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Copyright © 2007 Active-Semi, Inc.

ACT4075

Rev PrA, 13-Aug-07

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As seen in the Functional Block Diagram, the

ACT4075 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 the 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

output. This state continues until the cycle starts

again.

The High-Side Power Switch is driven by logic using

BS bootstrap pin as the positive rail. This pin is

Switch turns on.

The COMP voltage is the integration of the error

between 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 2.65V.

The Oscillator normally switches at 410kHz. How-

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

switching frequency decreases until it reaches a

Shutdown Control

The ACT4075 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 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.

Thermal Shutdown

The ACT4075 automatically turns off when its junc-

tion temperature exceeds 160°C and then restarts

once the temperature falls to 150°C.