25
LT1578/LT1578-2.5
APPLICATIONS INFORMATION
WUU
U
For the example above, with maximum load current of
0.25A:
IA
CONT
=
()()
+
()
++
()
=
55 15
455555505
038
22
..
.. .
.
This says that discontinuous mode can be used and the
minimum inductor needed is found from:
LH
MIN
=
()( )
()
=
25 025
200 10 1 5
56
3
2
.
•.
.µ
In practice, the inductor should be increased by about 30%
over the calculated minimum to handle losses and varia-
tions in value. This suggests a minimum inductor of 7.3µH
for this application, but looking at the ripple voltage chart
shows that output ripple voltage could be reduced by a fac-
tor of two by using a 30µH inductor. There is no rule of thumb
here to make a final decision. If modest ripple is needed and
the larger inductor does the trick, this is probably the best
solution. If ripple is noncritical use the smaller inductor. If
ripple is extremely critical, a second stage filter may have
to be added in any case, and the lower value of inductance
can be used. Keep in mind that the output capacitor is the
other critical factor in determining output ripple voltage.
Ripple shown on the graph (Figure 16) is with a capacitor’s
ESR of 0.1Ω. This is
reasonable for AVX type TPS “D” or
“E” size surface mount solid tantalum capacitors, but the
final capacitor chosen must be looked at carefully for ESR
characteristics.
inductor is therefore typically based on ensuring that peak
switch current rating is not exceeded. This gives the
lowest value of inductance that can be used, but in some
cases (lower output load currents) it may give a value that
creates unnecessarily high output ripple voltage. A com-
promise value is often chosen that reduces output ripple.
As you can see from the graph,
large
inductors will not
give arbitrarily low ripple, but
small
inductors can give
high ripple.
The difficulty in calculating the minimum inductor size
needed is that you must first know whether the switcher
will be in continuous or discontinuous mode at the critical
point where switch current is 1.5A. The first step is to use
the following formula to calculate the load current where
the switcher must use continuous mode. If your load
current is less than this, use the discontinuous mode
formula to calculate the minimum inductor value needed.
If the load current is higher, use the continuous mode
formula.
Output current where continuous mode is needed:
I
VI
VV VV V
CONT
IN P
IN OUT IN OUT F
=
()()
+
()
++
()
22
4
Minimum inductor discontinuous mode:
L
VI
fI
MIN
OUT OUT
P
=
()()
()( )
2
2
Minimum inductor continuous mode:
L
VV
fV V I I
VV
V
MIN
IN OUT
IN OUT P OUT
OUT
F
IN
=
()( )
()
+
()
−+
+
()
21
