LT1374CS8 datasheet(27/32 Pages) LINER

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LT1374

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APPLICATIONS INFORMATION

This says that discontinuous mode can be used and the

minimum inductor needed is found from:

MIN =

()( )







()

25 1

500 10

4 5

•.

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 1.3

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 15

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 two parallel capacitor’s ESR of 0.1

Ω.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.

Ripple Current in the Input and Output Capacitors

Positive-to-negative converters have high ripple current in

both the input and output capacitors. For long capacitor

lifetime, the RMS value of this current must be less than

the high frequency ripple current rating of the capacitor.

The following formula will give an

approximate value for

RMS ripple current.

This formula assumes continuous

mode and large inductor value. Small inductors will give

somewhat higher ripple current, especially in discontinu-

ous mode. The exact formulas are very complex and

appear in Application Note 44, pages 30 and 31. For our

purposes here I have simply added a fudge factor (ff). The

value for ff is about 1.2 for higher load currents and

≥10µH. It increases to about 2.0 for smaller inductors at

lower load currents.

Capacitor

ff I

OUT

IRMS =

()( )

ff = Fudge factor (1.2 to 2.0)

Diode Current

Average diode current is equal to load current. Peak diode

current will be considerably higher.

Peak diode current:

Continuous

Lf V

Discontinuous

OUT

Mode

Mode =

2IOUT

()

()( )

()( ) +

()

()( )

Keep in mind that during start-up and output overloads,

average diode current may be much higher than with

normal loads. Care should be used if diodes rated less than

3A are used, especially if continuous overload conditions

must be tolerated.

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