SC1109 Datasheet, PDF (12/17 Pages) Semtech Corporation – SYNCHRONOUS PWM CONTROLLER WITH DUAL LOW DROPOUT REGULATOR CONTROLLERS

SYNCHRONOUS PWM CONTROLLER WITH
DUAL LOW DROPOUT REGULATOR
CONTROLLERS
PRELIMINARY - October 16, 2000
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.
SC1109
3.3V Vin
2.5V Vout
1.8V Vout
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.
© 2000 SEMTECH CORP.
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