ACT4523
Rev 5, 14-Nov-12
FUNCTIONAL BLOCK DIAGRAM
IN
AVIN
PVIN
EN
BANDGAP,
REGULATOR,
&
SHUTDOWN
CONTROL
OSCILLATOR
V
REF
= 0.808V
EMI
CONTROL
HSB
Σ
PWM
CONTROLLER
SW
V
REF
= 0.808V
+
FB
-
CC
CONTROL
COMP
ISET
FUNCTIONAL DESCRIPTION
CV/CC Loop Regulation
As seen in
Functional Block Diagram,
the ACT4523
is a peak current mode pulse width modulation
(PWM) converter with CC and CV control. 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.
At this point, the SW side of the inductor swings to
a diode voltage 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 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 the
internal 0.808V reference. If FB is lower than the
reference voltage, COMP tends to go higher to
increase current to the output. Output current will
increase until it reaches the CC limit set by the ISET
resistor. At this point, the device will transition from
Innovative Power
TM
-5-
regulating output voltage to regulating output
current, and the output voltage will drop with
increasing load.
The Oscillator normally switches at 225kHz.
However, if FB voltage is less than 0.6V, then the
switching frequency decreases until it reaches a
typical value of 30kHz at V
FB
= 0.15V.
Enable Pin
The ACT4523 has an enable input EN for turning
the IC on or off. The EN pin contains a precision
1.6V comparator with 125mV hysteresis and a 4µA
pull-up current source. The comparator can be used
with a resistor divider from V
IN
to program a startup
voltage higher than the normal UVLO value. It can
be used with a resistor divider from V
OUT
to disable
charging of a deeply discharged battery, or it can be
used with a resistor divider containing a thermistor
to provide a temperature-dependent shutoff
protection for over temperature battery. The
thermistor should be thermally coupled to the
battery pack for this usage.
If left floating, the EN pin will be pulled up to roughly
5V by the internal 4µA current source. It can be
driven from standard logic signals greater than
1.6V, or driven with open-drain logic to provide
digital on/off control.
Thermal Shutdown
The ACT4523 disables switching when its junction
temperature exceeds 150°C and resumes when the
temperature has dropped by 20°C.
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