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参数资料
| 型号： | LT1952MPGN#PBF |
| 厂商： | LINEAR TECHNOLOGY CORP |
| 元件分类： | 稳压器 |
| 英文描述： | SWITCHING CONTROLLER, 560 kHz SWITCHING FREQ-MAX, PDSO16 |
| 封装： | 0.150 INCH, LEAD FREE, PLASTIC, SSOP-16 |
| 文件页数： | 3/28页 |
| 文件大小： | 357K |
| 代理商： | LT1952MPGN#PBF |
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LT1952/LT1952-1
11
19521fe
OPERATION
Introduction
The LT1952/LT1952-1 are current mode synchronous
PWM controllers optimized for control of the simplest
forward converter topology—using only one primary
MOSFET. The LT1952/LT1952-1 are ideal for 25W to 500W
power systems where very high efficiency and reliability,
low complexity and cost are required in a small space.
Key features of the LT1952/LT1952-1 include an adaptive
maximumdutycycleclampforthesingleprimaryMOSFET.
An additional output signal is included for synchronous
rectifier control. A precision 107mV threshold senses
overcurrent conditions and triggers Soft-Start for low
stress short-circuit protection and control. The key
functions of the LT1952/LT1952-1 are shown in the Block
Diagram in Figure 2.
Part Start-up
In normal operation the SD_VSEC pin must exceed 1.32V
and the VIN pin must exceed 14.25V (7.75V LT1952-1) to
allow the part to turn on. This combination of pin voltages
allows the 2.5V VREF pin to become active, supplying the
LT1952/LT1952-1 control circuitry and providing up to
2.5mA external drive. SD_VSEC threshold can be used for
externally programming an undervoltage lockout (UVLO)
threshold on the system input voltage. Hysteresis on
the UVLO threshold can also be programmed since the
SD_VSEC pin draws 11A just before part turn on and 0A
after part turn on.
With the LT1952/LT1952-1 turned on, the VIN pin can drop
as low as 8.75V (6.5V LT1952-1) before part shutdown
occurs. This VIN pin hysteresis (5.5V LT1952; 1.25V
LT1952-1) combined with low 460A (400A LT1952-1)
start-up input current allows low power start-up using
a resistor/capacitor network from system VIN to supply
the VIN pin (Figure 3). The VIN capacitor value is chosen
to prevent VIN falling below its turn off threshold before
an auxiliary winding in the converter takes over supply
to the VIN pin.
Output Drivers
The LT1952/LT1952-1 have two outputs, SOUT and OUT.
The OUT pin provides a
±1A peak MOSFET gate drive
clamped to 13V. The SOUT pin has a
±50mA peak drive
clamped to 12V and provides sync signal timing for syn-
chronous rectification control.
For SOUT and OUT turn on, a PWM latch is set at the start
of each main oscillator cycle. OUT turn on is delayed from
SOUT turn on by a time tDELAY (Figure 2). tDELAY is pro-
grammed using a resistor from the DELAY pin to ground
and is used to set the timing control of the secondary
synchronous rectifiers for optimum efficiency.
SOUT and OUT turn off at the same time each cycle by
one of three methods:
(1) MOSFET peak current sense at ISENSE pin
(2) Adaptive maximum duty cycle clamp reached during
load/line transients
(3) Maximum duty cycle reset of the PWM latch
During any of the following conditions—low VIN, low
SD_VSEC or overcurrent detection at the OC pin—a soft-
start event is latched and both SOUT and OUT turn off
immediately (Figure 1).
Leading Edge Blanking
TopreventMOSFETswitchingnoisecausingprematureturn
off of SOUT or OUT, programmable leading edge blanking
exists. This means both the current sense comparator
and overcurrent comparator outputs are ignored during
MOSFET turn on and for an extended period after the OUT
leading edge (Figure 6). The extended blanking period is
programmable by adjusting a resistor from the BLANK
pin to ground.
Adaptive Maximum Duty Cycle Clamp
(Volt-Second Clamp)
For forward converter applications using the simplest
topology of a single MOSFET on the primary, a maximum
switch duty cycle clamp which adapts to transformer input
voltageisnecessaryforreliablecontroloftheMOSFET.This
volt-second clamp provides a safeguard for transformer
reset that prevents transformer saturation. Instantaneous
load changes can cause the converter loop to demand
maximum duty cycle. If the maximum duty cycle of the
switch is too great, the transformer reset voltage can
exceedthevoltageratingoftheprimary-sideMOSFETwith
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