SG6741 Ver la hoja de datos (PDF) - Fairchild Semiconductor

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SG6741

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

Functional Description (Continued)

VDD Over-Voltage Protection (OVP)

VDD over-voltage protection has been built in to prevent

damage due to abnormal conditions. Once the VDD

voltage is over the VDD over-voltage protection voltage

(VDD-OVP), and lasts for tD-VDDOVP, the PWM pulses is

disabled until the VDD voltage drops below the UVLO,

then starts up again. Over-voltage conditions are

usually caused by open feedback loops.

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, the supply voltage VDD begins decreasing.

When VDD goes below the turn-off threshold (eg, 10.5V)

the controller totally shuts down. VDD is charged up to

the turn-on threshold voltage of 16V through the startup

resistor until PWM output is restarted. This protection is

activated as long as the overloading condition persists.

This prevents the power supply from overheating.

Noise Immunity

Noise on the current sense or control signal may cause

significant pulse width jitter, particularly in the

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 to the SG6741, and

increasing the power MO gate resistance improves

performance.

© 2008 Fairchild Semiconductor Corporation

SG6741 • Rev. 1.3.3

9

www.fairchildsemi.com

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