AAT1151
850kHz 700mA Synchronous Buck DC/DC Converter
Functional Block Diagram
VCC
V
P
= 2.7V- 5.5V
1.0V REF
FB
OP. AMP
CMP
DH
LOGIC
LX
1MΩ
DL
Temp.
Sensing
OSC
SGND
EN
PGND
Operation
Control Loop
The AAT1151 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 cur-
rent loop for the necessary inductor current to force
a constant output voltage for all load and line condi-
tions. The voltage feedback resistive divider is inter-
nal, dividing the output voltage to the error amplifier
reference voltage of 1.0V. The voltage error amplifi-
er does not have the large DC gain typical of most
error amplifiers. This eliminates the need for exter-
nal compensation components, while still providing
sufficient DC loop gain for load regulation. The volt-
age loop crossover frequency and phase margin are
set by the output capacitor value only.
PFM/PWM Operation
Light load efficiency is maintained by way of Pulse
Frequency Modulation (PFM) control. The AAT1151
PFM control forces the peak inductor current to a
minimum level regardless of load demand. At medi-
um to high load demand, this has no effect on circuit
operation and normal PWM controls take over. PFM
reduces the switching frequency at light loads, thus
reducing the associated switching losses.
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.
When pulled low, the enable input forces the
AAT1151 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 the noise
associated with the output MOSFET switching. The
low pass filter R1 and C2 in schematic Figure 1 fil-
ters the noise associated with the power switching.
1151.2005.11.1.4
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