ACT4012A
Rev PrA, 21-Nov-07
FUNCTIONAL BLOCK DIAGRAM
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.
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 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.
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.
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