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SG6859

Low Cost Green-Mode PWM Controller for Flyback Converters

Fairchild Semiconductor 

Low Cost Green-Mode PWM Controller for Flyback Converters

Gate Output

The SG6859 BiCMOS output stage is a fast totem

pole gate driver. Cross conduction has been avoided to

minimize heat dissipation, increase efficiency, and

enhance reliability. The output driver is clamped by an

internal 17V Zener diode in order to protect power

MOSFET transistors against undesired over-voltage gate

signals.

Built-in Slope Compensation

The sensed voltage across the current sense resistor

is used for current mode control and pulse-by-pulse

current limiting. Built-in slope compensation will

improve stability and prevent sub-harmonic oscillations

due to peak-current mode control. The SG6859 has a

synchronized, positively-sloped ramp built-in at each

switching cycle. The slope of the ramp is:

0.36 × Duty

Duty (max .)

Noise Immunity

Noise from the current sense or the control signal can

cause significant pulse width jitter, particularly in

continuous-conduction mode. While slope compensation

helps alleviate these problems, further precautions should

still be taken. Good placement and layout practices should

be followed. Avoiding long PCB traces and component

leads, locating compensation and filter components near

the SG6859, and increasing the power MOS gate

resistance is advised.

Product Specification

SG6859

© System General Corp.

Version 1.2.1(IAO33.0067.B2)

-9-

www.sg.com.tw • www.fairchildsemi.com

September 28, 2007

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