AAT1155(2004) Ver la hoja de datos (PDF) - Analog Technology Inc
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AAT1155 Datasheet PDF : 16 Pages
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AAT1155
1MHz 2.5A Buck DC/DC Converter
Diodes Inc.
B340LA
0.45V@3A
ROHM
RB050L-40
0.45@3A
Micro Semi
5820SM
0.46V@3A
3 Amp Surface Mount Schottky Diodes
Input Capacitor Selection
The primary function of the input capacitor is to pro-
vide a low impedance loop for the edges of pulsed
current drawn by the AAT1155. A low ESR/ESL
ceramic capacitor is ideal for this function. To mini-
mize the stray inductance the capacitor should be
placed as close as possible to the IC. This also
keeps the high frequency content of the input cur-
rent localized, minimizing the radiated and con-
ducted EMI while facilitating optimum performance
of the AAT1155. The proper placement of the input
capacitor C1 is shown in the layout in figure 1.
Ceramic X5R or X7R capacitors are ideal. The
size required will vary depending on the load, out-
put voltage, and input voltage source impedance
characteristics. Typical values range from 1µF to
10 µF. The input capacitor RMS current varies with
the input voltage and the output voltage. It is high-
est when the input voltage is double the output volt-
age where it is one half of the load current.
IRMS = IO ·
VO ·1- VO
VIN VIN
A high ESR tantalum 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 out any input oscillations
that may occur due to the source inductance res-
onating with the converter input impedance
Output Capacitor
With no external compensation components, the
output capacitor has a strong effect on the loop sta-
bility. 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 rip-
ple current is given by
IRMS =
2
1
·
3
·(VOUT+VFWD) ·(VIN
L · F · VIN
-
VOUT)
For a ceramic output capacitor the dissipation due
to the RMS current and output ripple associated
with 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
ESR ≤ VRIPPLE
∆I
Where ∆I is the peak to peak inductor ripple current.
Due to the ESR zero associated with the tantalum
capacitor, smaller values than those required with
ceramic capacitors provide more phase margin a
with greater loop crossover frequency.
Layout
Figures 6 and 7 display the suggested PCB layout
for the AAT1155. The following guidelines should
be used to help insure a proper layout.
1. The connection from the input capacitor to the
Schottky anode should be as short as possible.
2. The input capacitor should connect as closely as
possible to VPOWER (pins 5 and 8) and GND (pin 2).
3. C1, L1, and CR1 should be connected as
closely as possible. The connection from the cath-
ode of the Schottky to the LX node should be as
short as possible.
4. The feedback trace (pin 1) should be separate
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.
5. The resistance of the trace from the load
return to the gnd (pin 2) should be kept to a mini-
mum. This will help to minimize any error in DC
regulation due to differences in the potential of the
internal reference ground and the load rtn.
6. R1 and C3 are required in order to provide
a cleaner power source for the AAT1155 control
circuitry.
1155.2004.08.1.2
11
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