Datasheet

I
IN-RMS
= I
OUT
D(1 - D)
V
DROOP
= 'I
OUTSTEP
x R
ESR
+
L x 'I
OUTSTEP
2
C
OUT
x (V
IN
- V
OUT
)
'V
OUT
= 'i
L
x
1
8 x f
SW
x C
OUT
R
ESR
+
LM20123
SNVS524E –OCTOBER 2007–REVISED MARCH 2013 www.ti.com
OUTPUT CAPACITOR SELECTION (C
OUT
)
The output capacitor, C
OUT
, filters the inductor ripple current and provides a source of charge for transient load
conditions. A wide range of output capacitors may be used with the LM20123 that provide excellent performance.
The best performance is typically obtained using ceramic, SP, or OSCON type chemistries. Typical trade-offs are
that the ceramic capacitor provides extremely low ESR to reduce the output ripple voltage and noise spikes,
while the SP and OSCON capacitors provide a large bulk capacitance in a small volume for transient loading
conditions.
When selecting the value for the output capacitor the two performance characteristics to consider are the output
voltage ripple and transient response. The output voltage ripple can be approximated by using the formula shown
below.
where
• ΔV
OUT
(V) is the amount of peak to peak voltage ripple at the power supply output
• R
ESR
(Ω) is the series resistance of the output capacitor
• f
SW
(Hz) is the switching frequency
• C
OUT
(F) is the output capacitance used in the design (4)
The amount of output ripple that can be tolerated is application specific; however a general recommendation is to
keep the output ripple less than 1% of the rated output voltage. Keep in mind ceramic capacitors are sometimes
preferred because they have very low ESR; however, depending on package and voltage rating of the capacitor
the value of the capacitance can drop significantly with applied voltage. The output capacitor selection will also
affect the output voltage droop during a load transient. The peak droop on the output voltage during a load
transient is dependent on many factors; however, an approximation of the transient droop ignoring loop
bandwidth can be obtained using the following equation.
where
• C
OUT
(F) is the minimum required output capacitance
• L (H) is the value of the inductor
• V
DROOP
(V) is the output voltage drop ignoring loop bandwidth considerations
• ΔI
OUTSTEP
(A) is the load step change
• R
ESR
(Ω) is the output capacitor ESR
• V
IN
(V) is the input voltage
• V
OUT
(V) is the set regulator output voltage (5)
Both the tolerance and voltage coefficient of the capacitor needs to be examined when designing for a specific
output ripple or transient drop target.
INPUT CAPACITOR SELECTION (C
IN
)
Good quality input capacitors are necessary to limit the ripple voltage at the VIN pin while supplying most of the
switch current during the on-time. In general it is recommended to use a ceramic capacitor for the input as they
provide both a low impedance and small footprint. One important note is to use a good dielectric for the ceramic
capacitor such as X5R or X7R. These provide better over temperature performance and minimize the DC voltage
derating that occurs on Y5V capacitors. For many applications, a 22 µF, X5R, 6.3V input capacitor is sufficient;
however, additional capacitance may be required if the connection to the input supply bulk is far from the PVIN
pins. The input capacitor should be placed as close as possible PVIN and PGND pins of the device.
Non-ceramic input capacitors should be selected for RMS current rating and minimum ripple voltage. A good
approximation for the required ripple current rating is given by the relationship:
(6)
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