© 2008 Fairchild Semiconductor Corporation

www.fairchildsemi.com

SG6742HL/HR • Rev. 1.0.4

9

Functional Description

Startup Current

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

bulk capacitor through an external diode and resistor,

current drawn from pin HV is 2.3mA and charges the

hold-up capacitor through the diode and resistor. When

the auxiliary winding of the main transformer to provide

the operating current.

Operating Current

Operating current is around 2.7mA. The low operating

current enables better efficiency and reduces the

Green-Mode Operation

The proprietary green-mode function provides an off-

time modulation to reduce the switching frequency in

the light-load and no-load conditions. The on time is

which is derived from the voltage feedback loop, is

threshold voltage, switching frequency is continuously

decreased to the minimum green-mode frequency of

around 22KHz.

Current Sensing / PWM Current Limiting

Peak-current-mode control is utilized 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 is determined by this

When the voltage on SENSE pin reaches around

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 15.5V and 9.5V. During startup, the hold-up capacitor

must be charged to 15.5V through the startup resistor to

enable the IC. The hold-up capacitor continues to

drop below 9.5V during this startup process. This UVLO

hysteresis window ensures that hold-up capacitor is

Gate Output / Soft Driving

The 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.

Soft-Start

For many applications, it is necessary to minimize the

inrush current at startup. The built-in 6ms soft-start

circuit significantly reduces the startup current spike

and output voltage overshoot.

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.

SG6742HL/HR 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

results in a larger additional current and the output

power limit is higher than 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 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.

again. Over-voltage conditions are usually caused by

open feedback loops.