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LT1976

1976f

APPLICATIO S I FOR ATIO

To calculate actual peak switch current in continuous

mode with a given set of conditions, use:

Lf V

SW PK

OUT

()

–

()

()( )( )

If a small inductor is chosen which results in discontinous

mode operation over the entire load range, the maximum

load current is equal to:

If L V

OUT MAX

OUT

()

–

()( )( )

()(

)

CHOOSING THE INDUCTOR

For most applications the output inductor will fall in the

range of 15

µH to 100µH. Lower values are chosen to

reduce physical size of the inductor. Higher values allow

more output current because they reduce peak current

seen by the LT1976 switch, which has a 1.5A limit. Higher

values also reduce output ripple voltage and reduce core

loss.

When choosing an inductor you might have to consider

maximum load current, core and copper losses, allow-

able component height, output voltage ripple, EMI, fault

current in the inductor, saturation and of course cost.

The following procedure is suggested as a way of han-

dling these somewhat complicated and conflicting

requirements.

1. Choose a value in microhenries from the graph of

maximum load current. Choosing a small inductor with

lighter loads may result in discontinuous mode of

operation, but the LT1976 is designed to work well in

either mode.

Assume that the average inductor current is equal to

load current and decide whether or not the inductor

must withstand continuous fault conditions. If maxi-

mum load current is 0.5A, for instance, a 0.5A inductor

may not survive a continuous 2A overload condition.

For applications with a duty cycle above 50%, the

inductor value should be chosen to obtain an inductor

ripple current of less than 40% of the peak switch

current.

2. Calculate peak inductor current at full load current to

ensure that the inductor will not saturate. Peak current

can be significantly higher than output current, especially

with smaller inductors and lighter loads, so don’t omit

this step. Powdered iron cores are forgiving because they

saturate softly, whereas ferrite cores saturate abruptly.

Other core materials fall somewhere in between. The

following formula assumes continuous mode of opera-

tion, but it errs only slightly on the high side for discon-

tinuous mode, so it can be used for all conditions.

Table 3. Inductor Selection Criteria

VENDOR/

PART NUMBER

VALUE (

µH) IRMS(A) DCR (Ω) HEIGHT (mm)

Coiltronics

UP2B-150

2.4

0.041

UP2B-330

1.7

0.062

UP2B-470

1.4

0.139

UP2B-680

1.2

0.179

UP2B-101

100

0.95

0.271

UP3B-150

3.9

0.032

6.8

UP3B-330

2.4

0.069

6.8

UP3B-470

1.9

0.101

6.8

UP3B-680

1.6

0.156

6.8

UP3B-101

100

1.4

0.205

6.8

Sumida

CDRH8D28-150M

2.2

0.053

CDRH124-150M

3.2

0.05

4.5

CDRH127-150M

4.5

0.02

CDRH8D28-330M

1.4

0.122

CDRH124-330M

2.7

0.97

4.5

CDRH127-330M

3.0

0.048

CDRH8D28-470M

1.25

0.150

CDRH125-470M

1.8

0.058

CDRH127-470M

2.5

0.076

CDRH124-680M

1.5

0.228

4.5

CDRH127-680M

2.1

0.1

CDRH124-101M

100

1.2

0.30

4.5

CDRH127-101M

100

1.7

0.17

Coilcraft

DT3308P-153

2.0

0.1

DT3308P-333

1.4

0.3

DT3308P-473

0.47

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