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PDF LT1952 Data sheet ( Hoja de datos )
| Número de pieza | LT1952 | |
| Descripción | Single Switch Synchronous Forward Controller | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT1952 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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FEATURES
■ Synchronous Rectifier Control for High Efficiency
■ Programmable Volt-Second Clamp
■ Output Power Levels from 25W to 500W
■ Low Current Start-Up
■ True PWM Soft-Start
■ Low Stress Short Circuit Protection
■ Precision 100mV Current Limit Threshold
■ Adjustable Delay for Synchronous Timing
■ Accurate Shutdown Threshold with Programmable
Hysteresis
■ Programmable Slope Compensation
■ Programmable Leading Edge Blanking
■ Programmable Frequency (100kHz to 500kHz)
■ Synchronizable to an External Clock up to 1.5 • fOSC
■ Internal 1.23V Reference
■ 2.5V External Reference
■ Current Mode Control
■ Small 16-Pin SSOP Package
U
APPLICATIO S
■ Telecommunications Power Supplies
■ Industrial and Distributed Power
■ Isolated and Non-Isolated DC/DC Converters
LT1952www.DataSheet4U.com
Single Switch Synchronous
Forward Controller
DESCRIPTIO
The LT®1952 is a current mode PWM controller optimized
to control the forward converter topology, using one
primary MOSFET. The LT1952 provides synchronous
rectifier control, resulting in extremely high efficiency. A
programmable Volt-Second clamp provides a safeguard
for transformer reset that prevents saturation. This allows
a single MOSFET on the primary side to reliably run at
greater than 50% duty cycle for high MOSFET, trans-
former and rectifier utilization. The LT1952 includes
soft-start for controlled exit from shutdown, overcurrent
conditions and undervoltage lockout. A precision 100mV
current limit threshold, independent of duty cycle, com-
bines with soft-start to provide hiccup short circuit protec-
tion. Micropower start-up allows the LT1952 to be effi-
ciently started from high input voltages. Programmable
slope compensation and leading edge blanking allow
optimization of loop bandwidth with a wide range of
inductors and MOSFETs. The LT1952 can be programmed
over a 100kHz to 500kHz frequency range and the part can
be synchronized to an external clock. The error amplifier
is a true op amp, allowing a wide range of compensation
networks. The LT1952 is available in a small 16-pin SSOP
package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
TYPICAL APPLICATIO
36V to 72V Input, 12V at 20A Semi-Regulated Bus Converter
SUPPLY FROM BIAS
WINDING OF T1
VIN T1
40k PA0905
16V 10µF
VREF
VIN
52.3k
COMP
VIN
340k
SS_MAXDC
OUT
LT1952 OC
ISENSE
Si7450
0.005Ω
Si7370
×2
100k
SD_VSEC
LTC3900
13k FB
SYNC
SOUT
FG CG
T2
SYNC
GND
ROSC
220pF
560Ω
L1
PA1494.242
VOUT
12V
20A
PH4840
×2
47µF
16V
X5R
×2
0.1µF 0.1µF
PGND BLANK DELAY
40k 40k
178k
1952 TA01
12V Bus Converter
VOUT vs VIN
16
14
12
10
8
36 42 48 54 60 66 72
VIN (V)
1952 TA01b
1952f
1
1 page  
TYPICAL PERFOR A CE CHARACTERISTICS
LT1952www.DataSheet4U.com
FB Voltage vs Temperature
1.25
1.24
1.23
1.22
1.21
1.20
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G01
VIN Start-up Current vs
Temperature
600
SD_VSEC = 1.4V
550
500
450
400
350
300
250
200
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G04
SD_VSEC Pin Current vs
Temperature
15
PIN CURRENT BEFORE
PART TURN ON
10
5
0
–50 –25
0µA PIN CURRENT AFTER
PART TURN ON
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G07
Switching Frequency vs
Temperature
245
230
215
200
185
170
155
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G02
VIN IQ vs Temperature
6.5
OC = OPEN
6.0
5.5
5.0
4.5
4.0
3.5
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G05
VIN Turn ON/OFF Voltage vs
Temperature
18
16
VIN TURN ON VOLTAGE
14
12
10
VIN TURN OFF VOLTAGE
8
6
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G08
VIN Shutdown Current vs
Temperature
500
VIN = 15V
450 SD_VSEC = 0V
400
350
300
250
200
150
100
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G03
SD_VSEC Turn ON Threshold vs
Temperature
1.42
1.37
1.32
1.27
1.22
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G06
COMP Active Threshold vs
Temperature
1.6
RISENSE = 0k
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
–50 –25
0 25 50 75
TEMPERATURE (°C)
100 125
1952 G09
1952f
5
5 Page 
U
OPERATIO
by limiting the operational duty cycle of the MOSFET to
50% or less — or by using a fixed (non-adaptive) maxi-
mum duty cycle clamp with very large voltage rated
MOSFETs. The LT1952 provides a volt-second clamp to
allow MOSFET duty cycles well above 50%. This gives
greater power utilization for the MOSFET, rectifiers and
transformer resulting in less space for a given power
output. In addition, the volt-second clamp allows a re-
duced voltage rating on the MOSFET resulting in lower
RDSON for greater efficiency. The volt-second clamp de-
fines a maximum duty cycle ‘guard rail’ which falls when
system input voltage increases.
