AAT1154
1MHz 3A Step-Down DC/DC Converter
VIN 3.5V to 5.5V
VOUT 3.3V @ 3A
U1
AAT1154-3.3
R1
R2
100 100k
FB
VP
C4
100µF
L1
1.5µH
GND LX
EN LX
VCC VP
C1
+
-
C2
B340LA 120µF
10µF
D1
C3
0.1µF
rtn
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay120µF 6.3V 594D127X96R6R3C2T
C3 0.1µF 0603ZD104M AVX
C4 Vishay Sprague 100µF 16V 595D107X0016C 100µF 16V
D1 B340LA Diodes Inc.
L1 CDRH6D28-1.5µH Sumida
Options
C2 MuRata 100µF 6. 3V (two or three in parallel)
3V GRM43-2 X5R 107M 100µF 6.
C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel)
Figure 1: 3.3V, 3A Output.
Inductor
VOUT
IOUT · k · FS
VOUT
VIN(MAX)
⎛
⎞
L =
· 1 -
⎝
The output inductor is selected to limit the ripple
current to 20% to 40% of full load current at the
maximum input voltage. Manufacturer's specifica-
tions list both the inductor DC current rating, which
is a thermal limitation, and the peak current rating,
which is determined by the inductor saturation
characteristics. The inductor should not show any
appreciable saturation under normal load condi-
tions. During overload and short-circuit conditions,
the inductor can exceed its peak current rating
without affecting converter performance. Some
inductors may have sufficient peak and average
current ratings yet result in excessive losses due to
a high DC resistance (DCR). The losses associat-
ed with the DCR and its effect on the total convert-
er efficiency must be considered.
⎠
3.3V
3A · 0.3 · 1MHz
⎞
⎛
⎝
⎞
⎠
⎛
⎝
3.3V
5.25V⎠
=
· 1 -
= 1.36μH
The factor "k" is the fraction of the full load (30%)
selected for the ripple current at the maximum input
voltage.
The corresponding inductor RMS current is:
ΔI2
12
⎛
⎝
⎞
⎠
2
IRMS
=
IO
+
I = 3A
≈
O
ΔI is the peak-to-peak ripple current which is fixed by
the inductor selection above. For a peak-to-peak
current of 30% of the full load current, the peak cur-
rent at full load will be 115% of the full load. The
1.5µH inductor selected from the Sumida
CDRH6D38 series has a 11mΩ DCR and a 4.0A DC
current rating with a height of 4mm. At full load, the
inductor DC loss is 99mW for a 1% loss in efficiency.
For a 3A load and the ripple current set to 30% at the
maximum input voltage, the maximum peak-to-peak
ripple current is 0.9A. Assuming a 5V ±5% input volt-
age and 30% ripple, the output inductance required is:
10
1154.2006.09.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
