AAT1152
850kHz 1A Synchronous Buck DC/DC Converter
R5
Pok
100k
U1
R3
AAT1152-1.0
Vin+ 3.3V
Vo+ 1.25V1A
2.55k 1%
Vp
FB
Pok
LX
R1 100
R4
Vcc
EN
10k 1%
EN
R2
L1
100k
LX
C2
2.7µH
C1
10µF
Sgnd Pgnd
C3
0.1µF
100µF
V-
C1 Murata 10µF 6.3V X5R GRM42-6X 5R106K6.3
C2 MuRata 100µF 6.3V GRM43-2 X5R 107M 100µF6.3V
L1 Sumida CDRH4D28-2R 7µH
Figure 3: 3.3V to 1.25V converter
stabilize the output during large load transitions
and has ESR and ESL characteristics necessary
for low output ripple. The output capacitor rms rip-
ple current is given by:
Output Capacitor
Since there are no external compensation compo-
nents, the output capacitor has a strong effect on
loop stability. Larger output capacitance will reduce
the crossover frequency with greater phase mar-
gin. For the 1.5V 1A design using the 4.1 µH induc-
tor, a 47µF capacitor provides a stable loop with 35
degrees of phase margin at a crossover frequency
of 100 kHz. Doubling the capacitance to 100µF
reduces the crossover frequency to half while
increasing the phase margin to 60 degrees. In
addition to assisting stability, the output capacitor
limits the output ripple and provides holdup during
large load transitions. A 100µF X5R or X7R ceram-
ic capacitor provides sufficient bulk capacitance to
(VOUT + VFWD) (VIN - VOUT)
L F VIN
3
1
IRMS
=
2
For a ceramic capacitor the dissipation due to the
RMS current of the capacitor is not a concern.
Tantalum capacitors, with sufficiently low ESR to
meet output voltage ripple requirements, also have
an RMS current rating much greater than that
actually seen in this application.
12
1152.2003.01.0.9
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
