LT1952 datasheet(10/24 Pages) LINER

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LT1952

1952f

OPERATIO

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_VSEC pin must exceed 1.32V

and the VIN pin must exceed 14.25V to allow the part to

turn on. This combination of pin voltages allows the 2.5V

VREF pin to become active, supplying the LT1952 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 11µA just

before part turn on and 0µA after part turn on.

With the LT1952 turned on, the VIN pin can drop as low as

8.75V before part shutdown occurs. This VIN pin hyster-

esis (5.5V) combined with low 460µA start-up input

current allows low power start-up using a resistor/capaci-

tor network from system VIN to supply the VIN pin (Figure

3). The VIN capacitor value is chosen to prevent VIN falling

below 8.75V before an auxiliary winding in the converter

takes over supply to the VIN 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 tDELAY (Figure 2). tDELAY 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 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 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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