850kHz 1A Buck DC/DC Converter
Functional Block Diagram
VCC
VP = 2.7V to 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
Control Loop
The AAT1152 is a peak current mode buck convert-
er. 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-circuit and
overload protection. A fixed slope compensation sig-
nal is added to the sensed current to maintain sta-
bility. The loop appears as a voltage-programmed
current source in parallel with the output capacitor.
The voltage error amplifier output programs the cur-
rent loop for the necessary inductor current to force
a constant output voltage for all load and line con-
ditions. The feedback resistive divider is internal,
dividing the output voltage to the error amplifier ref-
erence voltage of 1V. The error amplifier does not
have the large DC gain typical of most error ampli-
fiers. This eliminates the need for external compen-
sation components, while still providing sufficient
DC loop gain for load regulation. The crossover fre-
quency 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
(see Figure 1). When pulled low, the enable input
forces the AAT1152 into a low-power, non-switching
state. The total input current during shutdown is
less than 1μA.
1152.2006.09.1.7
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