AAT1217
600mA, 1.2MHz, Micropower
Synchronous Step-Up Converter
Functional Block Diagram
VIN
+
V
IN
Start-Up
Oscillator
VOUT
GOOD
2.3V
–
Slope
MUX
Compensation
SW
L1
V
VOUT
OUT
Current
Sense
+
C
IN
–
C
OUT
R1
Antiringing
Control
To VIN
Bandgap
1.23V
R2
PWM
Logic
+
–
Comp
+
–
EA
FB
GND
SHDN
Shutdown
Control
Oscillator
1.2MHz
control circuitry that incorporates relatively compli-
cated timing concerns. An external Schottky diode
is required when the output voltage is greater than
4.5V.
Functional Description
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.
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 VOUT exceeds 2.3V. An internal com-
parator (VOUT GOOD) monitors the output voltage
and places the chip into normal operation once
VOUT exceeds 2.3V. The AAT1217’s control circuit-
ry is biased by VIN during start-up and biased by
VOUT once VOUT exceeds VIN. When VOUT exceeds
VIN, the AAT1217’s operation will be independent
of VIN.
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 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
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