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PDF LT1681 Data sheet ( Hoja de datos )
| Número de pieza | LT1681 | |
| Descripción | Dual Transistor Synchronous Forward Controller | |
| Fabricantes | Linear | |
| Logotipo |  |
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LT1681
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 Fixed Frequency Operation to 350kHz
s Adaptive and Adjustable Blanking
s Synchronous Rectifier Driver
s Local 1% Voltage Reference
s Undervoltage Lockout Protection with Hysteresis
s Input Overvoltage Protection
s Programmable Start Inhibit
s Transformer Primary Saturation Protection
s Optocoupler Feedback Support
s Soft-Start Control
U
APPLICATIO S
s Isolated Telecommunication Systems
s Personal Computers and Peripherals
s Lead Acid Battery Backup Systems
s Automotive and Heavy Equipment
DESCRIPTIO
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.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
36V-72V DC to 5V/7A Synchronous Forward Converter (Half-Brick Footprint)
L1
4.7µH
VIN+
+ C2
22µF
100V
C4
1.5µF
100V
C3
1.5µF
100V
MURS120T3
Q1
MURS120T3
Q3 0.025Ω
1/2W
6
1
7 2 10Ω
3 0.25W
5
1nF
8 10 1nF 100V
4
11 100V 10Ω
12 0.25W
9
T1 Q5
Q6
VIN–
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
73.2k
270k 1%
0.25W
20k
CMPZ5248B
18V
CMPZ-
5248B
15V
0.1µF
68µF +
20V
10k 56k
1.24k
1nF 24k 1%
ZVN3310F BAT54
1OV
BIAS
MMBD914LT1
0.1µF
100V
10k
330pF
BAS21
20 17 19 18 16 11 12 15
14
2
VCC
VBST
BLKSENS
TG
BSTREF
BG
SENSE
TMAX
PGND
SG
13
OVLO
LT1681
1 SHDN 5VREF FSET THERM SYNC SGND SS VC VFB 9
56
3 7 4 8 10
Q10 52.3k
100Ω
5V
OUT
3.3Ω
0.047µF
3.01k 51Ω
1%
1µF
150pF
4.7nF
0.01µF
1k
1%
1OV
BIAS FZT690
100Ω
4.7µF
16V
CMPZ5242B
12V
8 LTC1693-2 6
VCC1
VCC2
35
IN2 OUT2
1
IN1
7
OUT1
4
GND2
2
GND1
2k
0.22µF
50V
L2
4.1µH
VOUT = 5V
IOUT = 7A
VOUT+
MBR- +
0540T1
C5
330µF
10V
4.7Ω
VOUT–
1681 TA01
1681f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1681
IBST Boost Supply Current
vs Temperature
5.2
5.1
5.0
4.9
4.8
–55 –40
0 40 80
TEMPERATURE (°C)
125
1681 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 CURRENT (V)
1681 G05
5VREF Short-Circuit Current Limit
vs Temperature
60
0.6
0.5
–55 –40
0 40 80
TEMPERATURE (°C)
125
1681 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
1681 G07
VC Pin Short-Circuit Current Limit
vs Temperature
25
20
15
10
–55 –40
0 40 80
TEMPERATURE (°C)
125
1681 G10
30
–55 –40
0 40 80
TEMPERATURE (°C)
125
1681 G08
Soft-Start Output Current
vs Temperature
12
VSS = 2V
11
10
9
1.240
–55 –40
0 40 80
TEMPERATURE (°C)
125
1681 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
1681 G11
0
0 100 200 300 400 500
SOFT-START PIN VOLTAGE (mV)
1681 G12
1681f
5
5 Page 
LT1681
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 IMAX pin threshold
f) Exceeding the 1.25V fault detector threshold on either
the OVLO or THERM pins
The OVLO and THERM pins are used to directly trigger a
GFC. If either of these pins are not used, they can be
disabled by connecting the pin to SGND. The intention of
the OLVO pin is to allow monitoring of the input supply to
protect from an overvoltage condition. Monitoring of
system temperature (THERM) is possible through use of
a resistor divider using a thermistor as a resistor 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 exces-
sive pin input currents, low impedance pull-up devices
must not be used on these pins.
Undervoltage Lockout
The LT1681 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 LT1681 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
LT1681 supply current to < 1mA, simplifying 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 Amplifier Configurations
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. The
error amplifier is generally configured as an integrator and
is used to create the dominant pole for the main converter
feedback loop. The LT1681 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 (see Fig-
ure␣ 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
LT1681
1.25V
1681 F01
Figure 1. Nonisolated Error Amp Configuration
Another common error amplifier configuration is for
optocoupler use in fully isolated converters with second-
ary-side control (see Figure 2). In such a system, the
dominant pole for the feedback loop is created at the sec-
ondary-side controller, so the error amplifier needs only to
1681f
11
11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT1681.PDF ] | |
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