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PDF LT3781 Data sheet ( Hoja de datos )
| Número de pieza | LT3781 | |
| Descripción | Bootstrap Start Dual Transistor Synchronous Forward Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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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
10Ω
0.25W
22µF
100V
1.5µF
100V
MURS120T3
1nF
8 10 1nF 100V
C3
1.5µF
100V
Q3
R9
0.025Ω
4
11 100V 10Ω
12 0.25W
1/2W
9
Q5
VIN–
ZVN3310F BAT54
Q6
L2
4.7µH
VOUT = 5V
IOUT = 7A
VOUT+
MBR- +
0540T1
C5
330µF
10V
4.7Ω
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
100Ω
5V
OUT
3.3k
0.047µF
3.01k
1%
51Ω
6.8k
1µF
150pF
4.7nF
0.01µF
1k
1%
1OV
BIAS
FZT690
100Ω
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
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT3781
IBST Boost Supply Current
vs Temperature
5.2
5.1
5.0
4.9
4.8
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G04
5VREF Voltage vs Temperature
5.10
ICC Supply Current
vs SHDN Pin Voltage
60
TA = 25°C
UVLO ICC Supply Current
vs Temperature
1
40
0.8
20
0
0 0.2 0.4 0.6 0.8 1.0 1.2
SHDN PIN VOLTAGE (V)
3781 G05
5VREF Short-Circuit Current Limit
vs Temperature
60
0.6
0.5
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G06
Error Amp Reference
vs Temperature
1.260
5.05 1.255
50
5.00 1.250
40
4.95 1.245
4.90
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G07
VC Pin Short-Circuit Current Limit
vs Temperature
25
20
15
10
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G10
30
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G08
Soft-Start Output Current
vs Temperature
12
VSS = 2V
11
10
9
1.240
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G09
Soft-Start Output Current
vs Soft-Start Pin Voltage
60
TA = 25°C
40
20
8
–55 –40
0 40 80
TEMPERATURE (°C)
125
3781 G11
0
0 100 200 300 400 500
SOFT-START PIN VOLTAGE (mV)
3781 G12
3781f
5
5 Page 
LT3781
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 1.25V fault detector threshold on
either the OVLO or THERM pins
OVLO and THERM pins is used to directly trigger a GFC. If
either of these pins are not used, they can be disabled by
connecting the pin to ground. The intention of the OVLO
pin is to allow the monitoring of the input supply to protect
from an overvoltage condition though the use of a resistor
divider from the input supply. Monitoring of system tem-
perature (THERM) is possible through use of a resistor
divider using a thermistor as a 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 LT3781 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 LT3781 disables all internal functions with the
exception of the shutdown and UVLO circuitry. The exter-
nal 5VREF supply is also disabled during this condition.
Disabling of all switching control circuity reduces the
LT3781 supply current to <1mA, making for efficient
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
causing excessive power dissipation due to inadequate
gate drive during start-up.
Error Amplifer Configurations
The converter output voltage information is fed back to the
LT3781 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 LT3781 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 ( Figure␣ 1).
Placing a capacitor CERR from the VFB pin to the VC pin will
set the single-pole crossover frequency at (2πRFBCERR)-1.
Additional poles and zeros can be added by increasing the
complexity of the RC network.
VOUT
RFB
VFB
9
CERR
VC
10
LT3781
1.25V
3781 F01
Figure 1. Nonisolated Error Amp Configuration
3781f
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT3781.PDF ] | |
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