Star
PAGE 2
LT1681 U W U U W W W ABSOLUTE MAXIMUM RATINGS PACKAGE/ORDER INFORMATION (Note 1) 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 ........................
PAGE 3
LT1681 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 4.85 4.80 5 ● 5.10 5.
PAGE 4
LT1681 ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VTS = 0V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS 11 11.5 0.1 0.
PAGE 5
LT1681 U W TYPICAL PERFOR A CE CHARACTERISTICS IBST Boost Supply Current vs Temperature ICC Supply Current vs SHDN Pin Voltage UVLO ICC Supply Current vs Temperature 60 5.2 1 5.1 5.0 4.9 4.8 –55 –40 0 40 80 TEMPERATURE (°C) UVLO ICC SUPPLY CURRENT (mA) ICC SUPPLY CURRENT (µA) IBST BOOST SUPPLY CURRENT (mA) TA = 25°C 40 20 0 125 0 0.2 0.4 0.6 0.8 1.0 SHDN PIN CURRENT (V) 1681 G04 5.00 4.95 125 1.
PAGE 6
LT1681 U W TYPICAL PERFOR A CE CHARACTERISTICS Soft-Start Output Current vs Soft-Start Pin Voltage Current Sense Amplifier Bandwidth vs Temperature 60 8 CURRENT SENSE AMP BANDWIDTH (MHz) SOFT-START OUTPUT CURRENT (µA) TA = 25°C 40 20 0 0 1 2 3 4 SOFT-START PIN VOLTAGE (V) 5 1681 G13 7 6 5 4 3 2 –55 –35 –15 5 25 45 65 85 105 125 TEMPERATURE (°C) 1681 G14 U U U PI FU CTIO S SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding positive-going threshold of 1.25V enables the LT1681.
PAGE 7
LT1681 U U U 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    FSET  – 6 – 4  ≅ 0.5 • 10 + C FSET +  8 • 10 +      R 3  FSET    –1 SYNC (Pin 7): Oscillator Synchronization Input Pin with TTL-Level Compatible Input.
PAGE 8
LT1681 U U U 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.
PAGE 9
×4 ILIM 1.25V Q T – + SGND 4 OVLO 2 – + REFERENCE GENERATOR 1.25V UVL (<8V) 1.25V 350mV S 5VREF 5 1.
PAGE 10
LT1681 U W U U 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.
PAGE 11
LT1681 U W U U APPLICATIO S I FOR ATIO Events that trigger a GFC are: a) Exceeding the current limit of the 5VREF pin causing excessive power dissipation due to inadequate gate drive during start-up. b) Detecting an undervoltage condition on VCC Error Amplifier Configurations c) Detecting an undervoltage condition on 5VREF 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.
PAGE 12
LT1681 U W U U APPLICATIO S I FOR ATIO 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 OSCILLATOR FREQUENCY (kHz) translate the optocoupler information. The bandwidths of the optocoupler and amplifier should be as high as possible to simplify system compensation.
PAGE 13
LT1681 U W U U 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. The LT1681 enters an ultralow current shutdown mode when the SHDN pin is below 350mV.
PAGE 14
LT1681 U W U U 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.
PAGE 15
C3 1.5µF 100V 68µF 20V 0.1µF + 10k MMBZ5248B-7 18V MMBZ5245LT1 15V 267k 0.25W 1nF 24k 1.24k 56k 20k 73.2k BAS21 20 17 19 18 16 11 12 15 BAS21 BAS21 BAT54 330pF 10k BAT54 MMBT3906LT1 ZVN3310F 10Ω 5VREF 1µF MMBT3906LT1 2 6 82pF 3 4 4700pF 7 8 3k 3300pF 10Ω 10 3 1• 4 •6 1k 5 8 S 2 3 T1 5 4 6 2 14 5 10k 15 1 4700pF 1k 220pF S Q1 12 11 16 2 100Ω 0.25W 6 0.
PAGE 16
C3 1.5µF 100V C4 1.5µF 100V C26 68µF 20V 0.1µF + 10k MMBZ5248B-7 18V MMBZ5245LT1 15V 267k 0.25W 1nF 24k 1.24k 1% 56k 20k 73.2k 1% BAS21 20 17 19 18 16 11 12 15 BAS21 BAS21 BAT54 330pF 10k BAT54 MMBT3906LT1 Q12 ZVN3310F 10Ω 5VREF 1µF MMBT3906LT1 2 6 82pF 3 4 4700pF 7 8 3k 3300pF 10Ω 10 4 •6 1k 5 8 S 2 1 T1 4 2 14 5 10k 15 3 4700pF 1k 220pF S Q1 12 11 16 2 100Ω 2k 0.25W 6 0.1µF VAUX OPTODRV SYNC MARGIN OVPIN VFB 3 4 10 1.
PAGE 17
Q7 NDT410EL 0.1µF C26 68µF 20V + 10k MMBZ5245LT1 15V 267k 0.25W 4.7µF 47k 1.5k 0.25W MMBD914LT1 5VREF VIN– 20 17 19 18 16 11 12 BAT54 15 BAS21 BAS21 330pF 10k BAT54 MMBT3906LT1 Q12 ZVN3310F 10Ω 1µF MMBT3906LT1 5VREF 52.3k 1% 5 6 82pF 3 4 4700pF 7 8 3k 3300pF 10Ω 10 0.1µF 3 1• T2 4 •6 1k 5 8 S 2 S Q1 1 T1 4 2 14 5 10k 15 3 4700pF 1k 220pF ISO1 5VREF MOC207 4 3 1 7 6 3.3Ω BAT54 0.
PAGE 18
LT1681 U 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 EFFICIENCY (%) 90 48V 85 72V 80 75 70 65 60 1 2 3 4 5 6 7 CURRENT (A) 8 9 10 1681 TA04 LT1681/LTC1698 36V-72V VIN to 3.3V/20A Module (See Figure 9 for Application Schematic) LT1681/LTC1698 Isolated 3.
PAGE 19
LT1681 U PACKAGE DESCRIPTION SW Package 20-Lead Plastic Small Outline (Wide .300 Inch) (Reference LTC DWG # 05-08-1620) 0.496 – 0.512* (12.598 – 13.005) 20 19 18 17 16 15 14 13 12 11 0.394 – 0.419 (10.007 – 10.643) NOTE 1 0.291 – 0.299** (7.391 – 7.595) 0.010 – 0.029 × 45° (0.254 – 0.737) 1 2 3 4 5 6 7 8 9 0.093 – 0.104 (2.362 – 2.642) 10 0.037 – 0.045 (0.940 – 1.143) 0° – 8° TYP 0.009 – 0.013 (0.229 – 0.330) NOTE 1 0.016 – 0.050 (0.406 – 1.270) 0.050 (1.270) BSC 0.014 – 0.
PAGE 20
LT1681 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1158 Half-Bridge N-Channel MOSFET Driver Current Limit Protection, 100% of Duty Cycle LT1160 Half-Bridge N-Channel MOSFET Driver Up to 60V Input Supply, No Shoot-Through LT1162 Dual Half-Bridge N-Channel MOSFET Driver VIN to 60V, Good for Full-Bridge Applications LT1336 Half-Bridge N-Channel MOSFET Driver Smooth Operation at High Duty Cycle (95% to 100%) LT1339 High Power Synchronous DC/DC Controller 60V Dual N-Channel MOSFET Controlle