PRODUCT DATASHEET
AAT1217
SwitchReg
TM
600mA, 1.2MHz, Micropower Synchronous Step-Up Converter
Functional Block Diagram
V
IN
VIN
Start-Up
Oscillator
Slope
Compensation
L1
SW
+
VOUT
GOOD
2.3V
–
MUX
VOUT
V
OUT
+
C
IN
–
Current
Sense
C
OUT
To V
IN
Bandgap
1.23V
Antiringing
Control
PWM
Logic
R1
R2
+
+
FB
–
EA
–
Comp
GND
SHDN
Shutdown
Control
Oscillator
1.2MHz
Functional Description
The AAT1217 is a synchronous step-up DC-DC converter.
It utilizes internal MOSFET switches to achieve high effi-
ciency over the full load current range. It operates at a
fixed switching frequency of 1.2MHz, and uses the slope
compensated current mode pulse width modulation
(PWM) architecture. The device can operate with an input
voltage below 1V; the typical start-up voltage is 0.85V.
rectifier is realized by a P-channel MOSFET (PMOS) with
gate control circuitry that incorporates relatively compli-
cated timing concerns. An external Schottky diode is
required when the output voltage is greater than 4.5V.
Low Voltage Start-Up
The AAT1217 can start-up with supply voltages down to
0.85V. During start-up, the internal low voltage start-up
circuitry controls the internal NMOS switch. The AAT1217
leaves the start-up mode once V
OUT
exceeds 2.3V. An
internal comparator (VOUT GOOD) monitors the output
voltage and places the chip into normal operation once
V
OUT
exceeds 2.3V. The AAT1217’s control circuitry is
biased by V
IN
during start-up and biased by V
OUT
once
V
OUT
exceeds V
IN
. When V
OUT
exceeds V
IN
, the AAT1217’s
operation will be independent of V
IN
.
Synchronous Rectification
The AAT1217 integrates a synchronous rectifier to
improve efficiency as well as to eliminate the need for an
external Schottky diode. The synchronous rectifier is used
to reduce the conduction loss contributed by the forward
voltage of an external Schottky diode. The synchronous
1217.2008.03.1.3
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