LT1376CS8 datasheet(25/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

INDUCTOR VALUE

Unlike buck converters, positive-to-negative converters

cannot use large inductor values to reduce output ripple

voltage. At 500kHz, values larger than 25

µH make almost

no change in output ripple. The graph in Figure 19 shows

peak-to-peak output ripple voltage for a 5V to – 5V con-

verter versus inductor value. The criteria for choosing the

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.

INDUCTOR SIZE (

µH)

150

120

1375/76 F19

5V TO –5V CONVERTER

OUTPUT CAPACITOR

ESR = 0.1

Ω

ILOAD = 0.25A

ILOAD = 0.1A

Figure 19. Ripple Voltage on Positive-to-Negative Converter

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:

CONT =

() ( )

() ++

()

51 5

4 5 5 550 5

037

This says that discontinuous mode can be used and the

minimum inductor needed is found from:

MIN =

()( )







()

25 0 25

500 10

1 5

•.

In practice, the inductor should be increased by about

30% over the calculated minimum to handle losses and

variations in value. This suggests a minimum inductor of

µH for this application, but looking at the ripple voltage

chart shows that output ripple voltage could be reduced by

a factor of two by using a 15

µH inductor. There is no rule

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