AAT2500M
400mA Step-Down Converter and 300mA LDO
Equivalent Series Resistance: ESR is a very
important characteristic to consider when selecting a
capacitor. ESR is the internal series resistance asso-
ciated with a capacitor that includes lead resistance,
internal connections, size and area, material compo-
sition, and ambient temperature. Typically, capacitor
ESR is measured in milliohms for ceramic capacitors
and can range to more than several ohms for tanta-
lum or aluminum electrolytic capacitors.
0.35 ⋅ VO
0.35
⋅
VO
A
µsec
⋅ VO
A
L =
=
≈
1.5
m
0.24A
µsec
µsec
A
= 1.5
⋅ 2.5V = 3.75µH
In this case, a standard 4.7ꢀH value is selected.
For high output voltage fixed versions (2.5V and
above), m = 0.48A/ꢀsec. Table 1 displays inductor
values for the AAT2500M fixed and adjustable
options.
Step-Down Converter
Inductor Selection: The step-down converter
uses peak current mode control with slope com-
pensation to maintain stability for duty cycles
greater than 50%. The output inductor value must
be selected so the inductor current down slope
meets the internal slope compensation require-
ments. The internal slope compensation for the
adjustable and low-voltage fixed versions of the
AAT2500M is 0.24A/ꢀsec. This equates to a slope
compensation that is 35% of the inductor current
down slope for a 1.5V output and 2.2ꢀH inductor.
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 2.2ꢀH CDRH3D16 series inductor selected
from Sumida has a 59mΩ DCR and a 1.3A DC cur-
rent rating. At full load, the inductor DC loss is
9.4mW which gives a 1.5% loss in efficiency for a
400mA, 1.5V output.
0.35 ⋅ VO 0.35 ⋅ 1.5V
= 0.24
A
µsec
m =
=
L
2.2µH
This is the internal slope compensation for the
adjustable (VO = 0.6V) version or low output volt-
age fixed versions. When externally programming
the 0.6V version to 2.5V, the calculated inductance
is 3.75ꢀH.
Configuration
Output Voltage
0.6V to 2.0V
2.5V
Inductor
2.2ꢀH
Slope Compensation
0.24A/ꢀsec
0.6V Adjustable With
External Resistive Divider
4.7ꢀH
0.24A/ꢀsec
0.6V to 2.0V
2.5V to 3.3V
2.2ꢀH
0.24A/ꢀsec
Fixed Output
2.2ꢀH
0.48A/ꢀsec
Table 1: Inductor Values.
2500M.2007.06.1.0
13
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