Datasheet

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SGLS243A − APRIL 2004 − REVISED JUNE 2008
16
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
output capacitor selection (continued)
If an output capacitor is selected with an ESR value ≤ 120 mΩ, its RMS ripple current rating always meets the
application requirements. The RMS ripple current is calculated as:
I
RMS(C
o
)
+ V
O
1–
V
O
V
I
L ƒ
1
2 3
Ǹ
The overall output ripple voltage is the sum of the voltage spike caused by the output capacitor ESR plus the
voltage ripple caused by charge and discharging the output capacitor:
DV
O
+ V
O
1–
V
O
V
I
L ƒ
ǒ
1
8 C
o
ƒ
) ESR
Ǔ
Where the highest output voltage ripple occurs at the highest input voltage V
I
.
Table 3. Tested Capacitors
CAPACITOR VALUE ESR/mΩ COMPONENT SUPPLIER COMMENTS
10 µF 50 Taiyo Yuden
JMK316BJ106KL
Ceramic
47 µF 100 Sanyo 6TPA47M POSCAP
68 µF 100 Spraque
594D686X0010C2T
Tantalum
input capacitor selection
Because of the nature of the buck converter having a pulsating input current, a low ESR input capacitor is
required for best input voltage filtering and minimizing the interference with other circuits caused by high input
voltage spikes.
The input capacitor should have a minimum value of 10 µF and can be increased without any limit for better input
voltage filtering.
The input capacitor should be rated for the maximum input ripple current calculated as:
I
RMS
+ I
O(max)
V
O
V
I
ǒ1–
V
O
V
I
Ǔ
Ǹ
The worst case RMS ripple current occurs at D = 0.5 and is calculated as: I
RMS
+
I
O
2
.
Ceramic capacitors show a good performance because of their low ESR value and they are less sensitive
against voltage transients compared to tantalum capacitors.
Place the input capacitor as close as possible to the input pin of the IC for best performance.