850kHz 1A Synchronous Buck DC/DC Converter
Functional Block Diagram
V
CC
V
P
= 2.7V- 5.5V
AAT1152
1.0V REF
FB
OP. AMP
CMP
DH
LOGIC
OSC
LX
DL
Temp.
Sensing
Power
Good
SGND
POK
EN
PGND
Applications Information
850 kHz 1 Amp DC-DC Synchronous
Buck Converter
Control Loop
The AAT1152 is a peak current mode buck con-
verter. The inner, wide bandwidth loop controls the
peak current of the output inductor. The output
inductor current is sensed through the P-Channel
MOSFET (high side) and is also used for short cir-
cuit and overload protection. A fixed slope com-
pensation signal is added to the sensed current to
maintain stability. The loop appears as a voltage
programmed current source in parallel with the out-
put capacitor.
The voltage error amplifier output programs the
current loop for the necessary inductor current to
force a constant output voltage for all load and line
conditions. The feedback resistive divider is inter-
nal, dividing the output voltage to the error amplifi-
er reference voltage of 1.0V. The error amplifier
does not have a large DC gain typical of most error
amplifiers. This eliminates the need for external
compensation components while still providing suf-
ficient DC loop gain for load regulation. The
crossover frequency and phase margin are set by
the output capacitor value only.
Soft-Start/Enable
Soft start increases the inductor current limit point
in discrete steps when the input voltage or enable
input is applied. It limits the current surge seen at
the input and eliminates output voltage overshoot.
The enable input, when pulled low, forces the
AAT1152 into a low power non-switching state. The
total input current during shutdown is less that 1µA.
1152.2003.01.0.9
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