Datasheet

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×=D ESR
fCoutfL
Vin
Vout
VoutVout
8
1
1
32
1
1
×
×
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×=
fL
Vin
Vout
VoutI
RMSCout
TPS65217A, TPS65217B, TPS65217C, TPS65217D
www.ti.com SLVSB64F –NOVEMBER 2011–REVISED APRIL 2013
where:
f = Switching frequency (2.25 MHz 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 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. Also the resistance of the windings will
greatly affect the converter efficiency at high load. Please refer to Table 1 for recommended inductors.
Table 1. Recommended Inductors for DCDC1, 2, and 3
PART NUMBER SUPPLIER VALUE (µH) R
DS
(mΩ) MAX RATED CURRENT (A) DIMENSIONS (mm)
LQM2HPN2R2MG0L Murata 2.2 100 1.3 2 x 2.5 x 0.9
VLCF4018T-2R2N1R4-2 TDK 2.2 60 1.44 3.9 x 4.7 x 1.8
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 10 µF, 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.
If ceramic output capacitors are used, the capacitor RMS ripple current rating must always meet the application
requirements. For completeness the RMS ripple current is calculated as:
(5)
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:
(6)
Where the highest output voltage ripple occurs at the highest input voltage V
IN
.
At light load currents the converters operate in Power Save Mode and the output voltage ripple is dependent on
the output capacitor value. The output voltage ripple is set by the internal comparator delay and the external
capacitor. The typical output voltage ripple is less than 1% of the nominal output voltage.
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