SC4905AMSTR datasheet(8/20 Pages) SEMTECH

8 / 20 page

SC4905A/B

POWER MANAGEMENT

PRELIMINARY

 2002 Semtech Corp.

www.semtech.com

VDD UNDER VOLTAGE LOCK OUT

According to the application, and the voltages available,

the SC4905A (UVLO = 4.5V), or the SC4905B (UVLO =

12.5V) can be used to provide the VDD undervoltage lock

out function to ensure the converters controlled start up.

Before the VDD UVLO has been reached, the internal ref-

erence, oscillator, OUT driver, and logic are disabled.

REFERENCE

A 4V (SC4905A) or a 5V(SC4905B) reference voltage is

available that can be used to source a typical current of

3mA to the external circuitry. The REF can be used to

provide the feed back circuitry with a regulated bias.

OSCILLATOR

The oscillator frequency is set by connecting a RC network

as shown below.

SC4905

DMAX

VFF

REF

VDD

SYNC

ILIM

GND

OUT

8.25k

280k

Rosc

499k

Cosc

220p, 16V

Vin

The oscillator has a ramp voltage that will track the volt-

age at the VFF pin (1.2V<VFF<3.6V). The oscillator peak

voltage is derived by charging the oscillator capacitor (Cosc)

to the VFF voltage via the oscillator resistor (Rosc). The

bias current to charge the Cosc is controlled by the Rosc.

Once the RC pin has reached the VFF voltage, the oscil-

lator ramp is discharged by an internal switch hence cre-

ating the triangle oscillator ramp.

Since the Rosc is referenced to the input supply volt-

age, any variation in the supply is directly translated into

a variation in the duty cycle, while maintaining the fixed

frequency operation.

Application Information (Cont.)

Following equation can be used to calculate the oscilla-

tor frequency:

()

VFF

Vin

OSC

•













−

≅

The recommended range if timing resistors is between 10

kohm and 500kohm and range of timing capacitors is

between 100pF and 1000pF. Timing resistors less than

10 kohm should be avoided.

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