AAT1218 Datasheet, PDF (11/15 Pages) Advanced Analogic Technologies – 1A, 1.2MHz Synchronous Boost Converter

SwitchReg^TM
Load Disconnect in Shutdown
In a conventional synchronous step-up converter, a con-
duction path exists from battery to output through the
body diode of the PMOS during shutdown. To avoid out-
put side circuitry drawing current from V_IN, special appli-
cation circuitry such as that shown in Figure 1 is neces-
sary to disconnect the output from V_IN during shutdown.
PCB Layout Guidance
The AAT1218 typically operates at 1.2MHz. Since this is
a high frequency for DC-DC converters, PCB layout is
critical to guarantee satisfactory performance. It is rec-
ommended to make traces of the power loop as short
and wide as possible, especially where the switching
PRODUCT DATASHEET
AAT1218
1A, 1.2MHz Synchronous Boost Converter
node is involved. First, the inductor and input and output
capacitors should be placed close to the IC. Feedback
and shutdown traces should avoid proximity to large AC
signals, e.g. the power inductor and switching nodes.
The optional rectifier diode (D1 in Figure 1 and Figure 2)
can improve efficiency and alleviate the stress on the
integrated PMOS device. The diode should also be close
to the inductor and the chip to form the shortest possible
switching loop. While the two layer PCB shown in Figures
3 and 4 is sufficient for low to moderate power applica-
tions, large and integral multi-layer ground planes are
ideal for high power applications since the large area of
copper has lower resistance and higher thermal dissipa-
tion capability. The converter’s grounds should join at
one point only. Figure 1 illustrates the schematic for a
typical AAT1218 application.
L1
2.2μH
D1(Optional)
V_IN 1.2V
C_IN
10μF
LX
VIN
VOUT
AAT1218
EN
PGND
FB
GND
Q1
Si2305DS
V_OUT
3.3V, 300mA
R4
510kΩ
R1
1.02MΩ
R2
576kΩ
C_OUT
2x22μF
ON/OFF Control
R3
510kΩ
Q2
2N3904
Figure 1: AAT1218 Application Schematic with Load Disconnect Circuit.
1218.2008.10.1.1
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