Datasheet

'V
OUT
 'I
L
x
1
8 x f
SW
x C
OUT
R
ESR
+
L =
(V
IN
± V
OUT
)
D
üI
L
f
SW
V
IN
I
L AVG
= I
OUT
'I
L
Time
Time
I
L
V
SW
LM21215
www.ti.com SNVS625E –FEBRUARY 2011–REVISED MARCH 2013
Figure 31. Switch and Inductor Current Waveforms
Once the ripple current has been determined, the appropriate inductor size can be calculated using the following
equation:
(5)
OUTPUT CAPACITOR SELECTION
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 LM21215 that provide various advantages.
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 following
formula:
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 (6)
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:
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