ACT4060A
FUNCTIONAL BLOCK DIAGRAM
IN
EN
ENABLE
REGULATOR
&
REFERENCE
Rev 1, 25-Jun-09
BS
CURRENT SENSE
AMPLIFIER
COMP
1.293V
ERROR
AMPLIFIER
0.18Ω
HIGH-SIDE
POWER
SWITCH
PWM
COMP
FB
FOLDBACK
CONTROL
OSCILLATOR
&
RAMP
LOGIC
SW
THERMAL
SHUTDOWN
4.5Ω LOW-SIDE
POWER SWITCH
G
FUNCTIONAL DESCRIPTION
As seen in
Functional Block Diagram,
the
ACT4060A is a current mode pulse width
modulation (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
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. 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
BS as the positive rail. This pin is charged to V
SW
+
6V when the Low-Side Power Switch turns on.
The COMP voltage is the integration of the error
between FB input and the internal 1.293V
Innovative Power
TM
-4-
reference. 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 2.55V.
The Oscillator normally switches at 400kHz.
However, if FB voltage is less than 0.7V, then the
switching frequency decreases until it reaches a
typical value of 60kHz at V
FB
= 0.5V.
Shutdown Control
The ACT4060A has an enable input EN for turning
the IC on or off. When EN is less than 0.7V, the IC
is in 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 low voltage
input with a maximum voltage of 6V, it should never
be directly connected to IN.
Thermal Shutdown
The ACT4060A automatically turns off when its
junction temperature exceeds 160°C.
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