Datasheet

ADP2323 Data Sheet
Rev. A | Page 20 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 leads to a faster transient response but degrades
efficiency due to larger inductor ripple current, whereas a large
inductor value leads to 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 1/3
of the maximum load current. The inductor value can be
calculated using the following equation:
( )
IN OUT
L SW
VV D
L
If
−×
=
∆×
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 ADP2323 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
minimum inductor value.
For a duty cycle that is larger than 50%, the minimum inductor
value is determined by the following equation:
( )
1
2
OUT
SW
VD
f
×−
×
The inductor peak current is calculated using the following
equation:
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 getting into saturation.
The rms current of the inductor can be calculated by the
following equation:
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-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 5.8
MSS1048-222NL 2.2 8.4 9.78 7.2
MSS1048-332NL 3.3 7.38 7.22 10.4
MSS1048-472NL 4.7 6.46 6.9 12.3
MSS1048-682NL 6.8 5.94 6.01 18
Wurth
Elektronik
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
suddenly increased, the output capacitor supplies the load until
the control loop has a chance to 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:
( )
UVOUTOUT
IN
P
STE
UV
UVOUT
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 case 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
_
22
_
()
OV STEP
OUT OV
OUT OUT OV OUT
KI L
C
VV V
×∆ ×
=
+∆ −
where:
ΔV
OUT_OV
is the allowable overshoot on the output voltage.
K
OV
is a factor, typically setting K
OV
= 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:
RIPPLEOUT
SW
L
RIPPLEOUT
Vf
I
C
_
_
8 ∆××
∆
=
L
RIPPLEOUT
ESR
I
V
R
∆
∆
=
_