LT3781
“Bootstrap” Start Dual
Transistor Synchronous Forward Controller
FEATURES
s
s
DESCRIPTIO
s
s
s
s
s
s
s
s
s
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
Isolated Telecommunication Systems
Personal Computers and Peripherals
Distributed Power Step-Down Converters
Lead Acid Battery Backup Systems
Automotive and Heavy Equipment
The LT
®
3781 controller simplifies the design of high
power synchronous dual transistor forward DC/DC con-
verters. The part employs fixed frequency current mode
control and supports both isolated and nonisolated to-
pologies. 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 V
CC
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 accommo-
date multiple-converter systems. Internal logic guaran-
tees 50% maximum duty cycle operation to prevent trans-
former saturation.
The LT3781 is available in a 20-lead SSOP package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATIO S
s
s
s
s
s
TYPICAL APPLICATIO
V
IN+
L1
4.7µH
36V-72V to 5V/7A DC/DC
Synchronous Forward Converter (Quarter-Brick Footprint)
T1
6
Q1
MURS120T3
7
5
MURS120T3
R9
0.025Ω
1/2W
8
4
9
Q5
ZVN3310F
•
+
C2
22µF
100V
C3
1.5µF
100V
C4
1.5µF
100V
•
•
1
2
3
Q3
10
11
12
1nF
100V
•
V
IN–
BAT54
1OV
BIAS
MMBD914LT1
270k
0.25W
73.2k
1%
20k
13
2
1
0.1µF
68µF
20V
1nF
1.24k
1%
10k
1µF
BAS21
20
V
CC
V
BST
OVLO
SHDN 5V
REF
FSET
19
18
LT3781
15
11
14
TG BSTREF BG SENSE
0.1µF
100V
10k
330pF
5V
OUT
3.3k
0.047µF
LTC1693-2 6
V
CC1
V
CC2
5
3
IN2
OUT2
7
1
IN1
OUT1
2
4
GND2 GND1
8
1OV
BIAS
FZT690
4.7µF
16V
PGND 12
SG
9
THERM SYNC SGND SS V
C
V
FB
3
7
4
8
10
6
5
52.3k
150pF
+
100Ω
4.7nF
0.01µF
3.01k
1%
1k
1%
51Ω
6.8k
C2:SANYO 100MV22AX
C3, C4: VITRAMON VJ1825Y155MXB
C5: 4X KEMET T510X337KO10AS
L1: COILCRAFT DO1608C-472
U
L2
4.7µH
10Ω
0.25W
1nF
100V
10Ω
0.25W
V
OUT
= 5V
I
OUT
= 7A
V
OUT+
MBR-
0540T1
Q6
U
U
+
C5
330µF
10V
4.7Ω
V
OUT–
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
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SHDN
OVLO
THERM
SGND
5V
REF
FSET
SYNC
SS
V
FB
1
2
3
4
5
6
7
8
9
20 V
BST
19 TG
18 BSTREF
17 NC
16 NC
15 BG
14 PWRGND
13 V
CC
12 SG
11 SENSE
Power Supply (V
CC
)
Low Impedance Source Voltage ............. –0.3V to 20V
Shutdown Mode:
(Supply Self-Regulates to 18V)
Maximum Input Current ............................... 20mA
Topside Supply (V
BST
) ....................................................
V
BSTREF
– 0.3V to V
BSTREF
+20V (V
BST(MAX)
= 90V)
Topside Reference Pin (V
BSTREF
) ............... –0.6V to 75V
SHDN Pin Voltage ........................... –0.3V to V
CC
+ 0.3V
All Other Input Voltages .............. –0.3V to 5V
REF
+ 0.3V
5V
REF
Pin Sink Current ......................................... 10mA
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
ORDER PART
NUMBER
LT3781EG
LT3781IG
V
C
10
G PACKAGE
20-LEAD PLASTIC SSOP
T
JMAX
= 125°C,
θ
JA
= 90°C/W
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 T
A
= 25°C.
