AAT1155 AAT, AAT1155 Datasheet - Page 10

• Manufacturer Part Number: AAT1155
• Description: Step-Down DC-DC Converter
• Manufacturer: AAT

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.
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.
For a 2.5A load and the ripple current set to 40% at
the maximum input voltage, the maximum peak-to-
10
Figure 1: AAT1155 Evaluation Board with Adjustable Output.
100µF
Vin 2.7V-5.5V
C4
rtn
10µF
C1
R1
100
R2
100k
C1 Murata 10µF 6.3V X5R GRM42-6X5R106K6.3
C2 Vishay 120µF 6.3V 594D127X96R6R3C2T
C3 0. 1 µF 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)
C3
0.1µF
R4
10.0k
R3
2.55k
1MHz 2.5A Step-Down DC/DC Converter
AAT1155-1.0
FB
GND
EN
VCC
U1
LX
LX
VP
VP
peak ripple current is 1A. Assuming a 5V ±5% input
voltage and 40% ripple, the output inductance
required is:
The factor "k" is the fraction of the full load (40%)
selected for the ripple current at the maximum input
voltage.
The corresponding inductor RMS current is:
∆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.
L =
=
= 1.23µH
⎛
⎝
V
B340LA
OUT
2.5A · 0.4 · 1MHz
D1
I
I
RMS
OUT
1.25V @ 2.5A
L1
1.5µH
V
=
· k · F
C2
120µF
OUT
3.3V
⎛
⎝
I
O
2
SW
+
· 1 -
∆
12
⎛
⎝
I
2
⎞
⎠
⎞
⎠
≈
V
V
· 1 -
IN(MAX)
I
OUT
O
AAT1155
=
⎛
⎝
2.5A
⎞
⎠
5.25V
3.3V
1155.2005.11.1.6
⎞
⎠