ACT4455

Rev 2, 21-Nov-12

Innovative Power

TM

- 5 -

www.active-semi.com

Copyright © 2012 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

Operation

As seen in Functional Block Diagram, the ACT4455

is a current mode controlled regulator. The EA

output voltage (COMP voltage) is proportional to

the peak inductor current.

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. 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

inductor freewheels through the schottky diode

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

+ 5V when the Low-Side Power Switch turns on.

The Comp voltage is the integration of the error

between FB input and internal 0.808V reference. If

FB

is lower than the reference voltage, COMP

tends to go higher to increase current to the output.

Over Current and Short Circuit

Protection

CS pins are connected to the high side of current

sensing resistors to prevent output over current.

With independent CS1 and CS2 pins, two output

currents are detected. If the voltage at either CS

pins exceeds 116mV for more than 750µs. The

converter shuts down and goes into sleeping mode.

A new soft startup is triggered after 2.5s. If the fault

condition is un-cleared, the converter shuts down

again until over current condition is cleared. With

this long-waiting-time hiccup mode, the power

consumption at over loading or outputs short is

reduced to nearly zero.

Thermal Shutdown

The ACT4455 shuts down when its junction

temperature exceeds 150°C. The converter triggers

a soft-start when the temperature has dropped by

10°C. The soft-restart avoids output over voltage at

thermal hiccup.