LT1956IGN datasheet(26/28 Pages) LINER

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LT1956/LT1956-5

1956f

PACKAGE DESCRIPTIO

Minimum inductor continuous mode:

MIN

OUT





















()(

)

()(

)

–

()

For a 12V to –12V converter using the LT1956 with peak

switch current of 1.5A and a catch diode of 0.63V:

CONT >

() ( . )

()(

)

1 5

4 1212 1212 0 63

0 370

For a load current of 0.25A, this says that discontinuous

mode can be used and the minimum inductor needed is

found from:

MIN ==

212 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.

Ripple Current in the Input and Output Capacitors

Positive-to-negative converters have high ripple current

in the input capacitor. 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 some-

what higher ripple current, especially in discontinuous

mode. The exact formulas are very complex and appear

in Application Note 44, pages 29 and 30. For our pur-

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

value for ff is about 1.2 for higher load currents and L

≥15µH. It increases to about 2.0 for smaller inductors at

lower load currents.

Capacitor I

ff I

RMS

OUT

= ()(

)

ff = 1.2 to 2.0

The output capacitor ripple current for the positive-to-

negative converter is similar to that for a typical buck

regulator—it is a triangular waveform with peak-to-peak

value equal to the peak-to-peak triangular waveform of the

inductor. The low output ripple design in Figure 14 places

the input capacitor between VIN and the regulated negative

output. This placement of the input capacitor significantly

reduces the size required for the output capacitor (versus

placing the input capacitor between VIN and ground).

The peak-to-peak ripple current in both the inductor and

output capacitor (assuming continuous mode) is:

IP-P

P-P

DC V

DC Duty Cycle

RMS

OUT

COUT

•

()

The output ripple voltage for this configuration is as low as

the typical buck regulator based predominantly on the

inductor’s triangular peak-to-peak ripple current and the

ESR of the chosen capacitor (see Output Ripple Voltage in

Applications Information).

Diode Current

Average diode current is equal to load current. Peak diode

current will be considerably higher.

Peak diode current:

Continuous Mode

Lf V

Discontinuous Mode

OUT

()

(

)(

)

()( )(

)

()(

)

()( )

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.

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