LINEAR TECHNOLOGY LT3781 Technical data

Transistor Synchronous Forward Controller

FEATURES

■

High Voltage Operation up to 72V

■

Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems

■

Synchronous Switch Output

■

Undervoltage Lockout Protection with 6V Hysteresis
for Self-Biased Power

■

Fixed Frequency Operation to 350kHz

■

Local ±1% Voltage Reference

■

Input Overvoltage Protection

■

Low Start-Up Current

■

Programmable Start Inhibit for Power Supply
Sequencing and Protection

■

Optocoupler Support

■

Soft-Start Control

U

APPLICATIO S

■

Isolated Telecommunication Systems

■

Personal Computers and Peripherals

■

Distributed Power Step-Down Converters

■

Lead Acid Battery Backup Systems

■

Automotive and Heavy Equipment

U

TYPICAL APPLICATIO

L1

+

V

IN

–

V

IN

4.7µH

C2

+

22µF
100V

C3

1.5µF
100V

C4

1.5µF
100V

Synchronous Forward Converter (Quarter-Brick Footprint)

36V-72V to 5V/7A DC/DC

MURS120T3

Q3

ZVN3310F

BAT54

LT3781

“Bootstrap” Start Dual

U

DESCRIPTIO

The LT®3781 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 LT3781 is ideal for output derived power schemes,
through the use of a large undervoltage lockout hysteresis
range. The part is also equipped with an 18V VCC shunt
regulator, which prevents exceeding absolute maximum
ratings while in trickle start applications.

The LT3781’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 LT3781 is available in a 20-lead SSOP package.

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

L2

4.7µH

MBR­0540T1

4.7Ω

+

C5
330µF
10V

MURS120T3

0.025Ω

1/2W

T1

•

Q1

R9

6

1

7

2
3

•

•

5

8

10
11

•

4

12

9

Q5

1nF

100V

0.25W

1nF

100V

10Ω

0.25W

10Ω

Q6

V

= 5V

OUT

= 7A

I

OUT

+

V

OUT

–

V

OUT

68µF

20V

0.1µF

1OV
BIAS

MMBD914LT1

73.2k

270k

0.25W

+

1nF

10k

20k

1%

BAS21

13

2
1

1.24k
1%

0.1µF
100V

20

V

V

CC

BST

OVLO

SHDN

FSET

5V

REF

5

52.3k

1µF 150pF

19

BSTREF

TG

LT3781

THERM

6

11

15

18

SENSE

BG

SYNC SGND

374

4.7nF

PGND

V

8SS10

14

V

C

100Ω

10k

330pF

SG

FB

12

9

0.01µF

OUT

5V

0.047µF

3.01k
1%

1k

1%

3.3k

51Ω

6.8k

8

V

3

IN2

1

IN1

4

GND2

1OV

BIAS

LTC1693-2

CC1

FZT690

4.7µF

16V

6

V

CC2

5

OUT2

7

OUT1

2

GND1

C2:SANYO 100MV22AX
C3, C4: VITRAMON VJ1825Y155MXB
C5: 4X KEMET T510X337KO10AS
L1: COILCRAFT DO1608C-472

100Ω

CMPZ5242B

12V

2k

0.22µF

50V

L2: PANASONIC ETQP6F4R1LF4
Q1,Q3:100V SILICONIX SUD40N10-25
Q5,Q6: SILICONIX Si4450
T1:COILTRONICS VP5-1200

3781f

1

LT3781

WW

W

U

ABSOLUTE AXI U RATI GS

(Note 1)

Power Supply (VCC)

Low Impedance Source Voltage ............. –0.3V to 20V

Shutdown Mode:

(Supply Self-Regulates to 18V)

Maximum Input Current ............................... 20mA

Topside Supply (V

V

BSTREF

– 0.3V to V
Topside Reference Pin (V

SHDN Pin Voltage........................... –0.3V to V

All Other Input Voltages.............. –0.3V to 5V

5V

Pin Sink Current ......................................... 10mA

REF

FSET Pin Current ...................................... –2mA to 5mA

All Other Input Pin Currents...................... –2mA to 2mA

Operating Ambient

Temperature Range (Note 4) ...............–40°C to 85°C

Operating Junction

Temperature Range ...............................–40°C to 125°C

Storage Temperature Range ..................–65°C to 150°C

Lead Temperature (Soldering, 10 sec).................. 300°C

) ....................................................

