PRODUCT DATASHEET
AAT1217
SwitchRegTM
600mA, 1.2MHz, Micropower Synchronous Step-Up Converter
Output Capacitor
Load Disconnect in Shutdown
The output capacitor limits the output ripple and pro-
vides holdup during large load transitions. A 4.7ꢀF to
10ꢀF, X5R or X7R, ceramic capacitor is suggested for the
output capacitor. Typically the recommended capacitor
range provides sufficient bulk capacitance to stabilize the
output voltage during large load transitions and has the
ESR and ESL characteristics necessary for low output
voltage ripple.
In conventional synchronous step-up converters, a con-
duction path exists from input to output through the
backgate (body diode) of the P-channel MOSFET during
shutdown. Special application circuitry can disconnect the
load from the battery during shutdown (see Figure 1).
PCB Layout Guidance
The AAT1217 typically operates at 1.2MHz. This is a con-
siderably high frequency for DC-DC converters. PCB
layout is important to guarantee satisfactory perfor-
mance. It is recommended to make traces of the power
loop, especially where the switching node is involved, as
short and wide as possible. First of all, the inductor, input
and output capacitor should be as close as possible to
the device. Feedback and shutdown circuits should avoid
the proximity of large AC signals involving the power
inductor and switching node. The optional rectifier diode
(D1 in Figure 1) can improve efficiency and alleviate the
stress on the integrated MOSFETs. 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 2 and 3 is enough for most applications, large
and integral multi-layer ground planes are ideal for high
power applications. Large areas of copper have lower
resistance and help to dissipate heat. The converter’s
ground should join the system ground to which it sup-
plies power at one point only. Figure 1 is the schematic
for a highly efficient load disconnect application circuit
for the AAT1217. An example PCB layout for the AAT1217
is shown in Figures 2 and 3.
In addition, the output voltage droop during load tran-
sient is dominated by the capacitance of the ceramic
output capacitor. During a step increase in load current,
the ceramic output capacitor alone supplies the load cur-
rent until the loop responds. Within several switching
cycles, the loop responds and the inductor current
increases to match the load current demand. Larger out-
put capacitor values help to reduce the voltage droop
during large load current transients.
External Diode Selection
An external Schottky diode is required when the output
voltage is above 4.5V. The Schottky diode is optional for
output voltages ≤ 4.5V, but can improve efficiency by
about 2% to 3%.
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