ACT4070
Rev 2, 16-Sep-11
FUNCTIONAL BLOCK DIAGRAM
FUNCTIONAL DESCRIPTION
As seen in the
Functional Block Diagram,
the
ACT4070 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 its magnetic field. The inductor current
level is measured by the Current Sense Amplifier
and added to the Oscillator ramp signal. If the result-
ing 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 de-
crease 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.
Innovative Power
TM
The COMP voltage is the integration of the error
between FB input and the internal 1.222V 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 2.65V.
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 40kHz at V
FB
= 0V.
Shutdown Control
The ACT4070 has an enable input EN for turning
the IC on or off. When EN is less than 0.7V, the IC
is in 6μA 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.
Thermal Shutdown
The ACT4070 automatically turns off when its junc-
tion temperature exceeds 160°C and then restarts
once the temperature falls to 150°C.
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