LT3781 Datasheet PDF - Linear Technology

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LT3781
Linear Technology

Part Number LT3781
Description Bootstrap Start Dual Transistor Synchronous Forward Controller
Page 20 Pages


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LT3781
“Bootstrap” Start Dual
Transistor Synchronous Forward Controller
FEATURES
s High Voltage Operation up to 72V
s Synchronizable Operating Frequency and Output
Switch Phase for Multiple Controller Systems
s Synchronous Switch Output
s Undervoltage Lockout Protection with 6V Hysteresis
for Self-Biased Power
s Fixed Frequency Operation to 350kHz
s Local ±1% Voltage Reference
s Input Overvoltage Protection
s Low Start-Up Current
s Programmable Start Inhibit for Power Supply
Sequencing and Protection
s Optocoupler Support
s Soft-Start ControUl
APPLICATIO S
s Isolated Telecommunication Systems
s Personal Computers and Peripherals
s Distributed Power Step-Down Converters
s Lead Acid Battery Backup Systems
s Automotive and Heavy Equipment
DESCRIPTIO
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 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 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.
TYPICAL APPLICATIO
36V-72V to 5V/7A DC/DC
Synchronous Forward Converter (Quarter-Brick Footprint)
L1
4.7µH
T1
VIN+
6
Q1 1
+ C2
C4
MURS120T3
72
3
5
10
0.25W
22µF
100V
1.5µF
100V
MURS120T3
1nF
8 10 1nF 100V
C3
1.5µF
100V
Q3
R9
0.025
4
11 100V 10
12 0.25W
1/2W
9
Q5
VIN–
ZVN3310F BAT54
Q6
L2
4.7µH
VOUT = 5V
IOUT = 7A
VOUT+
MBR- +
0540T1
C5
330µF
10V
4.7
VOUT–
0.1µF
68µF +
20V
270k
0.25W
73.2k
1%
20k
1nF
10k
1OV
BIAS
MMBD914LT1
0.1µF
100V
10k
330pF
BAS21
20
13
VCC VBST
2 OVLO
1 SHDN 5VREF
19 18 15 11
14
TG BSTREF BG SENSE
PGND
SG
12
LT3781
FSET THERM SYNC SGND SS VC VFB 9
1.24k
1%
56
52.3k
3 7 4 8 10
100
5V
OUT
3.3k
0.047µF
3.01k
1%
51
6.8k
1µF
150pF
4.7nF
0.01µF
1k
1%
1OV
BIAS
FZT690
100
2k
0.22µF
50V
CMPZ5242B
4.7µF
12V
16V
8 LTC1693-2 6
VCC1 VCC2
35
IN2 OUT2
17
IN1 OUT1
42
GND2 GND1
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
3781f
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LT3781
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 (VBST) ....................................................
VBSTREF – 0.3V to VBSTREF +20V (VBST(MAX) = 90V)
Topside Reference Pin (VBSTREF) ............... –0.6V to 75V
SHDN Pin Voltage ........................... –0.3V to VCC + 0.3V
All Other Input Voltages .............. –0.3V to 5VREF + 0.3V
5VREF 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
UW U
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SHDN 1
OVLO 2
THERM 3
SGND 4
5VREF 5
FSET 6
SYNC 7
SS 8
VFB 9
VC 10
20 VBST
19 TG
18 BSTREF
17 NC
16 NC
15 BG
14 PWRGND
13 VCC
12 SG
11 SENSE
G PACKAGE
20-LEAD PLASTIC SSOP
TJMAX = 125°C, θJA = 90°C/W
ORDER PART
NUMBER
LT3781EG
LT3781IG
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 = 25°C.
VCC = VBST = 12V, VBSTREF = 0V, VVC = 2V, VFB = VREF = 1.25V, CTG = CBG = CSG = 1000pF.
SYMBOL PARAMETER
CONDITIONS
MIN TYP MAX
Supply and Protection
VCCUVLO Undervoltage Lockout Threshold
Falling Edge
Rising Edge
q 8.0 8.4 8.6
q 13 14.5 16
VCCSHDN Shutdown Mode Shunt Regulator
ICC DC Active Supply Current
100µA < IVCC 10mA
(Note 2)
q 16.5 18 19.9
17 22
q 25
VBST
VSHDN
DC Active UVLO Supply Current
DC Standby Supply Current
DC Active Supply Current
DC Standby Supply Current
Shutdown Rising Threshold
Shutdown Threshold Hysteresis
VSHDN = 1.35V, VCC = 8V
VSHDN < 0.3V
TG Logic High (Note 2)
VSHDN < 0.3V
q 800 1200
q 16 30
q 5.0 8.5
0.1
q 1.15 1.25 1.35
100 150 200
ISS
VSS
VBSTUVLO
Soft-Start Charge Current
Soft-Start Reset Threshold
Boost Undervoltage Lockout
(VBST-BSTREF)
Boost UVLO Hysteresis
VSS = 2V
Falling Edge
Rising Edge
q –14 – 10 – 6
225
q 5.7 6.4 7.1
q 6.5 7.0 7.5
q 0.3 0.6
5V External Reference
V5VREF 5V Reference Voltage
0 (I5VREF – IVC) < 20mA
4.85 5.0 5.10
q 4.80
5.15
I5VREFSC Short-Circuit Current
R5VREF Output Impedance
Source, IVC = 0
0 (I5VREF – IVC) < 20mA
q 20 45
1
2
UNITS
V
V
V
mA
mA
µA
µA
mA
µA
V
mV
µA
mV
V
V
V
V
V
mA
3781f



