SC4905AIMSTR datasheet(11/24 Pages) SEMTECH

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SC4905A/B

POWER MANAGEMENT

 2006 Semtech Corp.

www.semtech.com

Application Information (Cont.)

START UP SEQUENCE

Initially during the power up, the SC4905 is in the under

voltage lock out condition. As the VDD supply exceeds

the UVLO limit of the SC4905 and the VFF pin exceeds

the line under voltage lock out of about 1.2V, the inter-

nal reference, oscillator, and logic circuitry are powered

up.

The OUT driver is not enabled until the line under voltage

lock out limit is reached. At that point, once the FB pin

has reached above 600mV, the output driver is enabled.

As the output voltage starts to increase, the error signal

from the error amplifier starts to decrease. If isolation is

required, the error amplifier output can drive the LED of

the opto isolator. The output of the opto is connected in

a common emitter configuration with a pull up resistor to

a reference voltage connected to the FB pin of the

SC4905. The voltage level at the FB pin provides the duty

cycle necessary to achieve regulation.

SC4905AIMSTR Datasheet(PDF) 11 Page - Semtech Corporation