V
CC
= V
BST
= 12V, V
BSTREF
= 0V, V
VC
= 2V, V
FB
= V
REF
= 1.25V, C
TG
= C
BG
= C
SG
= 1000pF.
SYMBOL PARAMETER
Supply and Protection
V
CCUVLO
Undervoltage Lockout Threshold
V
CCSHDN
I
CC
Shutdown Mode Shunt Regulator
DC Active Supply Current
CONDITIONS
Falling Edge
Rising Edge
100µA < I
VCC
≤
10mA
(Note 2)
V
SHDN
= 1.35V, V
CC
= 8V
V
SHDN
< 0.3V
TG Logic High (Note 2)
V
SHDN
< 0.3V
q
q
q
q
MIN
8.0
13
16.5
TYP
8.4
14.5
18
17
800
16
5.0
0.1
1.25
150
– 10
225
6.4
7.0
0.6
5.0
45
1
MAX
8.6
16
19.9
22
25
1200
30
8.5
1.35
200
–6
7.1
7.5
UNITS
V
V
V
mA
mA
µA
µA
mA
µA
V
mV
µA
mV
V
V
V
V
V
mA
Ω
3781f
DC Active UVLO Supply Current
DC Standby Supply Current
V
BST
DC Active Supply Current
DC Standby Supply Current
V
SHDN
Shutdown Rising Threshold
Shutdown Threshold Hysteresis
I
SS
Soft-Start Charge Current
V
SS
Soft-Start Reset Threshold
V
BSTUVLO
Boost Undervoltage Lockout
(V
BST
-BSTREF)
Boost UVLO Hysteresis
5V External Reference
V
5VREF
5V Reference Voltage
I
5VREFSC
R
5VREF
Short-Circuit Current
Output Impedance
q
q
q
q
V
SS
= 2V
Falling Edge
Rising Edge
q
q
q
q
1.15
100
–14
5.7
6.5
0.3
4.85
4.80
20
0
≤
(I
5VREF
– I
VC
) < 20mA
q
5.10
5.15
Source, I
VC
= 0
0
≤
(I
5VREF
– I
VC
) < 20mA
q
2
U
W
U
U
W W
W
LT3781
ELECTRICAL CHARACTERISTICS
The
q
denotes specifications which apply over the full operating temperature range, otherwise specifications are T
A
= 25°C.
V
CC
= V
BST
= 12V, V
BSTREF
= 0V, V
VC
= 2V, V
TS
= 0V, V
FB
= V
REF
= 1.25V, C
TG
= C
BG
= C
SG
= 1000pF.
SYMBOL
Error Amp
V
FB
I
FB
A
V
I
VC
V
VC
GBW
A
V
I
SENSE
V
SENSE
t
D
t
MIN
V
THERM
/
V
OVLO
t
D
f
OSC
I
FSET
V
SYNC
f
SYNC
t
H, L
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
Switch Minimum On Time
Threshold (Rising Edge)
Threshold Hysteresis
Fault Delay to Output Disable
Oscillator Frequency, Free Run
Frequency Programming Error
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)
TG On Voltage
TG Off Voltage
TG Rise/Fall Time
BG On Voltage
BG Off Voltage
BG Rise/Fall Time
SG On Voltage
SG Off Voltage
SG Rise/Fall Time
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
PARAMETER
CONDITIONS
Measured at Feedback Pin
q
MIN
1.242
1.225
TYP
1.250
–50
72
MAX
1.258
1.265
UNITS
V
V
nA
dB
mA
mA
V
V
MHz
V/V
µA
V
FB
= V
REF
Source
Sink
q
q
10
0.5
25
1
1.4
3.2
(Note 3)
1
12
– 275
Current Sense
Measured at SENSE Pin
q
135
130
150
175
165
170
mV
mV
ns
ns
Measured at BG Output
q
q
250
1.2
20
1.25
40
650
700
q
THERM and OVLO Fault Detectors
1.3
60
V
mV
ns
kHz
%
nA
2
350
1/f
OSC
V
V
kHz
s
>50mV Overdrive
Measured at F
SET
Pin
f
OSC
≤
500kHz (Note 3)
F
SET
Charging, V
FSET
= 2V
Positive-Going Edge
Negative-Going Edge
f
OSC
= Oscillator Free-Run Frequency
q
q
q
Oscillator and Synchronization Decoder
–10
50
0.8
f
OSC
/2
1.4
1.4
5
Output Drivers
V
TG
t
TGr/f
V
BG
t
BGr/f
V
SG
t
SGr/f
t
SG-BG
t
TG-BG
q
q
11
11.5
0.1
35
11.5
0.1
35
11.5
0.1
35
150
100
0.5
12
0.5
12
0.5
300
V
V
ns
V
V
ns
V
V
ns
ns
ns
3781f
10% to 90%/90% to 10%
q
q
11
10% to 90%/90% to 10%
q
q
11
80
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 3:
Guaranteed but not tested.