BST

BSTREF

BSTREF

+20V (V

BST(MAX)

) ...............–0.6V to 75V

= 90V)

CC

+ 0.3V

REF

+ 0.3V

UUW

PACKAGE/ORDER I FOR ATIO

TOP VIEW

1

SHDN

2

OVLO

3

THERM

4

SGND

5

5V

REF

6

FSET

7

SYNC

8

SS

9

V

FB

10

V

C

G PACKAGE

20-LEAD PLASTIC SSOP

T

= 125°C, θJA = 90°C/W

JMAX

Consult LTC Marketing for parts specified with wider operating temperature ranges.

20
19
18
17
16
15
14
13
12
11

V

BST

TG
BSTREF
NC
NC
BG
PWRGND
V

CC

SG
SENSE

ORDER PART

NUMBER

LT3781EG
LT3781IG

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VCC = V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Supply and Protection

V

CCUVLO

V

CCSHDN

I

CC

V

BST

V

SHDN

I

SS

V

SS

V

BSTUVLO

5V External Reference

V

5VREF

I

5VREFSC

R

5VREF

= 12V, V

BST

Undervoltage Lockout Threshold Falling Edge ● 8.0 8.4 8.6 V

Shutdown Mode Shunt Regulator 100µA < I
DC Active Supply Current (Note 2) 17 22 mA

DC Active UVLO Supply Current V
DC Standby Supply Current V
DC Active Supply Current TG Logic High (Note 2) ● 5.0 8.5 mA
DC Standby Supply Current V
Shutdown Rising Threshold ● 1.15 1.25 1.35 V
Shutdown Threshold Hysteresis 100 150 200 mV
Soft-Start Charge Current VSS = 2V ● –14 –10 –6 µA
Soft-Start Reset Threshold 225 mV
Boost Undervoltage Lockout Falling Edge ● 5.7 6.4 7.1 V

(V

-BSTREF) Rising Edge ● 6.5 7.0 7.5 V

BST

Boost UVLO Hysteresis ● 0.3 0.6 V

5V Reference Voltage 0 ≤ (I

Short-Circuit Current Source, IVC = 0 ● 20 45 mA
Output Impedance 0 ≤ (I

= 0V, VVC = 2V, VFB = V

BSTREF

= 1.25V, CTG = CBG = CSG = 1000pF.

REF

Rising Edge

≤ 10mA ● 16.5 18 19.9 V

VCC

= 1.35V, VCC = 8V ● 800 1200 µA

SHDN

< 0.3V ● 16 30 µA

SHDN

< 0.3V 0.1 µA

SHDN

– IVC) < 20mA 4.85 5.0 5.10 V

5VREF

– IVC) < 20mA 1 Ω

5VREF

● 13 14.5 16 V

● 25 mA

● 4.80 5.15 V

3781f

2

LT3781

ELECTRICAL CHARACTERISTICS

The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C.
VCC = V

SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS
Error Amp

V

FB

I

FB

A

V

I

VC

V

VC

GBW Gain Bandwidth Product (Note 3) 1 MHz

Current Sense

A

V

I

SENSE

V

SENSE

t

D

t

MIN

THERM and OVLO Fault Detectors

V

THERM

V

OVLO

t

D

Oscillator and Synchronization Decoder

f

OSC

I

FSET

V

SYNC

f

SYNC

t

H, L

Output Drivers

V

TG

t

TGr/f

V

BG

t

BGr/f

V

SG

t

SGr/f

t

SG-BG

t

TG-BG

= 12V, V

BST

= 0V, VVC = 2V, VTS = 0V, VFB = V

BSTREF

= 1.25V, CTG = CBG = CSG = 1000pF.

