AAT1146 Analogic Corporation, AAT1146 Datasheet - Page 11

• Manufacturer Part Number: AAT1146
• Manufacturer: Analogic Corporation

Applications Information
Inductor Selection
The step-down converter uses peak current mode
control with slope compensation to maintain stability
for duty cycles greater than 50%. The output induc-
tor value must be selected so the inductor current
down slope meets the internal slope compensation
requirements. The internal slope compensation for
the adjustable and low-voltage fixed versions of the
AAT1146 is 0.24A/µsec. This equates to a slope
compensation that is 75% of the inductor current
down slope for a 1.5V output and 4.7µH inductor.
This is the internal slope compensation for the
adjustable (0.6V) version or low-voltage fixed ver-
sions. When externally programming the 0.6V ver-
sion to 2.5V, the calculated inductance is 7.5µH.
In this case, a standard 6.8µH value is selected.
For high-voltage fixed versions (≥2.5V), m = 0.48A/
µsec. Table 1 displays inductor values for the
AAT1146 fixed and adjustable options.
Manufacturer's specifications list both the inductor
DC current rating, which is a thermal limitation, and
the peak current rating, which is determined by the
saturation characteristics. The inductor should not
1146.2006.01.1.2
L =
0.6V Adjustable With
m =
External Feedback
= 3
Configuration
Fixed Output
0.75 ⋅ V
0.75 ⋅ V
µsec
A
m
L
⋅ 2.5V = 7.5µH
O
O
=
=
0.24A
0.75 ⋅ 1.5V
0.75 ⋅ V
4.7µH
µsec
A
O
≈
= 0.24
3
µsec
A
µsec
Table 1: Inductor Values.
Output Voltage
A
⋅ V
Fast Transient 400mA Step-Down Converter
0.6V to 3.3V
O
1.5V, 1.8V
2.5V, 3.3V
1V, 1.2V
show any appreciable saturation under normal load
conditions. Some inductors may meet the peak and
average current ratings yet result in excessive loss-
es due to a high DCR. Always consider the losses
associated with the DCR and its effect on the total
converter efficiency when selecting an inductor.
The 4.7µH CDRH3D16 series inductor selected
from Sumida has a 105mΩ DCR and a 900mA DC
current rating. At full load, the inductor DC loss is
17mW which gives a 2.8% loss in efficiency for a
400mA, 1.5V output.
Input Capacitor
Select a 4.7µF to 10µF X7R or X5R ceramic capac-
itor for the input. To estimate the required input
capacitor size, determine the acceptable input rip-
ple level (V
value varies with input voltage and is a maximum
when V
Always examine the ceramic capacitor DC voltage
coefficient characteristics when selecting the prop-
er value. For example, the capacitance of a 10µF,
6.3V, X5R ceramic capacitor with 5.0V DC applied
is actually about 6µF.
IN
V
V
is double the output voltage.
IN
O
PP
C
· 1 -
C
) and solve for C. The calculated
IN(MIN)
⎛
⎝
IN
=
V
V
=
⎛
⎝
IN
O
V
V
V
⎛
⎝
⎞
⎠
I
IN
O
O
V
PP
I
=
O
PP
· 1 -
Inductor
- ESR · F
1
4
⎛
⎝
- ESR · 4 · F
2.2µH
4.7µH
6.8µH
4.7µH
for V
1
V
V
⎞
⎠
IN
O
AAT1146
IN
⎞
⎠
⎞
⎠
= 2 × V
S
S
O
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