LT1578CS8 datasheet(26/28 Pages) LINER

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LT1578/LT1578-2.5

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

conduction mode and a large inductor value. Small induc-

tors will give somewhat higher ripple current, especially in

discontinuous mode. The exact formulas are very com-

plex and appear in Application Note 44, pages 30 and 31.

For our purposes here, a simple fudge factor (ff) is added.

The value for ff is about 1.2 for load currents above 0.38A

(in continuous conduction mode) and L

≥10µH. It in-

creases to about 2.0 for smaller inductors at lower load

currents (in discontinuous conduction mode).

Capacitor

ff I

OUT

IRMS =

()( )

ff = Fudge factor (1.2 to 2.0)

APPLICATIONS INFORMATION

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,

the average diode current may be much higher than with

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

1A are used, especially if continuous overload conditions

must be tolerated.

LT1578CS8 Datasheet(PDF) 26 Page - Linear Technology

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