Datasheet
Data Sheet ADP2325
Rev. A | Page 21 of 32
INDUCTOR SELECTION
The inductor value is determined by the operating frequency,
input voltage, output voltage, and inductor ripple current. Using
a small inductor provides faster transient response but degrades
efficiency due to larger inductor ripple current, whereas a large
inductor value provides smaller ripple current and better effi-
ciency but results in a slower transient response. Thus, there is a
trade-off between the transient response and efficiency. As a
guideline, the inductor ripple current, ΔI
L
, is typically set to
one-third of the maximum load current. The inductor value can
be calculated by using the following equation:
( )
SW
L
OUT
IN
fI
DVV
L
×∆
×−
=
where:
V
IN
is the input voltage.
V
OUT
is the output voltage.
ΔI
L
is the inductor ripple current.
f
SW
is the switching frequency.
D is the duty cycle.
IN
OUT
V
V
D =
The ADP2325 uses adaptive slope compensation in the current
loop to prevent subharmonic oscillations when the duty cycle is
larger than 50%. The internal slope compensation limits the min-
imum inductor value.
For a duty cycle that is larger than 50%, the minimum inductor
value is determined by the following equation:
( )
SW
OUT
f
DV
×
−×
2
1
The inductor peak current is calculated by
2
L
OUT
PEAK
I
II
∆
+=
The saturation current of the inductor must be larger than the
peak inductor current. For the ferrite core inductors with a
quick saturation characteristic, the saturation current rating of the
inductor should be higher than the current-limit threshold of the
switch to prevent the inductor from entering saturation.
The rms current of the inductor can be calculated by
12
2
2
L
OUT
RMS
I
II
∆
+=
Shielded ferrite core materials are recommended for low core
loss and low EMI.
Table 9. Recommended Inductors
Vendor Part No.
Value
(µH)
I
SAT
(A)
I
RMS
(A)
DCR
(mΩ)
Sumida CDRH105RNP-0R8N 0.8 13.5 9.5 4.3
CDRH105RNP-1R5N 1.5 10.5 8.3 5.8
CDRH105RNP-2R2N
2.2
9.25
7.5
7.2
CDRH105RNP-3R3N 3.3 7.8 6.5 10.4
CDRH105RNP-4R7N 4.7 6.4 6.1 12.3
CDRH105RNP-6R8N 6.8 5.4 5.4 18
Coilcraft MSS1048-152NL 1.5 10.5 10.8 5.1
MSS1048-222NL 2.2 8.4 9.78 7.2
MSS1048-332NL 3.3 7.38 7.22 10.1
MSS1048-472NL 4.7 6.46 6.9 11.4
MSS1048-682NL 6.8 5.94 6.01 15.4
Wurth
Elektronik
7447797110
1.1
16
7.6
14
7447797180 1.8 13.3 7.3 16
7447797300 3.0 10.5 7.0 18
7447797470 4.7 8.0 5.8 27
7447797620 6.2 7.5 5.5 30
OUTPUT CAPACITOR SELECTION
The output capacitor selection affects both the output voltage
ripple and the loop dynamics of the regulator. For example,
during load step transient on the output, when the load is sud-
denly increased, the output capacitor supplies the load until the
control loop can ramp up the inductor current, which causes an
undershoot of the output voltage. Use the following equation to
calculate the output capacitance that is required to meet the voltage
droop requirement:
( )
OUT_UVOUT
IN
STEP
UV
OUT_UV
VVV
LIK
C
∆×−×
×∆×
=
2
2
where:
ΔI
STEP
is the load step.
ΔV
OUT_UV
is the allowable undershoot on the output voltage.
K
UV
is a factor, typically setting K
UV
= 2.
Another example is when a load is suddenly removed from the
output and the energy stored in the inductor rushes into the
output capacitor, which causes the output to overshoot. The
output capacitance required to meet the overshoot requirement
can be calculated using the following equation:
( )
2
2
_
2
OUTOVOUTOUT
STEP
OV
OUT_OV
VVV
LIK
C
−∆+
×∆×
=
where:
ΔV
OUT_OV
is the allowable overshoot on the output voltage.
K
OV
is a factor, typically setting K
OV
= 2.