High Efficiency, Low Noise, Fast Transient
400mA Step-Down Converter
Functional Block Diagram
OUT
VIN
AAT1147
Err
Amp
.
DH
Voltage
Reference
Logic
LX
EN
INPUT
DL
PGND
AGND
Functional Description
The AAT1147 is a high performance 400mA
1.4MHz monolithic step-down converter. It has
been designed with the goal of minimizing external
component size and optimizing efficiency at heavy
load. Apart from the small bypass input capacitor,
only a small L-C filter is required at the output.
Typically, a 4.7µH inductor and a 4.7µF ceramic
capacitor are recommended (see table of values).
Only three external power components (C
IN
, C
OUT
,
and L) are required. Output voltage is programmed
with external resistors and ranges from 0.6V to the
input voltage. An additional feed-forward capacitor
can also be added to the external feedback to pro-
vide improved transient response (see Figure 1).
At dropout, the converter duty cycle increases to
100% and the output voltage tracks the input volt-
age minus the R
DS(ON)
drop of the P-channel high-
side MOSFET.
The input voltage range is 2.7V to 5.5V. The con-
verter efficiency has been optimized for heavy load
conditions up to 400mA.
The internal error amplifier and compensation pro-
vide excellent transient response, load, and line
regulation. Soft start eliminates any output voltage
overshoot when the enable or the input voltage is
applied.
8
1147.2006.05.1.0
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
