Texas Instruments TPS40090EVM-002 - Manual (Page 9)

SLUU195 − June 2004
9 TPS40090 Multi-Phase Buck Converter and TPS2834 Drivers Steps-Down from 12-V to 1.5-V at 100 A
0
0.1
0.2
0.5
0.6
0.4
0.3
0 10 20 30 40 50 60 70 80 90 100
Duty Cycle − %
I RMS_CIN(nom)
−
Normalized RMS Input − A
NPH = 1
NPH = 2
NPH = 3
NPH = 4
NPH = 6
Figure 4. Input Ripple Current RMS Value Overload Current
The maximum input ripple RMS current can be estimated as shown in (4).
I
^
I
OUT
D
I
IN(nom)
ǒ
4, D
min
Ǔ
+
3.18 A
It is also important to consider a minimum capacitance value which limits the voltage ripple to a
specified value if all the current is supplied by the onboard capacitor. For a typical ripple voltage
of 150 mV the maximum ESR is calculated in (5) as:
ESR
+ D V
D
I
+
150 mV
3.18 A
+
47 m
W
Two 68-
µ
F, 20-V Oscon capacitors (20SVP68M) from Sanyo are placed on the input side of the
board. The ESR is 40 m
Ω
for each capacitor.
4.4 Output Ripple Cancellation and Capacitor Selection
Due to the interleaving of channels, the total output ripple current is smaller than the ripple
current from a single phase. The ripple cancellation factor is expressed in equation (6).
D
I
OUT
ǒ
N
PH
, D
Ǔ
+
ǒ
N
PH P
i
+
1
Ť
i
*
N
PH
D
Ť
Ǔ
ƪ
N
PH
* 1
P
i
+
1
ǒŤ
i
*
N
PH
D
Ť
)
1
Ǔ
ƫ
k
ǒ
N
PH
, D
Ǔ
+
if
ǒ
N
PH
v
1,
D
I
OUT
(D),
D
I
OUT
ǒ
N
PH
, D
Ǔ Ǔ
(4)
(5)
(6)