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V
(DEFDCDC2)
V
O
R3 =R4 x -R4
()
R4
R3+R4
V =V
O (DEFDCDC2)
x
(5)
Output Filter Design (Inductor and Output Capacitor)
Inductor Selection
L x ¦
1-
V
I
2
I (max)=I (max)+
L O
DI =V x
L O
V
O
DI
L
(6)
TPS65050, TPS65051, TPS65052
TPS65054, TPS65056
SLVS710A – JANUARY 2007 – REVISED AUGUST 2007
V
(DEFDCDC2)
= 0.6 V
See Table 3 for typical resistor values:
Table 3. Typical Resistor Values
OUTPUT VOLTAGE R1 R2 NOMINAL VOLTAGE Typical CFF
3.3 V 680 k Ω 150 k Ω 3.32 V 47 pF
3 V 510 k Ω 130 k Ω 2.95 V 47 pF
2.85 V 560 k Ω 150 k Ω 2.84 V 47 pF
2.5 V 510 k Ω 160 k Ω 2.51 V 47 pF
1.8 V 300 k Ω 150 k Ω 1.8 v 47 pF
1.6 V 200 k Ω 120 k Ω 1.6 V 47 pF
1.5 V 300 k Ω 200 k Ω 1.5 V 47 pF
1.2 V 330 k Ω 330 k Ω 1.2 V 47 pF
The two converters operate with 2.2- μ H 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 directly influences the efficiency of
the converter. Therefore, an inductor with lowest dc resistance should be selected for highest efficiency. The
minimum inductor value is 1.5 μ H, but an output capacitor of 22 μ F minimum is needed in this case. For an
output voltage above 2.8 V, an inductor value of 3.3 μ H minimum is recommended. Lower values result in an
increased output voltage ripple in PFM mode.
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 6 . This is
recommended because during heavy load transient the inductor current rises above the calculated value.
with:
• f = Switching Frequency (2.25-MHz typical)
• L = Inductor Value
• Δ I
L
= Peak-to-peak inductor ripple current
• I
L
max = Maximum Inductor current
The highest inductor current occurs at maximum V
I
. Open core inductors have a soft saturation characteristic,
and they can normally 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. Consideration must be given to the difference in the core material from inductor to
inductor which has an impact on the efficiency especially at high switching frequencies. See Table 4 and the
typical applications for possible inductors.
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