AN007 Datasheet, PDF (1/11 Pages) Analog Intergrations Corporation – Using AIC1578

AN007
High Efficiency LCD Monitor Power Design
Using AIC1578
DESCRIPTION
The AIC1578 is a high performance step-down
DC/DC converter, designed to drive an external
P-channel MOSFET to generate programmable
output voltages. Two main schematics of
Pulse-Skipping and Pulse-Frequency Modulation
are employed to maintain low quiescent current
and high conversion efficiency under wide ranges
of input voltage and loading condition. A current
sense comparator with both inverting and
non-inverting input uncommitted is included to
provide the crucial function of either current limit
protection or constant output current control.
When the AIC1578 is used in a high-side current
source step-down constant current source, the
efficiency is typically greater than 90%. Duty cycle
can be adjusted to greater than 90% by
connecting a resistor from DUTY pin to VIN.
Switching frequency being in around 90KHZ to
280KHZ range small size switching components
are ideal for portable equipment.
In order to maintain good conversion efficiency
form light loads to full loads, the AIC1578 uses the
intermittent switch control method of PFM
(Pulse-Frequency Modulation) rather than the
conventional PWM control method, Fig1. shows
its basic structure.
When the feedback voltage is greater than the
reference voltage (1.22V),the Err Amp. output is
Low, and DRI ( Pin6 ) is Hi Level, turn off outside
drive device(P MOSFET), Whereas when the
feedback voltage is lower than the reference
voltage, the Err Amp is Hi, and DRI( Pin6 ) is Low
Level, turn off outside drive device. The kind of
control method works similar to PWM at full load,
with a stable switch waveform, Whereas when at
light load it uses intermittent switching to efficiently
sustain output loading requirements.
March 1999
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