ACT4072

Rev2, 27-May-08

Active-Semi

Innovative Power

TM

- 4 -

www.active-semi.com

Copyright © 2008 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As seen in the Functional Block Diagram, the

ACT4072 is a current mode pulse width modulation

(PWM) converter. The converter operates as fol-

lows:

A switching cycle starts when the rising edge of the

Oscillator clock output causes the High-Side Power

Switch to turn on and the Low-Side Power Switch to

turn off. With the SW side of the inductor now con-

nected to IN, the inductor current ramps up to store

energy in its magnetic field. The inductor current

level is measured by the Current Sense Amplifier

and added to the Oscillator ramp signal. If the result-

ing summation is higher than the COMP voltage, the

output of the PWM Comparator goes high. When

this happens or when Oscillator clock output goes

low, the High-Side Power Switch turns off and the

Low-Side Power Switch turns on. At this point, the

SW side of the inductor swings to a diode voltage

below ground, causing the inductor current to de-

crease and magnetic energy to be transferred to the

output. This state continues until the cycle starts

again.

The High-Side Power Switch is driven by logic using

the BS bootstrap pin as the positive rail. This pin is

Switch turns on.

The COMP voltage is the integration of the error

between the FB input and the internal 1.222V refer-

ence. If FB is lower than the reference voltage,

COMP tends to go higher to increase current to the

output. Current limit happens when COMP reaches

its maximum clamp value of 2.55V.

The Oscillator normally switches at 420kHz. How-

ever, if the FB voltage is less than 0.7V, then the

switching frequency decreases until it reaches a

Shutdown Control

The ACT4072 has an enable input EN for turning

the IC on or off. When EN is less than 0.7V, the IC

is in 6µA low current shutdown mode and the out-

put is discharged through the Low-Side Power

Switch. When EN is higher than 1.3V, the IC is in

normal operation mode. EN is internally pulled up

with a 2µA current source and can be left uncon-

nected for always-on operation.

Thermal Shutdown

The ACT4072 automatically turns off when its junc-

tion temperature exceeds 160°C.

IN

EN

COMP

FB

BS

SW

LOGIC

THERMAL

SHUTDOWN

REGULATOR

&

REFERENCE

+

-

OSCILLATOR

&

RAMP

FOLDBACK

CONTROL

1.222V

ERROR

AMPLIFIER

2µA

ENABLE

0.13O

HIGH-SIDE

POWER

SWITCH

G

+- + -

-

+

PWM

COMP

CURRENT SENSE

AMPLIFIER

10O LOW-SIDE

POWER SWITCH

Ω

Ω