AAT1151 View Datasheet(PDF) - Advanced Analogic Technologies
Part Name
Description
MFG CO.
'AAT1151' PDF : 16 Pages View PDF
Functional Block Diagram
AAT1151
850kHz 700mA Synchronous
Buck DC/DC Converter
VCC
VP = 2.7V to 5.5V
1.0V REF
FB
OP. AMP
CMP
DH
LOGIC
LX
1MΩ
DL
Temp.
Sensing
OSC
SGND
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.
8
EN
PGND
PFM/PWM Operation
Light load efficiency is maintained by way of PFM
control. The AAT1151 PFM control forces the peak
inductor current to a minimum level regardless of
load demand. At medium to high load demand, this
has no effect on circuit operation and normal PWM
controls take over. PFM reduces the switching fre-
quency 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.
1151.2007.01.1.6
Share Link: 
