Datasheet

ADP2384 Data Sheet
Rev. 0 | Page 16 of 24
For a duty cycle that is larger than 50%, the minimum inductor
value is determined using the following equation:
L (Minimum) =
SW
OUT
f
DV
×
−×
2
)1(
The peak inductor current is calculated by
I
PEAK
= I
OUT
+
2
L
I∆
The saturation current of the inductor must be larger than the peak
inductor current. For 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. This
prevents the inductor from reaching saturation.
The rms current of the inductor is calculated as follows:
I
RMS
=
12
2
2
L
OUT
I
I
∆
+
Shielded ferrite core materials are recommended for low core
loss and low EMI. Table 7 lists some recommended inductors.
OUTPUT CAPACITOR SELECTION
The output capacitor selection affects the output ripple voltage
load step transient and the loop stability of the regulator.
For example, during a load step transient where the load is
suddenly increased, the output capacitor supplies the load until
the control loop can ramp up the inductor current. The delay
caused by the control loop causes output undershoot. The
output capacitance that is required to satisfy the voltage droop
requirement can be calculated using the following equation:
C
OUT_UV
=
UVOUTOUTIN
STEPUV
VVV
LIK
_
2
)(2 ∆×−×
×∆×
where:
K
UV
is a factor, with a typical setting of K
UV
= 2.
ΔI
STEP
is the load step.
ΔV
OUT_UV
is the allowable undershoot on the output voltage.
Another example occurs when a load is suddenly removed from
the output, and the energy stored in the inductor rushes into
the output capacitor, causing the output to overshoot.
The output capacitance that is required to meet the overshoot
requirement can be calculated using the following equation:
C
OUT_OV
=
2
2
_
2
)(
OUTOVOUTOUT
STEPOV
VVV
LIK
−∆+
×∆×
where:
ΔV
OUT_OV
is the allowable overshoot on the output voltage.
K
OV
is a factor, with a typical setting of K
OV
= 2.
The output ripple is determined by the ESR and the value of the
capacitance. Use the following equation to select a capacitor
that can meet the output ripple requirements:
C
OUT_RIPPLE
=
RIPPLEOUTSW
L
Vf
I
_
8 ∆××
∆
R
ESR
=
L
RIPPLEOUT
I
V
∆
∆
_
where:
ΔV
OUT_RIPPLE
is the allowable output ripple voltage.
R
ESR
is the equivalent series resistance of the output capacitor
in ohms (Ω).
Table 7. Recommended Inductors
Vendor Part No. Value (µH) I
SAT
(A) I
RMS
(A) DCR (mΩ)
Toko FDVE1040-1R5M 1.5 13.7 14.6 4.6
FDVE1040-2R2M 2.2 11.4 11.6 6.8
FDVE1040-3R3M 3.3 9.8 9.0 10.1
FDVE1040-4R7M 4.7 8.2 8.0 13.8
FDVE1040-6R8M 6.8 7.1 7.1 20.2
FDVE1040-100M
10
6.1
5.2
34.1
Vishay IHLP4040DZ-1R0M-01 1.0 36 17.5 4.1
IHLP4040DZ-1R5M-01 1.5 27.5 15 5.8
IHLP4040DZ-2R2M-01 2.2 25.6 12 9
IHLP4040DZ-3R3M-01 3.3 18.6 10 14.4
IHLP4040DZ-4R7M-01 4.7 17 9.5 16.5
IHLP4040DZ-6R8M-01 6.8 13.5 8.0 23.3
IHLP4040DZ-100M-01 10 12 6.8 36.5
Wurth Elektronik 744325120 1.2 25 20 1.8
744325180 1.8 18 16 3.5
744325240 2.4 17 14 4.75
744325330 3.3 15 12 5.9
744325420 4.2 14 11 7.1
744325550 5.5 12 10 10.3