AAT1155 AAT, AAT1155 Datasheet - Page 12

• Manufacturer Part Number: AAT1155
• Description: Step-Down DC-DC Converter
• Manufacturer: AAT

A high ESR tantalum capacitor with a value about 10
times the input ceramic capacitor may also be
required when using a 10µF or smaller ceramic input
bypass capacitor. This dampens any input oscilla-
tions that may occur due to the source inductance
resonating with the converter input impedance.
Output Capacitor
With no external compensation components, the out-
put capacitor has a strong effect on the loop stability.
Larger output capacitance will reduce the crossover
frequency with greater phase margin. A 200µF
ceramic capacitor provides sufficient bulk capacitance
to stabilize the output during large load transitions and
has ESR and ESL characteristics necessary for very
low output ripple. The RMS ripple current is given by:
For a ceramic output capacitor, the dissipation due
to the RMS current and associated output ripple
are negligible.
Tantalum capacitors with sufficiently low ESR to
meet output ripple requirements generally have an
RMS current rating much greater than that actually
seen in this application. The maximum tantalum
output capacitor ESR is:
where ∆I is the peak-to-peak inductor ripple current.
12
Figure 3: Evaluation Board Top Side.
I
RMS
=
2
·
1
ESR ≤
3
·
(
V
OUT
V
RIPPLE
+
∆I
V
L F V
FWD
· ·
)
·
(
V
IN
IN
-
V
OUT
)
1MHz 2.5A Step-Down DC/DC Converter
Due to the ESR zero associated with the tantalum
capacitor, smaller values than those required with
ceramic capacitors provide more phase margin
with a greater loop crossover frequency.
Layout
Figures 3 and 4 display the suggested PCB layout
for the AAT1155. The following guidelines should
be used to help ensure a proper layout.
1. The connection from the input capacitor to the
2. The input capacitor should connect as closely
3. C1, L1, and CR1 should be connected as
4. The feedback trace (Pin 1) should be separate
5. The resistance of the trace from the load
6. R1 and C3 are required in order to provide
Schottky anode should be as short as possible.
as possible to V
(Pin 2).
closely as possible. The connection from the
cathode of the Schottky to the LX node
should be as short as possible.
from any power trace and connect as closely
as possible to the load point. Sensing along a
high-current load trace can degrade DC load
regulation.
return to the ground (Pin 2) should be kept to
a minimum. This will help to minimize any
error in DC regulation due to differences in
the potential of the internal reference ground
and the load return.
a cleaner power source for the AAT1155 con-
trol circuitry.
Figure 4: Evaluation Board Bottom Side.
P
(Pins 5 and 8) and GND
AAT1155
1155.2005.11.1.6