LT1375CS8 datasheet(26/28 Pages) LINER

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LT1375/LT1376

APPLICATIONS INFORMATION

of thumb here to make a final decision. If modest ripple is

needed and the larger inductor does the trick, go for it. If

ripple is noncritical use the smaller inductor. If ripple is

extremely critical, 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 determining output ripple voltage. Ripple shown

on the graph (Figure 19) is with a capacitor ESR of 0.1

Ω.

This is reasonable for an AVX type TPS “D” or “E” size

surface mount solid tantalum capacitor, 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 factor1 (1.2 to 2.0)

1Normally, Jamoca Almond

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

1A are used, especially if continuous overload conditions

must be tolerated.

Dual Output SEPIC Converter

The circuit in Figure 20 generates both positive and

negative 5V outputs with a single piece of magnetics. The

two inductors shown are actually just two windings on a

standard Coiltronics inductor. The topology for the 5V

output is a standard buck converter. The – 5V topology

would be a simple flyback winding coupled to the buck

converter if C4 were not present. C4 creates the SEPIC

(Single-Ended Primary Inductance Converter) topology

which improves regulation and reduces ripple current in

L1. For details on this circuit see Design Note 100.

LT1375CS8 Datasheet(PDF) 26 Page - Linear Technology

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