Datasheet

f
OUT
IN
L OUT
V
1 -
V
I = V x
L x
D
L
Lmax OUTmax
I
I = I +
2
D
C
1
f
OUT
OUT
= with L = 2.2 H, C = 10 F
2 L x C
m m
p
TPS65000, TPS65001, TPS650001
TPS650003, TPS650006, TPS650061
www.ti.com SLVS810B –JUNE 2009–REVISED AUGUST 2010
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
Inductor Selection
The typical value for the converter inductor is 2.2mH output inductor. Larger or smaller inductor values in the
range of 1.5mH to 3.3mH can be used to optimize the performance of the device for specific operation conditions.
The selected inductor has to be rated for its dc resistance and saturation current. The dc resistance of the
inductance will influence directly the efficiency of the converter. Therefore, an inductor with lowest dc resistance
should be selected for highest efficiency. See SLVA157 for more information on inductor selection.
Equation 5 calculates the maximum inductor current under static load conditions. The saturation current of the
inductor should be rated higher than the maximum inductor current as calculated with Equation 6. This is
recommended because during heavy load transient the inductor current will rise above the calculated value.
(5)
(6)
With:
f = Switching Frequency (2.25MHz typical)
L = Inductor Value
ΔI
L
= Peak to Peak inductor ripple current
I
Lmax
= Maximum Inductor current
The highest inductor current will occur at maximum V
IN
.
Open core inductors have a soft saturation characteristic, and can usually handle higher inductor currents versus
a comparable shielded inductor.
A more conservative approach is to select the inductor current rating just for the maximum switch current of the
corresponding converter. It must be considered, that the core material from inductor to inductor differs and will
have an impact on the efficiency especially at high switching frequencies.
Notice that the step down converter has internal loop compensation. As the internal loop compensation is
designed to work with a certain output filter corner frequency calculated as follows:
(7)
This leads to the fact the selection of external L-C filter has to be coped with the above formula. The product of
L × C
OUT
should be constant while selecting smaller inductor or increasing output capacitor value.
See Table 2 , and the typical applications for possible inductors.
Table 2. INDUCTORS
INDUCTOR TYPE Inductance mH SUPPLIER Max Dimensions (mm)
MIPS2520D2R2 2.0 FDK 2.5 × 2.0 × 1.0
MIPSA2520D2R2 2.0 FDK 2.5 × 2.0 × 1.2
KSLI-252010AG2R2 2.2 Htachi Metals 2.5 × 2.0 × 1.0
LQM2HPN2R2MJ0L 2.2 Murata 2.5 × 2.0 × 1.2
LPS15222 2.2 Coilcraft 3.0 × 3.0 × 1.5
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