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LT3437

3437f

APPLICATIO S I FOR ATIO

more output current because they reduce peak current

seen by the LT3437 switch, which has a 0.5A 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 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 LT3437 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 0.25A, for instance, a 0.25A

inductor may not survive a continuous minimum peak

switch current 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 do not 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.

fL V

PEAK

OUT

()

()( )( )

–

VIN = maximum input voltage

f = switching frequency, 200kHz

3. Decide if the design can tolerate an “open” core geom-

etry like a rod or barrel, which has high magnetic field

radiation, or whether it needs a closed core like a toroid,

to prevent EMI problems. This is a tough decision

because the rods or barrels are temptingly cheap and

small, and there are no helpful guidelines to calculate

when the magnetic field radiation will be a problem.

4. After making an initial choice, consider the secondary

things like output voltage ripple, second sourcing, etc.

Use the experts in Linear Technology’s applications

department if you feel uncertain about the final choice.

They have experience with a wide range of inductor

types and can tell you about the latest developments in

low profile, surface mounting, etc.

Table 3. Inductor Selection Criteria

VENDOR/

VALUE

IDC(MAX)

DCR

HEIGHT

PART NO.

(

µH)

(mA)

(Ohms)

(mm)

Coiltronics

UP1B-101

100

530

1.11

5.0

UP1B-151

150

460

1.61

5.0

UP2B-221

220

380

1.96

5.0

Coilcraft

D01605T-473MX

450

1.1

1.8

D01605T-104MX

100

300

2.3

1.8

D03308P-154

150

600

0.94

3.0

D03308P-224

220

500

1.6

3.0

Sumida

CDRH4D28-470

480

0.387

3.0

CDRH4D28-101

100

290

1.02

3.0

CDRH5D28-101

100

420

0.520

3.0

Short-Circuit Considerations

The LT3437 is a current mode controller. It uses the VC

node voltage as an input to a current comparator which

turns off the output switch on a cycle-by-cycle basis as

this peak current is reached. The internal clamp on the VC

node, nominally 1.5V, then acts as an output switch peak

current limit. This action becomes the switch current limit

specification. The maximum available output power is

then determined by the switch current limit.

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