6742MLTPM Просмотр технического описания (PDF) - Fairchild Semiconductor

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6742MLTPM

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

Functional Description (Continued)

Limited Power Control

The FB voltage increases every time the output of the

power supply is shorted or overloaded. If the FB voltage

remains higher than a built-in threshold for longer than

tD-OLP, PWM output is turned off. As PWM output is

turned off, VDD begins decreasing.

When VDD goes below the turn-off threshold (~9.5V) the

controller is totally shut down. VDD is charged up to the

turn-on threshold voltage of 15.5V through the startup

resistor until PWM output is restarted. This protection

feature continues as long as the overloading condition

persists. This prevents the power supply from

overheating due to overloading conditions. When VRT is

less than 1.05V (VRTTH1), the PWM is turned off after

12ms (tD-OTP1). If VRT is less than 0.7V (VRTTH2), PWM is

turned off after 100µs (tD-OTP2).

Noise Immunity

Noise on the current sense or control signal may cause

significant pulse-width jitter, particularly in continuous-

conduction mode. Slope compensation helps alleviate

this problem. Good placement and layout practices

should be followed. Avoiding long PCB traces and

component leads, locating compensation and filter

components near the SG6742ML/MR, and increasing

the power MOS gate resistance improve performance.

© 2008 Fairchild Semiconductor Corporation

SG6742ML/MR • Rev. 1.0.4

10

www.fairchildsemi.com

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