LT1576CS8 datasheet(25/28 Pages) LINER

25 / 28 page

LT1576/LT1576-5

APPLICATIONS INFORMATION

CONT =

() ( )

() ++

()

4 5 55 5 550 5

038

This says that discontinuous mode can be used and the

minimum inductor needed is found from:

MIN =

()( )







()

25 0 25

200 10

1 5

•.

.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 7.3

µH

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

µHinductor.Thereisnoruleofthumb

here to make a final decision. If modest ripple is needed and

the larger inductor does the trick, go for it. If ripple is non-

critical use the smaller inductor. If ripple is extremely criti-

cal, a second 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 deter-

mining output ripple voltage. Ripple shown on the graph

(Figure 16) is with a capacitor’s ESR of 0.1

Ω. This is rea-

sonable 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 minimum inductor needed. If load

current is higher, use the continuous mode formula.

Output current where continuous mode is needed:

CONT

OUT

() ( )

() ++

()

Minimum inductor discontinuous mode:

MIN

OUT

= ()(

)

()( )

Minimum inductor continuous mode:

MIN

OUT

()( )

() +

() −+

()

























For the example above, with maximum load current of

0.25A:

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