Datasheet

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SLVS426 − MAY 2002
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19
OUTPUT CAPACITOR SELECTION
Selection of the output capacitor is basically dependent on the amount of peak-to-peak ripple voltage allowed
on the output and the ability of the capacitor to dissipate the RMS ripple current. Assuming that the ESR of the
output filter sees the entire inductor ripple current then:
V
pp
+ I
(ripple)
R
(esr)
And a suitable capacitor must be chosen so that the peak-to-peak output ripple is within the limits allowable
for the application.
OUTPUT CAPACITOR RMS CURRENT
Assuming the inductor ripple current totally goes through the output capacitor to ground, the RMS current in
the output capacitor can be calculated as:
I
O
(rms) +
I
(ripple)
12
Ǹ
where I
O
(rms) is maximum RMS current in the output capacitor (A); I
(ripple)
is the peak-to-peak inductor ripple
current (A).
Example: I
(ripple)
= 1.57 A, then, I
O
(rms) = 0.45 A
INPUT CAPACITOR RMS CURRENT
Since the SBRC3 of the TPS5130 operates 180 degree off phase against the SBRC1 and SBRC2, total RMS
current in the input capacitor is calculated as follows, assuming the input current totally goes into the input
capacitor to the power ground, and ignoring ripple current in the inductor.
When the duty cycle of the SBRC2 (D2) is over 50 %,
I
I
(rms) + (D1 I
O
1
2
) ) (D2 I
O
2
2
) ) (D3 I
O
3
2
) ) (2D1 I
O
1 I
O
2) )
(
2D2 * 1
)
I
O
2 I
O
3 * I
O
x
2
Ǹ
I
O
x + (D1 I
O
1) ) (D2 I
O
2) ) (D3 I
O
3) D2 w 0.5 w D1 w D3
I
I
(rms) is the input RMS current in the input capacitor; DX is duty cycles, defined as V
O
/V
I
in this case, of the
SBRCx.
When D2 is less than 50%,
I
I
(rms) + (D1 I
O
1
2
) ) (D2 I
O
2
2
) ) (D3 I
O
3
2
) ) (2D1 I
O
1 I
O
2) * I
O
x
2
Ǹ
Example: VIN = 12 V, V
O
1 = 3.3 V, V
O
2 = 5 V (D2 = 0.42), V
O
3 = 1.8V, I
O
1 = I
O
2 = 4 A, I
O
3 = 6 A
Then, I
I
(rms) = 3.44 A
On the contrary, if three SBRCs operate in a same phase the RMS current is calculated as follows.
I
I
(rms) + (D1 I
O
1
2
) ) (D2 I
O
2
2
) ) (D3 I
O
3
2
) ) (2D1 I
O
1 I
O
2) ) (2D3 I
O
3)
ǒ
I
O
1 ) I
O
2
Ǔ
* I
O
x
2
Ǹ
Under the same operation condition, I
I
(rms) = 5.13 A
Therefore, 180 degree out of phase operation is effective in reducing input RMS current, and it allows a smaller
input capacitance value. The input capacitors must be chosen so that together they can safely handle the input
ripple current. Depending on the input filtering and the dc input voltage source, not all the ripple current flows
through the input capacitors, but some may be present on the input leads to the EVM.