Data Sheet
ADP2325
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.
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:
For a duty cycle that is larger than 50%, the minimum inductor
value is determined by the following equation:
VOUT
1 D
2 fSW
The inductor peak current is calculated by
K
UV ISTEP 2 L
VIN VOUT VOUT_UV
COUT_UV
IL
IPEAK IOUT
2
2
where:
ΔISTEP is the load step.
ΔVOUT_UV is the allowable undershoot on the output voltage.
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.
KUV is a factor, typically setting KUV = 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:
The rms current of the inductor can be calculated by
2
IL
12
IRMS
IOUT 2
K
OV ISTEP2 L
Shielded ferrite core materials are recommended for low core
loss and low EMI.
COUT_OV
2 VOUT
2
VOUT VOUT _OV
where:
Table 9. Recommended Inductors
ΔVOUT_OV is the allowable overshoot on the output voltage.
KOV is a factor, typically setting KOV = 2.
Value ISAT
(A)
IRMS
(A)
DCR
(mΩ)
Vendor
Part No.
(μH)
0.8
1.5
2.2
3.3
4.7
6.8
1.5
2.2
3.3
4.7
6.8
1.1
1.8
3.0
4.7
6.2
The output ripple is determined by the ESR of the output
capacitor and its capacitance value. Use the following equation to
select a capacitor that can meet the output ripple requirements:
Sumida
CDRH105RNP-0R8N
CDRH105RNP-1R5N
CDRH105RNP-2R2N
CDRH105RNP-3R3N
CDRH105RNP-4R7N
CDRH105RNP-6R8N
MSS1048-152NL
MSS1048-222NL
MSS1048-332NL
MSS1048-472NL
MSS1048-682NL
7447797110
13.5 9.5
10.5 8.3
9.25 7.5
7.8
6.4
5.4
4.3
5.8
7.2
10.4
12.3
18
6.5
6.1
5.4
IL
COUT_RIPPLE
8 fSW VOUT_RIPPLE
Coilcraft
10.5 10.8 5.1
8.4 9.78 7.2
7.38 7.22 10.1
6.46 6.9 11.4
5.94 6.01 15.4
VOUT_RIPPLE
IL
RESR
where:
ΔVOUT_RIPPLE is the allowable output voltage ripple.
RESR is the equivalent series resistance of the output capacitor.
Wurth
Elektronik
16
7.6
14
16
18
27
30
7447797180
7447797300
7447797470
7447797620
13.3 7.3
10.5 7.0
8.0
7.5
Select the largest output capacitance given by COUT_UV, COUT_OV
and COUT_RIPPLE to meet both load transient and output ripple
performance.
,
5.8
5.5
The selected output capacitor voltage rating must be greater
than the output voltage. The minimum rms current rating of
the output capacitor is determined by the following equation:
IL
12
IC
OUT _rms
Rev. 0 | Page 21 of 32
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