AAT1155
1MHz 2.5A Step-Down DC/DC Converter
Vin 2.7V-5.5V
VOUT 1.25V @ 2.5A
R3
2.55k
U1
AAT1155-1.0
R1 R2
100 100k
FB
VP
C4
100µF
L1
1.5µH
GND LX
EN LX
VCC VP
C1
C2
D1
B340LA 120µF
10µF
C3
0.1µF
R4
10.0k
rtn
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay 120µF 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 100uF 6.3V GRM43-2 X5R 107M 100µF 6.3V (two or three in parallel)
C2 TDK 100µF 6.3V C3325X5R0J107M 100µF 6.3V (two or three in parallel)
Figure 1: AAT1155 Evaluation Board with Adjustable Output.
peak ripple current is 1A. Assuming a 5V ±5% input
voltage and 40% ripple, the output inductance
required is:
Current Limit and Over-Temperature
Protection
Over-temperature and current limit circuitry pro-
tects the AAT1155 and the external Schottky diode
during overload, short-circuit, and excessive ambi-
ent temperature conditions. The junction over-tem-
perature threshold is 140°C nominal and has 15°C
of hysteresis. Typical graphs of the over-tempera-
ture load current vs. input voltage and ambient
temperature are shown in the Typical Character-
istics section of this document.
VOUT
IOUT · k · FSW
VOUT
VIN(MAX)
⎛
⎞
⎠
L =
· 1 -
⎝
3.3V
2.5A · 0.4 · 1MHz
⎞
⎛
⎝
⎞
⎠
⎛
⎝
3.3V
5.25V⎠
=
· 1 -
= 1.23µH
The factor "k" is the fraction of the full load (40%)
selected for the ripple current at the maximum input
voltage.
Inductor
The output inductor is selected to limit the ripple cur-
rent to 20% to 40% of the full load current at the max-
imum input voltage. Manufacturer's specifications list
both the inductor DC current rating, which is a ther-
mal limitation, and the peak current rating, which is
determined by the inductor saturation characteristics.
The inductor should not show any appreciable satu-
ration under normal load conditions. During overload
and short-circuit conditions, the inductor can exceed
its peak current rating without affecting the converter
performance. Some inductors may have sufficient
peak and average current ratings yet result in exces-
sive losses due to a high DCR. The losses associat-
ed with the DCR and its effect on the total converter
efficiency must be considered.
The corresponding inductor RMS current is:
∆I2
12
⎛
⎝
⎞
⎠
2
IRMS
=
IO
+
I = 2.5A
≈
O
∆I is the peak-to-peak ripple current which is fixed
by the inductor selection above. For a peak-to-
peak current of 40% of the full load current, the
peak current at full load will be 120% 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 70mW for a 0.84% loss in
efficiency.
For a 2.5A load and the ripple current set to 40% at
the maximum input voltage, the maximum peak-to-
10
1155.2005.11.1.6
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