REF

Error Amplifier Reference Voltage Measured at Feedback Pin 1.242 1.250 1.258 V

● 1.225 1.265 V

Feedback Input Current VFB = V

REF

–50 nA

Error Amplifier Voltage Gain 72 dB
Error Amplifier Current Limit Source ● 10 25 mA

Sink

● 0.5 1 mA

Zero Current Output Voltage 1.4 V
Maximum Output Voltage 3.2 V

Amplifier DC Gain 12 V/V
Input Bias Current –275 µA
Current Limit Threshold Measured at SENSE Pin 135 150 165 mV

● 130 170 mV

Current Sense to Switch Delay 175 ns
Switch Minimum On Time Measured at BG Output 250 ns

/ Threshold (Rising Edge) ● 1.2 1.25 1.3 V

Threshold Hysteresis ● 20 40 60 mV
Fault Delay to Output Disable >50mV Overdrive 650 ns

Oscillator Frequency, Free Run Measured at F
Frequency Programming Error f
FSET Input Bias Current F

≤ 500kHz (Note 3) ● –10 5 %

OSC

Charging, V

SET

Pin 700 kHz

SET

= 2V 50 nA

FSET

SYNC Logic High Input Threshold Positive-Going Edge ● 1.4 2 V
SYNC Logic Low Input Threshold Negative-Going Edge ● 0.8 1.4 V

SYNC Frequency ● f
Maximum SYNC Pulse Width f

= Oscillator Free-Run Frequency 1/f

OSC

/2 350 kHz

OSC

OSC

(Logic High or Logic Low)

TG On Voltage ● 11 11.5 V
TG Off Voltage

● 0.1 0.5 V

TG Rise/Fall Time 10% to 90%/90% to 10% 35 ns
BG On Voltage ● 11 11.5 12 V

BG Off Voltage

● 0.1 0.5 V

BG Rise/Fall Time 10% to 90%/ 90% to 10% 35 ns
SG On Voltage ● 11 11.5 12 V

SG Off Voltage

● 0.1 0.5 V

SG Rise/Fall Time 10% to 90%/ 90% to 10% 35 ns
SG to BG Enable Lag Time 4V On/Off Thresholds ● 80 150 300 ns
TG to BG Enable Lag Time 4V On/Off Thresholds 100 ns

s

3781f

3

LT3781

ELECTRICAL CHARACTERISTICS

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 4: The LT3781E is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
ambient temperature range are assured by design, characterization and
correlation with statistical process controls. For guaranteed performance
to specifications over the –40°C to 85°C operating ambient temperature
range, the LT3781I is available.

Note 3: Guaranteed but not tested.

UW

TYPICAL PERFOR A CE CHARACTERISTICS

Shutdown Mode: VCC vs I

18.4

TA = 25°C

18.2

(V)

18.0

CC

V

17.8

CC

18.20

18.15

(V)

18.10

CC

V

18.05

Shutdown Mode: VCC vs
Temperature (ICC = 1mA)

17.6

100µ

300µ

ICC Supply Current
vs Temperature

20

19

18

17

SUPPLY CURRENT (mA)

CC

I

16

15

–55

–40

0

TEMPERATURE (°C)

1m

ICC (A)

3m

3781 • G01a

10m

18.00
–55

–40

40

0

TEMPERATURE (°C)

80 125

3781 • G01b

ICC Supply Current
vs VCC Supply Voltage

18

VCC = 12V

40

12580

3781 G01

TA = 25°C

17

16

SUPPLY CURRENT (mA)

CC

I

15

9

12

10

SUPPLY VOLTAGE (V)