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LT3781
ELECTRICAL CHARACTERISTICS
The q 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
Error Amp
VFB Error Amplifier Reference Voltage
Measured at Feedback Pin
1.242 1.250 1.258
q 1.225
1.265
IFB Feedback Input Current
AV Error Amplifier Voltage Gain
IVC Error Amplifier Current Limit
VFB = VREF
Source
Sink
– 50
72
q 10 25
q 0.5
1
VVC Zero Current Output Voltage
Maximum Output Voltage
1.4
3.2
GBW Gain Bandwidth Product
(Note 3)
1
Current Sense
AV
ISENSE
VSENSE
Amplifier DC Gain
Input Bias Current
Current Limit Threshold
Measured at SENSE Pin
12
– 275
135 150 165
q 130
170
tD Current Sense to Switch Delay
tMIN Switch Minimum On Time
THERM and OVLO Fault Detectors
Measured at BG Output
175
250
VTHERM/ Threshold (Rising Edge)
VOVLO Threshold Hysteresis
tD Fault Delay to Output Disable
Oscillator and Synchronization Decoder
>50mV Overdrive
q 1.2 1.25 1.3
q 20 40 60
650
fOSC
IFSET
VSYNC
Oscillator Frequency, Free Run
Frequency Programming Error
FSET Input Bias Current
SYNC Logic High Input Threshold
SYNC Logic Low Input Threshold
Measured at FSET Pin
fOSC 500kHz (Note 3)
FSET Charging, VFSET = 2V
Positive-Going Edge
Negative-Going Edge
700
q –10
5
50
q 1.4
q 0.8 1.4
2
fSYNC
tH, L
SYNC Frequency
Maximum SYNC Pulse Width
(Logic High or Logic Low)
fOSC = Oscillator Free-Run Frequency
q fOSC/2
350
1/fOSC
Output Drivers
VTG TG On Voltage
TG Off Voltage
q 11 11.5
q 0.1 0.5
tTGr/f TG Rise/Fall Time
VBG BG On Voltage
BG Off Voltage
10% to 90%/90% to 10%
35
q 11 11.5 12
q 0.1 0.5
tBGr/f BG Rise/Fall Time
VSG SG On Voltage
SG Off Voltage
10% to 90%/90% to 10%
35
q 11 11.5 12
q 0.1 0.5
tSGr/f
tSG-BG
tTG-BG
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
35
q 80 150 300
100
UNITS
V
V
nA
dB
mA
mA
V
V
MHz
V/V
µA
mV
mV
ns
ns
V
mV
ns
kHz
%
nA
V
V
kHz
s
V
V
ns
V
V
ns
V
V
ns
ns
ns
3781f
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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: VCC vs ICC
18.4
TA = 25°C
18.2
Shutdown Mode: VCC vs
Temperature (ICC = 1mA)
18.20
18.15
18.0 18.10
17.8 18.05
17.6
100µ
300µ
1m
ICC (A)
ICC Supply Current
vs Temperature
20
19
3m 10m
3781 • G01a
VCC = 12V
18
17
16
15
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G01
18.00
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 • G01b
ICC Supply Current
vs VCC Supply Voltage
18
TA = 25°C
17
16
15
9 10
12 14 16
SUPPLY VOLTAGE (V)
18
3781 G03
3781f
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