AAT2505 Analogic Corporation, AAT2505 Datasheet - Page 15

• Manufacturer Part Number: AAT2505
• Manufacturer: Analogic Corporation

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 and above),
m = 0.48A/µsec. Table 1 displays inductor values
for the AAT2505 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
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.
2505.2006.06.1.1
L =
0.6V Adjustable With
External Feedback
= 3
Configuration
Fixed Output
0.75 ⋅ V
μsec
A
m
⋅ 2.5V = 7.5μH
O
=
0.24A
0.75 ⋅ V
μsec
A
O
≈
3
μsec
A
Table 1: Inductor Values.
Output Voltage
⋅ V
0.6V to 3.3V
O
1.5V, 1.8V
2.5V, 3.3V
1V, 1.2V
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.
The maximum input capacitor RMS current is:
V
OBUCK
V
I
IN
RMS
C
IN
C
IN(MIN)
IN
Converter/Linear Regulator
is double the output voltage.
· 1 -
= I
=
Dual Channel, Step-Down
⎛
⎝
PP
OBUCK
=
V
) and solve for C. The calculated
⎛
⎝
OBUCK
V
⎛
⎝
I
V
OBUCK
V
IN
I
OBUCK
OBUCK
V
V
·
PP
IN
PP
· 1 -
V
- ESR · F
⎞
⎠
⎛
⎝
OBUCK
V
- ESR · 4 · F
=
IN
1
4
Inductor
V
1
⎞
⎠
OBUCK
2.2µH
4.7µH
6.8µH
4.7µH
V
for V
· 1 -
⎞
⎠
IN
⎛
⎝
S
⎞
⎠
IN
V
= 2 × V
OBUCK
S
V
IN
⎞
⎠
OBUCK
15