ACT4455
Rev 2, 21-Nov-12
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
HSB as the positive rail. This pin is charged to V
SW
+ 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
Innovative Power
TM
-5-
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.
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