ACT4065
IN
2μA
EN
ENABLE
REGULATOR
&
REFERENCE
BS
CURRENT SENSE
AMPLIFIER
COMP
ERROR
AMPLIFIER
+
–
1.293V
FB
+
–
+
PWM
COMPARATOR
–
+
–
0.2Ω
HIGH-SIDE
POWER
SWITCH
SW
FOLDBACK
CONTROL
OSCILLATOR
&
RAMP
LOGIC
8Ω LOW-SIDE
POWER SWITCH
THERMAL
SHUTDOWN
G
Figure 2. Functional Block Diagram
FUNCTIONAL DESCRIPTION
As seen in Figure 2,
Functional Block
Diagram,
the ACT4065 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 its
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 bootstrap pin 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
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 200kHz.
However, if FB voltage is less than 0.7V, then the
switching frequency decreases until it reaches a
minimum of 50kHz at V
FB
= 0.5V.
SHUTDOWN CONTROL
The ACT4065 has an enable input EN for
turning the IC on or off. When EN is less than
0.7V, the IC is in 8μA low current shutdown
mode. 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 ACT4065 automatically turns off when its
junction temperature exceeds 160°C.
Active-Semi, Inc.
-4-
www.active-semi.com
ACTIVE-SEMI [ ACTIVE-SEMI, INC ]
