AAT1152 Datasheet, PDF (12/18 Pages) Advanced Analogic Technologies – 850kHz 1A Synchronous Buck DC/DC Converter

AAT1152
850kHz 1A Synchronous Buck DC/DC Converter
R5
100k
Pok
Vin+ 3.3V
C1
10µF
R1 100
R2
EN
100k
C3
0.1µF
U1
AAT1152-1.0
Vp FB
Vcc Pok
EN LX
Sgnd Pgnd
R3
2.55k 1%
Vo+ 1.25V1A
R4
10k 1%
LX
L1
2.7µH
C2
100µF
V-
C1 Murata 10µF 6.3V X5R GRM42-6X 5R106K6.3
C2 MuRata 100µF 6.3V GRM43-2 X5R 107M 100µF 6.3V
L1 Sumida CDRH4D28-2R 7µH
Figure 3: 3.3V to 1.25V converter
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
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:
I_RMS
=
2
1
⋅
3^â‹
(V_OUT
+
V_FWD)
L⋅F
⋅
⋅
(V_IN
V_IN
-
V_OUT)
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