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
LM21215A
www.ti.com SNOSB87B –MARCH 2011–REVISED MARCH 2013
Figure 32. 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 LM21215A-1 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)
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