Datasheet

DI
L
+ Vout
1 *
Vout
Vin
L ƒ
I
Lmax
+ I
outmax
)
DI
L
2
I
RMSCout
+ Vout
1 *
Vout
Vin
L ƒ
1
2 3
Ǹ
TPS62420-Q1
www.ti.com SLVSA56 –DECEMBER 2009
Inductor Selection
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 6 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 7. This is
recommended because during heavy load transient the inductor current will rise above the calculated value.
(6)
(7)
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 5 and the typical applications for possible inductors.
Table 5. List of Inductors
DIMENSIONS [mm
3
] INDUCTOR TYPE SUPPLIER
3.2×2.6×1.0 MIPW3226 FDK
3×3×0.9 LPS3010 Coilcraft
2.8×2.6×1.0 VLF3010 TDK
2.8x2.6×1.4 VLF3014 TDK
3×3×1.4 LPS3015 Coilcraft
3.9×3.9×1.7 LPS4018 Coilcraft
Output Capacitor Selection
The advanced fast response voltage mode control scheme of the two converters allows 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 X7R/X5R capacitors having low ESR values result in lowest output voltage ripple and
are therefore recommended.
If ceramic output capacitors are used, the capacitor RMS ripple current rating will always meet the application
requirements. The RMS ripple current is calculated as:
(8)
At nominal load current the inductive converters operate in PWM mode and the overall output voltage ripple is
the sum of the voltage spike caused by the output capacitor ESR plus the voltage ripple caused by charging and
discharging the output capacitor:
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