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參數(shù)資料
| 型號： | LT1952 |
| 廠商： | Linear Technology Corporation |
| 英文描述： | Single Switch Synchronous Forward Controller |
| 中文描述： | 單開關(guān)同步前進(jìn)控制器 |
| 文件頁數(shù)： | 10/24頁 |
| 文件大小： | 252K |
| 代理商： | LT1952 |
LT1952
10
1952f
OPERATIOU
Introduction
The LT1952 is a current mode synchronous PWM control-
ler optimized for control of the simplest forward converter
topology — using only one primary MOSFET. The LT1952
is 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
include an adaptive maximum duty cycle clamp for the
single primary MOSFET. An additional output signal is
included for synchronous rectifier control. A precision
100mV threshold senses over-current conditions and
triggers Soft-Start for low stress short circuit protection
and control. The key functions of the LT1952 are shown in
the Block Diagram in Figure 2.
Part Startup
In normal operation the SD_V
SEC
pin must exceed 1.32V
and the V
IN
pin must exceed 14.25V to allow the part to
turn on. This combination of pin voltages allows the 2.5V
V
REF
pin to become active, supplying the LT1952 control
circuitry and providing up to 2.5mA external drive. SD_V
SEC
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_V
SEC
pin draws 11
μ
A just
before part turn on and 0
μ
A after part turn on.
With the LT1952 turned on, the V
IN
pin can drop as low as
8.75V before part shutdown occurs. This V
IN
pin hyster-
esis (5.5V) combined with low 460
μ
A start-up input
current allows low power start-up using a resistor/capaci-
tor network from system V
IN
to supply the V
IN
pin (Figure
3). The V
IN
capacitor value is chosen to prevent V
IN
falling
below 8.75V before an auxiliary winding in the converter
takes over supply to the V
IN
pin.
Output Drivers
The LT1952 has 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 synchronous 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 t
DELAY
(Figure 2). t
DELAY
is
programmed 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 I
SENSE
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 V
IN
, low
SD_V
SEC
or over-current detection at the OC pin — a
soft-start event is latched and both SOUT and OUT turn off
immediately (Figure 1).
Leading Edge Blanking
To prevent MOSFET switching noise causing premature
turn off of SOUT or OUT, programmable leading edge
blanking exists. This means both the current sense com-
parator and over-current 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
voltage is necessary for reliable control of the MOSFET.
This volt-second clamp provides a safeguard for trans-
former reset that prevents transformer saturation. Instan-
taneous 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
exceed the voltage rating of the primary-side MOSFET with
catastrophic damage. Many converters solve this problem
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相關(guān)代理商/技術(shù)參數(shù) |
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| LT1952-1 | 制造商:LINER 制造商全稱:Linear Technology 功能描述:Single Switch Synchronous Forward Controller |
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| LT1952EGN#PBF | 功能描述:IC REG CTRLR ISO PWM CM 16-SSOP RoHS:是 類別:集成電路 (IC) >> PMIC - 穩(wěn)壓器 - DC DC 切換控制器 系列:- 標(biāo)準(zhǔn)包裝:2,500 系列:- PWM 型:電流模式 輸出數(shù):1 頻率 - 最大:500kHz 占空比:96% 電源電壓:4 V ~ 36 V 降壓:無 升壓:是 回掃:無 反相:無 倍增器:無 除法器:無 Cuk:無 隔離:無 工作溫度:-40°C ~ 125°C 封裝/外殼:24-WQFN 裸露焊盤 包裝:帶卷 (TR) |
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