Datasheet

f
S
x 0.4 x I
L(MAX)
(1 ± D)
x V
OUT
f
S
x 0.2 x I
L(MAX)
(1 ± D)
x V
OUT
7 L 7
'i
Lp-p
=
f
S
x L
(1 - D) x V
OUT
|
V
OUT
f
S
x L
(1 - )
V
OUT
V
PVIN
R
FRQ
(k:)
FREQUENCY (kHz)
1800
1600
1400
1200
1000
800
600
400
200
0
0 20 40 60 80 100 120 140 160 180
LM21305
www.ti.com SNVS639F –DECEMBER 2009–REVISED MARCH 2013
rate of inductor current) and reduces the DCR losses. The optimal switching frequency is usually a tradeoff in a
given application and thus needs to be determined on a case-by-case basis. It is related to the input voltage,
output voltage, most common load current level, external component choices, and circuit size requirements. The
choice of switching frequency may also be limited if an operating condition triggers T
ON-MIN
or T
OFF-MIN
. Please
refer to the aforementioned MINIMUM ON-TIME CONSIDERATIONS section.
The following equation or Figure 26 can be used to calculate the resistance to obtain a desired frequency of
operation:
f
s
[kHz] = 31000 x R
FRQ
-0.9
[kΩ] (11)
Figure 26. External Resistor Selection to Set
the Switching Frequency
INDUCTOR
A general recommendation for the filter inductor in an LM21305 application is to keep a peak-to-peak ripple
current between 20% and 40% of the maximum DC load current of 5A. It also should have a sufficiently high
saturation current rating and a DCR as low as possible.
The peak-to-peak current ripple can be calculated by:
(12)
It is recommended to choose L such that:
(13)
The inductor should be rated to handle the maximum load current plus the ripple current:
I
L(MAX)
= I
LOAD(MAX)
+ Δi
L(MAX)
/2 (14)
An inductor with saturation current higher than the over-current protection limit is a safe choice. In general, it is
desirable to have lower inductance in switching power supplies, because it usually corresponds to faster
transient response, smaller DCR, and reduced size for more compact designs. But too low of an inductance can
generate too large of an inductor ripple current such that over-current protection at the full load could be falsely
triggered. It also generates more conduction loss, since the RMS inductor current is slightly higher relative to that
with lower current ripple at the same DC current. Larger inductor ripple current also implies larger output voltage
ripple with the same output capacitors. With peak current-mode control, it is recommended to not have too small
of an inductor current ripple so that the peak current comparator has enough signal-to-noise ratio.
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