The LT1952 SD_VSEC and SS_MAXDC pins provide a
capacitorless, programmable volt-second clamp solution.
Some controllers with volt-second clamps control switch
maximum duty cycle by using an external capacitor to
program maximum switch ON time. Such techniques have
a volt-second clamp inaccuracy directly related to the
error of the external capacitor/pin capacitance and the
error/drift of the internal oscillator. The LT1952 uses
simple resistor ratios to implement a volt-second clamp
without the need for an accurate external capacitor and
with an order of magnitude less dependency on oscillator
error.
An increase of voltage at the SD_VSEC pin causes the
maximum duty cycle clamp to decrease. If SD_VSEC is
resistively divided down from transformer input voltage, a
volt-second clamp is realised. To adjust the initial maxi-
mum duty cycle clamp, the SS_MAXDC pin voltage is
programmed by a resistor divider from the 2.5V VREF pin
to ground. An increase of programmed voltage on
SS_MAXDC pin provides an increase of switch maximum
duty cycle clamp.
Soft-Start
The LT1952 provides true PWM soft-start by using the
SS_MAXDC pin to control soft-start timing. The propor-
tional relationship between SS_MAXDC voltage and switch
maximum duty cycle clamp allows the SS_MAXDC pin to
slowly ramp output voltage by ramping the maximum
switch duty cycle clamp — until switch duty cycle clamp
seamlessly meets the natural duty cycle of the converter.
LT1952www.DataSheet4U.com
A soft-start event is triggered whenever VIN is too low,
SD_VSEC is too low (UVLO), or a 100mV over-current
threshold at OC pin is exceeded. Whenever a soft-start
event is triggered, switching at SOUT and OUT is stopped
immediately.
The SS_MAXDC pin is discharged and only released for
charging when it has fallen below it’s reset threshold of
0.45V and all faults have been removed. Increasing volt-
age on the SS_MAXDC pin above 0.8V will increase switch
maximum duty cycle. A capacitor to ground on the
SS_MAXDC pin in combination with a resistor divider
from VREF, defines the soft-start timing.
Current Mode Topology (ISENSE Pin)
The LT1952 current mode topology eases frequency com-
pensation requirements because the output inductor does
not contribute to phase delay in the regulator loop. This
current mode technique means that the error amplifier
(nonisolated applications) or the optocoupler (isolated
applications) commands current (rather than voltage) to
be delivered to the output. This makes frequency compen-
sation easier and provides faster loop response to output
load transients.
A resistor divider from the application’s output voltage
generates a voltage at the inverting FB input of the LT1952
error amplifier (or to the input of an external optocoupler)
and is compared to an accurate reference (1.23V for
LT1952). The error amplifier output (COMP) defines the
input threshold (ISENSE) of the current sense comparator.
COMP voltages between 0.8V (active threshold) and 2.5V
define a maximum ISENSE threshold from 0mV to 220mV.
By connecting ISENSE to a sense resistor in series with the
source of an external power MOSFET, the MOSFET peak
current trip point (turn off) can be controlled by COMP
level and hence by the output voltage. An increase in
output load current causing the output voltage to fall, will
cause COMP to rise, increasing ISENSE threshold, increas-
ing the current delivered to the output. For isolated appli-
cations, the error amplifier COMP output can be disabled
to allow the optocoupler to take control. Setting FB = VREF
disables the error amplifier COMP output, reducing pin
current to (COMP – 0.7)/40k.
1952f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1952.PDF ] | |
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