14

1816

3781 G03

4

3781f

UW

SOFT-START PIN VOLTAGE (mV)

0

SOFT-START OUTPUT CURRENT (µA)

20

40

60

1000 200 300 400

3781 G12

500

TA = 25°C

TYPICAL PERFOR A CE CHARACTERISTICS

I

Boost Supply Current

BST

vs Temperature

5.2

5.1

5.0

ICC Supply Current
vs SHDN Pin Voltage

60

TA = 25°C

40

LT3781

UVLO ICC Supply Current
vs Temperature

1

0.8

4.9

BOOST SUPPLY CURRENT (mA)

BST

I

4.8
–55

–40

5V

REF

5.10

5.05

5.00

VOLTAGE (V)

REF

5V

4.95

4.90
–55

–40

0

TEMPERATURE (°C)

Voltage vs Temperature

0

TEMPERATURE (°C)

20

SUPPLY CURRENT (µA)

CC

I

40

12580

3781 G04

0

0.2 0.4 0.6 0.8

0

5V

SHDN PIN VOLTAGE (V)

Short-Circuit Current Limit

REF

1.0

1.2

3781 G05

vs Temperature

60

50

40

SHORT-CIRCUIT CURRENT LIMIT (mA)

REF

5V

40

12580

3781 G07

30

–55

–40

0

40

TEMPERATURE (°C)

12580

3781 G08

SUPPLY CURRENT (mA)

CC

0.6

UVLO I

0.5
–55

–40

0

TEMPERATURE (°C)

Error Amp Reference
vs Temperature

1.260

1.255

1.250

1.245

ERROR AMP REFERENCE (V)

1.240
–55

–40

0

TEMPERATURE (°C)

40

40

12580

3781 G06

12580

3781 G09

VC Pin Short-Circuit Current Limit
vs Temperature

25

20

15

PIN SHORT-CIRCUIT CURRENT LIMIT (mA)

C

V

10

–55

–40

0

40

TEMPERATURE (°C)

12580

3781 G10

Soft-Start Output Current
vs Temperature

12

VSS = 2V

11

10

9

SOFT-START OUTPUT CURRENT (µA)

8

–55

–40

0

TEMPERATURE (°C)

40

12580

3781 G11

Soft-Start Output Current
vs Soft-Start Pin Voltage

3781f

5

LT3781

TEMPERATURE (°C)

–55

2

CURRENT SENSE AMP BANDWIDTH (MHz)

3

5

6

7

–15

25

45 125

3781 G14

4

–35 5

65

85

105

8

UW

TYPICAL PERFOR A CE CHARACTERISTICS

U

PI FU CTIO S

SHDN (Pin 1): Shutdown Pin. Pin voltages exceeding
positive going threshold of 1.25V enables the LT3781.
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.

An 18V clamp on the VCC pin is enabled during shutdown
mode, preventing a trickle start circuit from pulling that pin
above maximum operational levels. All other 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, LT3781 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.

6

Soft-Start Output Current
vs Soft-Start Pin Voltage

60

40

20

SOFT-START OUTPUT CURRENT (µA)

0

10234

SOFT-START PIN VOLTAGE (V)

UU

TA = 25°C

3781 G13

Current Sense Amplifier
Bandwidth vs Temperature

5

THERM (Pin 3): System Thermal Shutdown. Auxiliary
shutdown pin that is typically used for system thermal
protection. If pin voltage exceeds 1.25V, LT3781
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
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 LT3781 signal ground reference.

5V

(Pin 5): 5V Local Reference. Allows connection of

REF

external loads up to 20mA DC. Typically bypassed with
1µF 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,
LT3781 switching function is disabled and the soft-start
function is reset.

FSET (Pin 6): Oscillator Timing Pin. Connect a resistor
(R

) from the 5V

FSET

(C

) from this pin to ground.

FSET

pin to this pin and a capacitor

REF

3781f