Datasheet

L
Lmax outmax
I
I = I +
2
D
L
Vout
1
Vin
I = Vout
L
-
D ´
´ ¦
TPS65070, TPS65072, TPS65073
TPS650731, TPS650732, TPS650701, TPS650702, TPS650721
www.ti.com SLVS950G –JULY 2009–REVISED MAY 2013
APPLICATION INFORMATION
STEP-DOWN CONVERTERS
OUTPUT FILTER DESIGN (INDUCTOR AND OUTPUT CAPACITOR)
Inductor Selection
The step-down converters operate typically with 2.2μH output inductor. Larger or smaller inductor values 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.
Equation 4 can be used to calculate 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 4.
This is recommended because during heavy load transient the inductor current will rise above the calculated
value.
(4)
(5)
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 Vin.
Open core inductors have a soft saturation characteristic and they 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.
Refer to Table 4 and the typical applications for possible inductors.
Table 4. Tested Inductors
INDUCTOR TYPE RECOMMENDED INDUCTOR VALUE SUPPLIER
MAXIMUM DC
CURRENT
LPS3010 0.6 A 2.2 μH Coilcraft
LPS3015 1.2 A 2.2 μH Coilcraft
LPS4018 1.5 A 2.2 μH Coilcraft
VLCF4020 1.5 A 2.2 μH TDK
Output Capacitor Selection
The advanced Fast Response voltage mode control scheme of the two converters allow the use of small ceramic
capacitors with a typical value of 10μF, without having large output voltage under and overshoots during heavy
load transients. Ceramic capacitors having low ESR values result in lowest output voltage ripple and are
therefore recommended. Please refer to for recommended components.
If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application
requirements. Just for completeness the RMS ripple current is calculated as:
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