ACT4060SH datasheet(4/9 Pages) ACTIVE-SEMI

4 / 9 page

ACT4060

Innovative Products. Active Solutions.

- 4 -

www.active-semi.com

Rev8, 24-Jul-07

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As seen in Functional Block Diagram, the ACT4060

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 con-

nected 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 re-

sulting 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 volt-

age 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

BS as the positive rail. This pin is charged to VSW +

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 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 FB voltage is less than 0.7V, then the

switching frequency decreases until it reaches a

typical value of 50kHz at VFB = 0.5V.

Shutdown Control

The ACT4060 has an enable input EN for turning

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

is in 8

μA 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

μA 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 ACT4060 automatically turns off when its junc-

tion temperature exceeds 160°C.

COMP

LOGIC

THERMAL

SHUTDOWN

REGULATOR

REFERENCE

OSCILLATOR

RAMP

FOLDBACK

CONTROL

1.293V

ERROR

AMPLIFIER

ENABLE

0.2

HIGH-SIDE

POWER

SWITCH

- +

PWM

COMP

CURRENT SENSE

AMPLIFIER

4.7

Ω LOW-SIDE

POWER SWITCH

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