© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

SG6741 • Rev. 1.3.3

8

Functional Description

Startup Current

For startup, the HV pin is connected to the line input or

is recommended as 100KΩ. Typical startup current

drawn from pin HV is 2mA and it charges the hold-up

auxiliary winding of the main transformer to carry on

provide the operating current.

Operating Current

Operating current is around 4mA. The low operating

current enables better efficiency and reduces the

Green-Mode Operation

The patented green-mode function provides an off-time

modulation to reduce switching frequency in light-load

and no-load conditions. The on time is limited for better

from the voltage feedback loop, is used as the

switching frequency is continuously decreased to the

minimum

green-mode

frequency,

around

22KHz

Oscillator Operation

A resistor connected from the RI pin to the GND pin

generates a constant current source for the controller.

This current is used to determine the center PWM

frequency. Increasing the resistance reduces PWM

corresponding 65KHz PWM frequency. The relationship

(KHz)

)

(K

I

R

1690

PWM

f

Ω

=

(1)

The range of the PWM oscillation frequency is designed

as 47kHz ~ 109kHz.

Current Sensing / PWM Current Limiting

Peak-current-mode control is utilized in SG6741 to

regulate output voltage and provide pulse-by-pulse

current limiting. The switch current is detected by a

sense resistor into the SENSE pin. The PWM duty cycle

feedback voltage. When the voltage on the SENSE pin

to a variable voltage around 0.85V for output power

limit.

Leading-Edge Blanking (LEB)

Each time the power MOSFET is switched on, a turn-on

spike occurs on the sense-resistor. To avoid premature

termination of the switching pulse, a leading-edge

blanking time is built in. During this blanking period, the

current-limit comparator is disabled and cannot switch

off the gate driver.

Under-Voltage Lockout (UVLO)

The turn-on and turn-off thresholds are fixed internally

at 16.5V/10.5V. During startup, the hold-up capacitor

must be charged to 16.5V through the startup resistor

so that IC is enabled. The hold-up capacitor continues

drop below 10.5V during this startup process. This

UVLO hysteresis window ensures that hold-up capacitor

Gate Output / Soft Driving

The SG6741 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 18V Zener diode to protect power MOSFET

transistors against undesirable gate over-voltage. A soft

driving waveform is implemented to minimize EMI.

Built-in Slope Compensation

The sensed voltage across the current-sense resistor is

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

current limiting. Built-in slope compensation improves

stability and prevents sub-harmonic oscillation. SG6741

inserts a synchronized positive-going ramp at every

switching cycle.

Constant Output Power Limit

When the SENSE voltage, across the sense resistor

This delay introduces an additional current, proportional

larger additional current and the output power limit is

also higher than that under low input line voltage. To

compensate this variation for wide AC input range, a

sawtooth power-limiter is designed to solve the unequal

power-limit problem. The power limiter is designed as a

positive ramp signal and is fed to the inverting input of

the OCP comparator. This results in a lower current limit

at high-line inputs than at low-line inputs.