AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Functional Block Diagram
VCC
V
P
= 2.5V to 5.5V
REF
FB
OP. AMP
CMP
DH
LOGIC
LX
OSC
Temp.
Sensing
EN
Applications Information
Main Control Loop
The AAT1155 is a peak current mode step-down
converter. The inner wide bandwidth loop controls
the inductor peak current. The inductor current is
sensed as it flows through the internal P-channel
MOSFET. A fixed slope compensation signal is
then added to the sensed current to maintain sta-
bility for duty cycles greater than 50%. The inner
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 1.0V. The error amplifier has a
limited DC gain. This eliminates the need for exter-
nal compensation components, while still providing
sufficient DC loop gain for good load regulation.
The crossover frequency and phase margin are set
by the output capacitor value.
Duty cycle extends to 100% as the input voltage
approaches the output voltage. Thermal shutdown
protection disables the device in the event of a
short-circuit or overload condition.
Soft Start/Enable
Soft start controls the current limit when the input
voltage or enable is applied. It limits the current
surge seen at the input and eliminates output volt-
age overshoot.
When pulled low, the enable input forces the device
into a low-power, non-switching state. The total
input current during shutdown is less than 1µA.
Power and Signal Source
Separate small signal ground and power supply
pins isolate the internal control circuitry from
switching noise. In addition, the low pass filter R1
and C3 (shown in Figure 1) filters noise associated
with the power switching.
1155.2005.11.1.6
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