ACT4060A

Rev 1, 25-Jun-09

Innovative Power

TM

- 4 -

www.active-semi.com

Copyright © 2009 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As seen in Functional Block Diagram, the

ACT4060A is a current mode pulse width

modulation

(PWM)

converter.

The

converter

operates as follows:

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

connected to IN, the inductor current ramps up to

store energy in the magnetic field. The inductor

current level is measured by the Current Sense

Amplifier and added to the Oscillator ramp signal. If

the resulting 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 decrease and magnetic energy to be transferred

to output. This state continues until the cycle starts

again.

The High-Side Power Switch is driven by logic using

6V when the Low-Side Power Switch turns on.

The COMP voltage is the integration of the error

between FB input and the internal 1.293V

reference. 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 400kHz.

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

switching frequency decreases until it reaches a

Shutdown Control

The ACT4060A has an enable input EN for turning

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

is in 10μA low current shutdown mode and output 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 unconnected for

always-on operation. Note that EN is a low voltage

input with a maximum voltage of 6V, it should never

be directly connected to IN.

Thermal Shutdown

The ACT4060A automatically turns off when its

junction temperature exceeds 160°C.

IN

EN

COMP

FB

BS

SW

LOGIC

THERMAL

SHUTDOWN

REGULATOR

&

REFERENCE

OSCILLATOR

&

RAMP

FOLDBACK

CONTROL

1.293V

ERROR

AMPLIFIER

ENABLE

0.18Ω

HIGH-SIDE

POWER

SWITCH

G

PWM

COMP

CURRENT SENSE

AMPLIFIER

4.5Ω LOW-SIDE

POWER SWITCH