ACT4012ASH datasheet(4/10 Pages) ACTIVE-SEMI

4 / 10 page

ACT4012A

Innovative Products. Active Solutions.

- 4 -

www.active-semi.com

Rev PrA, 21-Nov-07

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As seen in Functional Block Diagram, the

ACT4012A is a current mode pulse width modula-

tion (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 2.6V.

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

minimum of 50kHz at VFB = 0.5V.

Shutdown Control

The ACT4012A has an enable input EN for turning

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

is 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 high voltage

input that can with stand voltages up to VIN.

Thermal Shutdown

The ACT4012A automatically turns off when its

junction temperature exceeds 160°C.

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