LT1578 Linear Technology, LT1578 Datasheet - Page 25

• Manufacturer Part Number: LT1578
• Description: 1.5A/ 200kHz Step-Down Switching Regulator
• Manufacturer: Linear Technology

APPLICATIONS
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:
Minimum inductor discontinuous mode:
Minimum inductor continuous mode:
I
L
L
CONT
MIN
MIN
2
2
V
f V
4
OUT
f I
V
IN
IN
P
I
U
2
OUT
V
V
OUT
OUT
V
INFORMATION
IN
U
V
I
2
V
P
IN
IN
I
P
I
2
OUT
V
V
OUT
OUT
W
1
V
F
V
OUT
V
IN
U
V
F
For the example above, with maximum load current of
0.25A:
This says that discontinuous mode can be used and the
minimum inductor needed is found from:
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 7.3 H
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 30 H 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 0.1 . 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.
I
L
CONT
MIN
200 10
4 5 5 5 5 5 5 0 5
2 5 0 25
5 5 1 5
3
LT1578/LT1578-2.5
1 5
2
2
2
5 6
H
0 38
A
25