SDC608 datasheet(8/11 Pages) SDC

8 / 11 page

Datasheet

December, 2013 Rev. 1.2

8/11

www.sdc-semi.com

Current Mode PWM Controller

SDC608

Function Description

Startup control

During Startup phase, reference voltage, the oscillator

and all protection circuits are OFF. Startup current of

SDC608 is designed to be very low so that VCC could be

charged up above UVLO threshold level and device starts

up quickly. A large startup resistor can therefore be used

to minimize the power loss yet achieve a reliable startup

in application.

PWM control

The peak current (sensed on the IS pin) is set by the

voltage on FB pin. By comparing the voltage on FB pin and

the IS ramp voltage, the duty-cycle of the PWM

modulator is thus adjusted to provide the necessary load

current at the desired output voltage. FB can be

controlled by internal control circuit and external

feedback circuit.

VCC over voltage protection

VCC over voltage protection circuit is integrated into IC.

When VCC voltage reaches 9.8V(TYP), FB voltage is pulled

down via internal control circuit, then the PWM switching

is shut off. When VCC voltage goes down below 9.8V(TYP),

the switching is reactivated. The VCC over voltage

protection ensures IC to operate reliably.

Current limit

The output is shut off to limit the power when voltage of

IS Pin exceeds Current sense threshold voltage.

Green mode control

Under no-load and light-load condition, the switching

frequency internally decreases to lower the switching

power loss and improve the conversion efficiency. If FB is

less than 2.7V(Typ), the cycle of the oscillator will increase

with it, the less FB is, the wider the cycle of the oscillator

is, until the oscillation stop.

Power transistor drive

During the ON cycle, OB pin supplies base current for the

power transistor, OE pulls down the emitter of the power

transistor to IS, and OB is adaptive to the IS current. If the

current of IS exceeds the specified current of FB, SDC608

will turn into the OFF cycle. During the OFF cycle, OB is

pulled down, the power transistor will shut off.

Over temperature protection

When IC’s internal temperature reaches 150°C, FB

voltage will be pulled down by internal control circuit,

the switching frequency decreases or shut off. This

protection protects the IC from over temperature.

SDC

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