AAT1157
1MHz 1.2A Buck DC/DC Converter
Functional Block Diagram
V
CC
V
P
= 2.7V to 5.5V
1.0V REF
FB
OP. AMP
CMP
DH
LOGIC
LX
1MΩ
DL
Temp.
Sensing
OSC
SGND
EN
PGND
Applications Information
Control Loop
The AAT1157 is a peak current mode buck converter.
The inner wide bandwidth loop controls the inductor
peak current. The 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 signal is added to the sensed current
to maintain stability for duty cycles greater than 50%.
The loop appears as a voltage-programmed current
source in parallel with the output 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 external voltage feedback resistive
divider divides the output voltage to the error ampli-
fier reference voltage of 0.6V. The low-DC gain
voltage error amplifier eliminates the need for
external compensation components while provid-
ing sufficient DC loop gain for good load regulation.
The voltage loop crossover frequency and phase
margin are set by the output capacitor.
input is applied. It limits the current surge seen at
the input and eliminates output voltage overshoot.
When pulled low, the enable input forces the
AAT1157 into a non-switching shutdown 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 the
noise associated with the output power MOSFET
switching. The low-pass filter R1 and C2 shown in
the Figure 1 schematic filters the input noise asso-
ciated with the power switching.
Current Limit and Over-Temperature
Protection
For overload conditions, the peak input current
sensed through the high-side P-channel MOSFET
is limited. Thermal protection completely disables
switching when internal dissipation becomes
excessive, protecting the device from damage. The
junction over-temperature threshold is 140°C with
15°C of hysteresis. Once the over-temperature or
over-current fault is removed, the AAT1157 auto-
matically recovers.
Soft Start/Enable
Soft start increases the inductor current limit point
in discrete steps once the input voltage or enable
1157.2005.11.1.4
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