AAT2505
Dual Channel, Step-Down
Converter/Linear Regulator
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.
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 (VPP) and solve for C. The calculated
value varies with input voltage and is a maximum
when VIN is double the output voltage.
0.75 ⋅ VO
0.75
⋅
VO
A
μsec
A
L =
=
≈
3
⋅ VO
m
0.24A
μsec
V
⎛
VOBUCK
VIN
⎞
μsec
OBUCK · 1
VIN
-
= 3
⋅ 2.5V = 7.5μH
⎝
⎠
A
CIN =
⎛
⎝
VPP
IOBUCK
⎞
⎠
- ESR · FS
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.
VOBUCK
VIN
⎛
· 1
⎝
VOBUCK
VIN
⎞
1
4
-
=
for VIN = 2 × VOBUCK
⎠
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.
1
CIN(MIN)
=
⎛
⎝
VPP
IOBUCK
⎞
⎠
- ESR
·
4
·
FS
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:
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.
VOBUCK
VIN
⎛
· 1
⎝
VOBUCK
VIN
⎞
IRMS = IOBUCK
·
-
⎠
Configuration
Output Voltage
Inductor
2.2µH
1V, 1.2V
1.5V, 1.8V
2.5V, 3.3V
0.6V to 3.3V
0.6V Adjustable With
External Feedback
4.7µH
6.8µH
Fixed Output
4.7µH
Table 1: Inductor Values.
2505.2006.06.1.1
15
ANALOGICTECH [ ADVANCED ANALOGIC TECHNOLOGIES ]
