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LT3434

3434f

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 15µH 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 LT3434 switch, which has a minimum 3A 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 such that the maximum

load current plus half of the inductor ripple current is

less than the minimum peak switch current (IPK).

Choosing a small inductor with lighter loads may result

in discontinuous mode of operation, but the LT3434 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 1A, for instance, a 1A inductor may

not survive a continuous 4A 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/

VALUE

IDC

DCR

HEIGHT

PART NO.

(

µH)

(Amps)

(Ohms)

(mm)

Sumida

CDRH104R-150

3.6

0.050

CDRH104R-220

2.9

0.073

CDRH104R-330

2.3

0.093

CDRH124-220

2.9

0.066

4.5

CDRH124-330

2.7

0.097

4.5

CDRH127-330

3.0

0.065

CDRH127-470

2.5

0.100

CEI122-220

2.3

0.085

Coiltronics

UP3B-330

0.069

6.8

UP3B-470

2.4

0.108

6.8

UP4B-680

4.3

0.120

7.9

Coilcraft

DO3316P-153

0.046

5.2

DO5022p-683

3.5

0.130

7.1

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