AAT1152 Advanced Analogic Technologies, AAT1152 Datasheet - Page 12

• Manufacturer Part Number: AAT1152
• Description: 850kHz 1A Synchronous Buck DC/DC Converter
• Manufacturer: Advanced Analogic Technologies

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
12
10 F
C1
Vin+ 3.3V
C1 Murata 10 F 6.3V X5R GRM42-6X 5R106K6.3
C2 MuRata 100 F 6.3V GRM43-2 X5R 107M 100 F 6.3V
L1 Sumida CDRH4D28-2R 7 H
Figure 3: 3.3V to 1.25V converter
R2
100k
0.1 F
R1 100
850kHz 1A Synchronous Buck DC/DC Converter
C3
EN
U1
AAT1152-1.0
Vp
Vcc
EN
Sgnd
R5
100k
V-
Pgnd
Pok
LX
FB
Pok
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:
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.
LX
R3
2.55k 1%
R4
10k 1%
I
RMS
2.7 H
L1
=
Vo+ 1.25V 1A
2
1
C2
3
100 F
(V
OUT
+ V
L F V
FWD
) (V
AAT1152
IN
IN
1152.2003.01.0.9
- V
OUT
)