Datasheet

)2/(1
max_min_ COOO
FRC ´´´= p
ú
û
ù
ê
ë
é
+
´´
-
=
E S R
OSW
LP PO P P
R
CF
D
IV
4
)5.0(
( )
OPPMAX
max
LPP SW O
D 0.5
V
ESR =
I 4 F C
-
-
´ ´
( )
12
OUT IN(MAX) OUT
COUT(RMS)
IN(MAX) OUT SW C
V × V V
1
I = ×
V × L × F × N
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TPS54233
www.ti.com SLUS859B –OCTOBER 2008– REVISED FEBRUARY 2011
Capacitor Selection
The important design factors for the output capacitor are dc voltage rating, ripple current rating, and equivalent
series resistance (ESR). The dc voltage and ripple current ratings cannot be exceeded. The ESR is important
because along with the inductor current it determines the amount of output ripple voltage. The actual value of the
output capacitor is not critical, but some practical limits do exist. Consider the relationship between the desired
closed loop crossover frequency of the design and LC corner frequency of the output filter. In general, it is
desirable to keep the closed loop crossover frequency at less than 1/5 of the switching frequency. With high
switching frequencies such as the 300 kHz frequency of this design, internal circuit limitations of the TPS54233
limit the practical maximum crossover frequency to about 25 kHz. In general, the closed loop crossover
frequency should be higher than the corner frequency determined by the load impedance and the output
capacitor. This limits the minimum capacitor value for the output filter to:
(11)
Where R
O
is the output load impedance (V
O
/I
O
) and f
CO
is the desired crossover frequency. For a desired
maximum crossover of 25 kHz the minimum value for the output capacitor is around 3.8μF. This may not satisfy
the output ripple voltage requirement. The output ripple voltage consists of two components; the voltage change
due to the charge and discharge of the output filter capacitance and the voltage change due to the ripple current
times the ESR of the output filter capacitor. The output ripple voltage can be estimated by:
(12)
Where N
C
is the number of output capacitors in parallel.
The maximum ESR of the output capacitor is determined by the amount of allowable output ripple as specified in
the initial design parameters; so the maximum specified ESR as listed in the capacitor data sheet is given by
Equation 13:
(13)
Where ΔV
p-p
is the desired peak-to-peak output ripple.
To meet the 100 mV p-p ripple requirement, a single 470 μF aluminum electrolytic output capacitor is chosen for
C9. This is a Panasonic, EEVFK1A471P rated at 10 V with a maximum ESR of 160 mΩ and a ripple current
rating of 600 mA.
The maximum RMS output ripple current can be calculated using Equation 14
(14)
The calculated total RMS ripple current is 216 mA and the maximum total ESR required is 43 mΩ. These output
capacitors exceed the requirements by a wide margin and will result in a reliable, high-performance design. The
selected output capacitor must be rated for a voltage greater than the desired output voltage plus = the ripple
voltage. Any derating amount must also be included.
Other capacitor types work well with the TPS54233, depending on the needs of the application.
COMPENSATION COMPONENTS
The external compensation used with the TPS54233 allows for a wide range of output filter configurations. A
large range of capacitor values and types of dielectric are supported. The design example uses ceramic X5R
dielectric output capacitors, but other types are supported.
A Type II compensation scheme is recommended for the TPS54233. The compensation components are chosen
to set the desired closed loop cross over frequency and phase margin for output filter components. The type II
compensation has the following characteristics; a dc gain component, a low frequency pole, and a mid frequency
zero / pole pair.
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