SYNCHRONOUS PWM CONTROLLER WITH

DUAL LOW DROPOUT REGULATOR

CONTROLLERS

© 2000 SEMTECH CORP.

PRELIMINARY - October 16, 2000

SC1109

12

TEL:805-498-2111 FAX:805-498-3804 WEB:http://www.semtech.com

DUAL LDO CONTROLLERS

SC1109 also provides two low drop out linear regula-

tor controllers that can be used to generate a 1.8V

(LDO2) and 2.5V (LDO1) outputs. The LDO output

voltage is achieved by controlling the voltage drop

across an external Mosfet from a 3.3V supply voltage.

The output voltage is sensed at the LDOS pin of the

SC1109 and compared to an internal reference. The

gate drive to the external Mosfet is then adjusted until

regulation is achieved. In order to have sufficient

voltage to the gate drives of the external Mosfet, an

internal charge pump is utilized to boost the gate

drive voltage to about two times the VSTBY.

The internal charge pump charges an external Bucket

capacitor to VSTBY and then connects it in series

with VSTBY to the LDOs supply at a frequency of

about 200kHz. This ensures sufficient gate drive

voltage for the LDOs independent of the VCC or the

12V external supply being available due to start up

timing sequence from the silver box.

The LDO1, and LDO2 output voltages are forced to

track the 3.3V input supply. This feature ensures that

during the start up application of the 3.3V, the 1.8V,

and 2.5V outputs track the 3.3V within 200mV typical

until regulation is achieved. However, the VSTBY

should be established at least 500us, to allow the

charge pump to reach its maximum voltage, before

the linear section will track within 200mV. This track-

ing will sequence the correct start up timing for the

external Chipset and Clock circuitry.

3.3V Vin

2.5V Vout

1.8V Vout