AN024 datasheet(2/7 Pages) AIC

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AN024

Application

Fig. 2 shows the application circuit for LCD power

management. This application circuit provides 5V,

3.3V and 2.5V output voltage. It uses AIC1563 to get

in the 12V-5V1A outputs. AIC1563 is a versatile

DC/DC converter, designed to drive an external

P-MOSFET to generate a programmable output

voltage. It consists of an internal temperature

compensated

reference,

comparator

and

bootstrapped driver, which can drive the NPN output

switch to saturation for higher efficiency and less heat

dissipation. Fig. 3 shows the block diagram, and it has

following features:

1. 3V to 30V input voltage operation.

2. 5A continuous output current.

3. Bootstrapped driver.

4. High side current sense capability.

5. Frequency operation from 100Hz to 100kHz.

6. High efficiency (up to 90%).

7. PSM (Pulse Skipping Modulation) control.

The operate principle is when the power turn on; the P

channel MOSFET Q2 doesn’t turn on yet and there is

no feedback voltage. The comparator in AIC1563

delivers a high signal to turn on the bootstrapped

driver. At this time, PIN 1 connects to ground through

a NPN transistor and makes the transistor Q1 turns off

and the P channel MOSFET Q2 turns on. The input

voltage stores energy on inductor through Q2 and the

output voltage starts rise until the feedback voltage

between R3 and R4 get in 1.25V. When the feedback

voltage gets bigger than the reference voltage, the

comparator in IC turns the bootstrapped driver off. At

this time PIN1 is floating, the voltages at collector and

base terminal of transistor Q1 are the same. It turns

Q1 on and Q2 off, and then the inductor supplies the

energy to output terminal.

During Q2 turns on, the voltage across the input

parasitism capacitor of Q2 is approximately zero. If

there is no charge path for the parasitism capacitor,

Q2 will get over temperature. Therefore during IC

turns off, the transistor Q1 turns on and provides a

charge path for the input parasitism capacitor of Q2

during Q2 turns off.

Concerning the 3.3V/1A output, we can use

AIC1084-33CT to achieve easily. The function of D3

and D4 are to reduce the dropout voltage from 5V

output of AIC1563. Due to there is no LDO has such

low dropout voltage can be used, we use AIC431 with

an N channel MOSFET to construct a LDO structure

with 2.5V output voltage from 3.3V output.

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