SG6840 Просмотр технического описания (PDF) - Unspecified

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SG6840

High-integrated Green Mode PWM Controller

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SG6840

Gate Output / Soft Driving

The SG6840 BiCMOS output stage is a fast totem pole gate

driver, which is designed to avoid cross conduction current.

This minimizes heat dissipation, increases efficiency and

enhances reliability. The output driver is clamped by an

internal 18V Zener diode in order to improve the control of the

power MOSFET transistors and protect them against unde-

sirable gate over-voltage. By controlling the rising time of the

switch-on waveform and falling shape of the switch-off wave-

form, the output stage is optimized to reduce switching noise,

improve EMI, and to provide a stable MOSFET gate drive.

Built-in Slope Compensation

Current mode control regulates the peak transformer/inductor

current via the current control loop. In a continuous mode

operation, the current is the average current, and composed

of both AC and DC components. Since the output is propor-

tional to the average, not the peak current, this causes oscil-

lation when input voltage is changed. Adding the slope com-

pensation to the current loop (reduce the current loop gain) to

correct the problem is a simple approach. The SG6840 in-

serts a synchronized 0.33V positive-going ramp at every

switching cycle to stabilize the current loop. Vs-comp = 0.33V.

Product Specification

Noise immunity

Noise on the current sense or control signal can cause sig-

nificant pulse width jitter, particularly with the continuous-

mode operation. While slope compensation helps alleviate

this problem. Note that the SG6840 has a single ground pin.

High sink current in the output therefore cannot be returned

separately. Good high frequency or RF layout practices

should be followed. Avoid long PCB traces and component

leads. Locate components such as Ri, Rt and VDD capacitor

near to the SG6840. The noise, which often causes the

problem, is caused by the output (pin 8) being pulled below

ground at turn-off by external parasitic. This is particularly true

when driving MOSFET. A resistor (10 ~ 20 ohms) series

connected from the output (pin 8) to the gate of MOSFET will

prevent such output noise.

Constant Output Power Limit

Every time when the SENSE voltage, across the sense re-

sistor Rs, is larger than the threshold voltage around 0.85V,

the output GATE drive is turned off after a small propagation

delay tD. Since the propagation delay is constant regardless

the input line voltage VIN, the output power would not be equal

for the wide input voltage VIN of 90Vrms to 265Vrms. To

compensate the different output power limit between high line

voltage and low line voltage, the internal threshold voltage is

adjusted dependent on the input line voltage VIN through the

VIN pin. The threshold voltage is decreased from 0.85V to a

smaller voltage when input line voltage VIN increases. Smaller

threshold voltage, at higher input line voltage, forces the

output GATE drive to terminate earlier, thus reduce the total

PWM turn-on time and make the output power equal to that of

low input line voltage.

Thermal Protection

A constant current IRT is output from pin RT. The resistor in pin

Ri decides the current IRT.

IRT = 2 x (1.3V / Ri);

An NTC thermistor Rntc in series with a resistor Ra can be

connected from pin RT to ground. The over-temperature

2002 System General Corporation

-8-

Ver1.0

http://www.sg.com.tw

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