ACT4060A
FUNCTIONAL BLOCK DIAGRAM
IN
EN
ENABLE
REGULATOR
&
REFERENCE
Rev 0, 28-Apr-09
BS
CURRENT SENSE
AMPLIFIER
COMP
1.293V
+
ERROR
AMPLIFIER
+
-
+
-
FB
FOLDBACK
CONTROL
OSCILLATOR
&
RAMP
-
PWM
COMP
+
-
0.18Ω
HIGH-SIDE
POWER
SWITCH
SW
LOGIC
4.5Ω LOW-SIDE
POWER SWITCH
THERMAL
SHUTDOWN
G
FUNCTIONAL DESCRIPTION
As seen in
Functional Block Diagram,
the AC-
T4060A is a current mode pulse width modulation
(PWM) converter. The converter operates as fol-
lows:
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 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 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 2.55V.
The Oscillator normally switches at 400kHz. How-
ever, 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.
Innovative Power
TM
-4-
www.active-semi.com
Copyright © 2009 Active-Semi, Inc.
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
