600mA, 1.2MHz, Micropower
Synchronous Step-Up Converter
Functional Block Diagram
V
IN
VIN
Slope
Compensation
Start-Up
Oscillator
MUX
VOUT
GOOD
2.3V
–
AAT1217
+
L1
SW
+
–
C
IN
Current
Sense
Antiringing
Control
VOUT
C
OUT
V
OUT
R1
R2
Bandgap
1.23V
FB
To V
IN
+
–
+
EA
–
Comp
PWM
Logic
GND
SHDN
Shutdown
Control
Oscillator
1.2MHz
The AAT1217 is a synchronous step-up DC-DC con-
verter. It utilizes internal MOSFET switches to
achieve high efficiency 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.
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 con-
tributed by the forward voltage of an external
Schottky diode. The synchronous rectifier is real-
ized by a P-channel MOSFET (PMOS) with gate
1217.2007.07.1.0
Functional Description
Synchronous Rectification
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 com-
parator (VOUT GOOD) monitors the output voltage
and places the chip into normal operation once
V
OUT
exceeds 2.3V. The AAT1217’s control circuit-
ry 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
.
7
Low Voltage Start-Up
control circuitry that incorporates relatively compli-
cated timing concerns. An external Schottky diode
is required when the output voltage is greater than
4.5V.
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
