SG6742 데이터 시트보기 (PDF) - Fairchild Semiconductor

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SG6742

Highly Integrated Green-Mode PWM Controller

Fairchild Semiconductor 

Highly Integrated Green-Mode PWM Controller

Product Specification

SG6742

VDD Over-Voltage Protection

VDD over-voltage protection has been built in to prevent

damage due to abnormal conditions. Once the VDD

voltage is over the over-voltage protection voltage

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

disabled until the VDD voltage drops below the UVLO,

then start-up again. Over-voltage conditions are usually

caused by open feedback loops.

Thermal Protection

An NTC thermistor RNTC in series with a resistor RA can

be connected from pin RT to ground. A constant current

IRT is output from pin RT. The voltage on RT pin can be

expressed as VRT = IRT × (RNTC + Ra), in which IRT = 2 x

(1.3V / RI). At high ambient temperature, RNTC is smaller,

such that VRT decreases. 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 should be turned off

immediately after 100µs (tD-OTP2).

Limited Power Control

The FB voltage increases every time the output of the

power supply is shorted or over-loaded. 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 is totally shut down. VDD is charged up to

the turn-on threshold voltage of 16V through the start-up

resistor until PWM output is restarted. This protection

feature is activated as long as the over-loading condition

persists. This prevents the power supply from overheating

due to over loading conditions.

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 SG6742, and increasing the power

MOS gate resistance improves performance.

© System General Corp.

Version 1.0.1 (IAO33.0083.B0)

- 10 -

www.sg.com.tw • www.fairchildsemi.com

September 24, 2007

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