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SLUU195 − June 2004
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TPS40090 Multi-Phase Buck Converter and TPS2834 Drivers Steps-Down from 12-V to 1.5-V at 100 A
4.2 Inductance Value
The output inductor value for each phase can be calculated from the volt-second during off time,
shown in equation (2).
L +
V
OUT
f I
RIPPLE
ǒ1 *
V
OUT
V
IN(max)
Ǔ
where
• I
RIPPLE
is usually chosen to be between 10% and 40% of maximum phase current I
PH(max)
.
With I
RIPPLE
= 20% of I
PH(max)
, there is a ripple current of 5 A, and the inductance value is found
to be 0.63 µH. Using SPM12550−R62M300 inductors from TDK, each had inductance of 0.6µH
and resistance of 1.75-mΩ.
In multi-phase high current buck converter design, due to the ripple cancellation factor from
interleaving, the inductor value could be smaller than that in a single phase operation. But from
conduction loss point of view, the inductor value tends to be big to reduce the ripple current, thus
losses.
4.3 Input Capacitor Selection
The bulk input capacitor selection is based on the input voltage ripple requirements. Due to the
interleaving of multi phase, the input RMS current is reduced. The input ripple current RMS
value over load current is calculated in equation (3).
D I
IN(nom)
ǒ
N
PH
,D
Ǔ
+
ȧ
ȱ
Ȳ
ǒ
D *
k
ǒ
N
PH
,D
Ǔ
N
PH
Ǔ
ǒ
k
ǒ
N
PH
,D
Ǔ
) 1
N
PH
* D
Ǔ
ȧ
ȳ
ȴ
)
ǒ
N
PH
12 D
2
Ǔ
ƪ
V
OUT
(
1 * D
)
L f
ǒ
I
OUT
Ǔ
ƫ
2
ȧ
ȱ
Ȳ
ǒ
k
ǒ
N
PH
,D
Ǔ
) 1
Ǔ
2
ǒ
D *
k
ǒ
N
PH
,D
Ǔ
N
PH
Ǔ
3
) k
ǒ
N
PH
,D
Ǔ
2
ǒ
k
ǒ
N
PH
,D
Ǔ
) 1
N
PH
* D
Ǔ
3
ȧ
ȳ
ȴ
where
• k
ǒ
N
PH
,D
Ǔ
+ floor
ǒ
N
PH
D
Ǔ
• floor(x) is the function to return the greatest integer less than N
PH
× D
• N
PH
is the number of active phases
Figure 4 shows the input ripple current RMS value over the load current versus duty cycle with
different number of active phases.
(2)
(3)