Datasheet

'V
OUT
= 'i
L
x
1
8 x f
SW
x C
OUT
R
ESR
+
V
IN
I
L AVG
= I
OUT
'i
L
Time
Time
I
L
V
SW
LM20333
www.ti.com SNVS558D –MAY 2008–REVISED APRIL 2013
Figure 26. Switch and Inductor Current Waveforms
If needed, slightly smaller value inductors can be used, however, the peak inductor current, I
OUT
+ Δi
L
/2, should
be kept below the peak current limit of the device. In general, the inductor ripple current, Δi
L
, should be more
than 10% of the rated output current to provide adequate current sense information for the current mode control
loop. If the ripple current in the inductor is too low, the control loop will not have sufficient current sense
information and can be prone to instability.
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 LM20333 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 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 (3)
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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