ACT2113

Rev 3, 23-May-12

®

- 5 -

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Copyright © 2012 Active-Semi, Inc.

FUNCTIONAL BLOCK DIAGRAM

FUNCTIONAL DESCRIPTION

As seen in Function Block Diagram, the ACT2113 is

peak current mode controlled synchronous Buck

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. The

High-Side Power Switch is driven by logic using

HSB as the positive rail. This pin is charged to VSW

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

The COMP voltage is the integration of the error

between FB input and the internal 0.923V

reference. If FB is lower than the reference voltage,

COMP tends to go higher to increase current to the

output to keep the output voltage regulated. The

Oscillator normally switches at 510kHz.

Pulse Skipping Mode

To decrease the power recycling at very light load,

the low-side FET current is sensed to emulate a

diode. When the low-side FET current decreases to

zero, the FET is turned off to avoid negative

inductor current. At no load and very light load,

ACT2113 skips pulse automatically and thus

achieve very high light load efficiency. With load

increasing, ACT2113 goes into Discontinuous

Current Mode (DCM) and then Continuous Current

Mode (CCM).

Soft Startup

The ACT2113 builds in internal soft startup

function. The internal FB reference voltage rises to

steady state of 0.923V in 2ms to avoid inrush input

current during startup.

Under Voltage Protection (UVP)

At output short circuit or over current, the FB

voltage is usually pulled low. To protect the IC at

over current and short circuit, the ACT2113 builds

in Under Voltage Protection (UVP) function. When

ACT2113 detects the FB voltage below 75% of the

0.923V reference, it pulls low COMP voltage and

discharges internal soft-start capacitor and goes

into hiccup mode. The IC restarts in 32ms after

going into hiccup mode. If the short circuit or over

current is clear, the IC restarts back to normal

mode. The UVP is disabled for 6ms starting from

startup. If the output is short at startup, the output

voltage never rises to nominal voltage. During the

6ms period of time, the output current is limited by

cycle-by-cycle current limit. With 32ms shutdown

period, the average input and output current at

short circuit is significantly reduced and the IC is

more reliable.