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.
TYPICAL PERFOR A CE CHARACTERISTICS
Shutdown Mode: V
CC
vs I
CC
18.4
T
A
= 25°C
18.20
18.2
V
CC
(V)
18.0
17.8
17.6
100µ
300µ
1m
I
CC
(A)
V
CC
(V)
I
CC
Supply Current
vs Temperature
20
V
CC
= 12V
I
CC
SUPPLY CURRENT (mA)
I
CC
SUPPLY CURRENT (mA)
19
18
17
16
15
–55 –40
40
80
TEMPERATURE (°C)
4
U W
0
Shutdown Mode: V
CC
vs
Temperature (I
CC
= 1mA)
18.15
18.10
18.05
3m
10m
3781 • G01a
18.00
–55 –40
40
80
0
TEMPERATURE (°C)
125
3781 • G01b
I
CC
Supply Current
vs V
CC
Supply Voltage
18
T
A
= 25°C
17
16
125
3781
G01
15
9
10
12
14
16
SUPPLY VOLTAGE (V)
18
3781
G03
3781f
LT3781
TYPICAL PERFOR A CE CHARACTERISTICS
I
BST
Boost Supply Current
vs Temperature
5.2
I
BST
BOOST SUPPLY CURRENT (mA)
5.1
40
UVLO I
CC
SUPPLY CURRENT (mA)
I
CC
SUPPLY CURRENT (µA)
5.0
4.9
4.8
–55 –40
40
80
TEMPERATURE (°C)
0
5V
REF
Voltage vs Temperature
5V
REF
SHORT-CIRCUIT CURRENT LIMIT (mA)
5.10
60
5.05
ERROR AMP REFERENCE (V)
5V
REF
VOLTAGE (V)
5.00
4.95
4.90
–55 –40
40
80
TEMPERATURE (°C)
0
V
C
Pin Short-Circuit Current Limit
vs Temperature
V
C
PIN SHORT-CIRCUIT CURRENT LIMIT (mA)
25
12
SOFT-START OUTPUT CURRENT (µA)
11
20
SOFT-START OUTPUT CURRENT (µA)
15
10
–55 –40
40
80
TEMPERATURE (°C)
0
U W
3781
G04
3781
G07
3781
G10
I
CC
Supply Current
vs SHDN Pin Voltage
60
T
A
= 25°C
1
UVLO I
CC
Supply Current
vs Temperature
0.8
20
0.6
125
0
0
0.2
0.4
0.6
0.8
1.0
SHDN PIN VOLTAGE (V)
1.2
3781
G05
0.5
–55 –40
40
80
TEMPERATURE (°C)
0
125
3781
G06
5V
REF
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
3781
G08
1.240
–55 –40
40
80
TEMPERATURE (°C)
0
125
3781
G09
Soft-Start Output Current
vs Temperature
60
Soft-Start Output Current
vs Soft-Start Pin Voltage
T
A
= 25°C
V
SS
= 2V
40
10
20
9
125
8
–55 –40
0
40
80
TEMPERATURE (°C)
0
125
3781
G11
0
100
200
300
400
SOFT-START PIN VOLTAGE (mV)
500
3781
G12
3781f
5
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