LT3430 datasheet(25/28 Pages) LINER

25 / 28 page

LT3430

sn3430 3430is

APPLICATIO S I FOR ATIO

INDUCTOR VALUE

The criteria for choosing the inductor is 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.

The difficulty in calculating the minimum inductor size

needed is that you must first decide whether the switcher

will be in continuous or discontinuous mode at the critical

point where switch current reaches 3A. The first step is to

use the following formula to calculate the load current

above which 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 a 40V to –12V converter using the LT3430 with peak

switch current of 3A and a catch diode of 0.52V:

CONT =

() ( )

()(

)

4 4012 4012 0 52

1 148

For a load current of 0.5A, this says that discontinuous

mode can be used and the minimum inductor needed is

found from:

MIN ==

212 0 5

200 10

()( . )

(•)( )

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 for-

mula will give an

approximate value for RMS ripple cur-

rent.

This formula assumes continuous mode and large

inductor value. Small inductors will give somewhat 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 purposes 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

OUTPUT**

–12V, 0.5A

INPUT†

5.5V TO

48V

3430 F15

0.68

µF

30BQ060

36.5k

100

µF

16V TANT

4.7

µF

100V

CER

D2†

MMSD914TI

L1*

µH

BOOST

LT3430

VIN

VSW

GND

4.12k

* INCREASE L1 FOR HIGHER CURRENT APPLICATIONS.

SEE APPLICATIONS INFORMATION

** MAXIMUM LOAD CURRENT DEPENDS ON MINIMUM INPUT VOLTAGE

AND INDUCTOR SIZE. SEE APPLICATIONS INFORMATION

† FOR V

IN > 44V AND VOUT = –12V, ADDITIONAL VOLTAGE DROP IN THE

PATH OF D2 IS REQUIRED TO ENSURE BOOST PIN MAXIMUM RATING IS

NOT EXCEEDED. SEE APPLICATIONS INFORMATION (BOOST PIN VOLTAGE)

Figure 15. Positive-to-Negative Converter

LT3430 Datasheet(PDF) 25 Page - Linear Technology

LT3430 Datasheet(HTML) 25 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL