OUTPUT-CAPACITOR SELECTION
O
I
RMS
O
V
1
V
1
I
(C )
V
L f
2
3
-
=
(4)
O
I
O
ESR
O
V
1
V
1
V
R
L
f
8
C
f
-
D
=
+
÷
è
(5)
INPUT-CAPACITOR SELECTION
O
RMS
O
I
V
I
I max
1
V
=
-
÷
è
(6)
FEEDFORWARD-CAPACITOR SELECTION
RECOMMENDED CAPACITORS
www.ti.com .................................................................................................................................................... SLVS585A – JULY 2005 – REVISED FEBRUARY 2009
A 22-
F (typical) output capacitor is needed with a 6.8-H inductor. For an output voltage greater than 5 V, a
33-
F (minimum) output capacitor is required for stability. For best performance, a low-ESR ceramic output
capacitor is needed.
Just for completeness, the RMS ripple current is calculated as:
The overall output ripple voltage is the sum of the voltage spike caused by the output capacitor ESR plus the
voltage ripple caused by charging and discharging the output capacitor:
where the highest output-voltage ripple occurs at the highest input voltage VI.
The nature of the buck converter is a pulsating input current; therefore, a low ESR input capacitor is required for
best input voltage filtering and for minimizing the interference with other circuits caused by high input-voltage
spikes. The input capacitor should have a minimum value of 10 F 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:
The worst-case RMS ripple current occurs at D = 0.5 and is calculated as: IRMS = IO/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.
The feedforward capacitor (Cff) is needed to compensate for parasitic capacitance from the feedback pin to GND.
Typically, a value of 4.7 pF to 22 pF is needed for an output voltage divider with a equivalent resistance (R1 in
parallel with R2) in the 150-k
range. The value can be chosen based on best transient performance and lowest
output-voltage ripple in PFM mode.
It is recommended that only X5R or X7R ceramic capacitors be used as input/output capacitors. Ceramic
capacitors show a dc-bias effect. This effect reduces the effective capacitance when a dc-bias voltage is applied
across a ceramic capacitor, as on the output and input capacitor of a dc/dc converter. The effect may lead to a
significant capacitance drop, especially for high input/output voltages and small capacitor packages. See the
manufacturer's data sheet about the performance with a dc bias voltage applied. It may be necessary to choose
a higher voltage rating or nominal capacitance value in order to get the required value at the operating point. The
capacitors listed in
Table 3 have been tested with the TPS62110 with good performance.
Table 3. List of Capacitors
MANUFACTURER
PART NUMBER
SIZE
VOLTAGE
CAPACITANCE
TYPE
TMK316BJ106KL
1206
25 V
10 F
Taiyo Yuden
Ceramic
EMK325BJ226KM
1210
16 V
22 F
Copyright 2005–2009, Texas Instruments Incorporated
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