8

LT1372/LT1377

Choosing the Inductor

For most applications the inductor will fall in the range of

2.2

µH to 22µH. Lower values are chosen to reduce physi-

cal size of the inductor. Higher values allow more output

current because they reduce peak current seen by the

power switch, which has a 1.5A limit. Higher values also

reduce input ripple voltage and reduce core loss.

When choosing an inductor you might have to consider

maximum load current, core and copper losses, allowable

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 handling

these somewhat complicated and conflicting requirements.

1. Assume that the average inductor current for a boost

decide whether or not the inductor must withstand

continuous overload conditions. If average inductor

current at maximum load current is 0.5A, for instance,

a 0.5A inductor may not survive a continuous 1.5A

overload condition. Also be aware that boost convert-

ers are not short circuit protected, and that under

output short conditions, inductor current is limited only

by the available current of the input supply.

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, espe-

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

omit this step. Powdered iron cores are forgiving be-

cause they saturate softly, whereas ferrite cores satu-

rate abruptly. Other core materials fall in between

somewhere. The following formula assumes continu-

ous mode operation but it errors only slightly on the

high side for discontinuous mode, so it can be used for

all conditions.

f = 500kHz Switching Frequency (LT1372) or

1MHz Switching Frequency (LT1377)

+

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

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

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

to prevent EMI problems. One would not want an open

core next to a magnetic storage media for instance!

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 radia-

tion will be a problem.

4. Start shopping for an inductor which meets the re-

quirements of core shape, peak current (to avoid

saturation), average current (to limit heating) and fault

current. If the inductor gets too hot, wire insulation will

melt and cause turn-to-turn shorts. Keep in mind that

all good things like high efficiency, low profile and high

temperature operation will increase cost, sometimes

dramatically.

5. After making an initial choice, consider the secondary

things like output voltage ripple, second sourcing, etc.

Use the experts in the Linear Technology application

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.

Output Capacitor

The output capacitor is normally chosen by its effective

series resistance, (ESR), because this is what determines

output ripple voltage. At 500kHz, any polarized capacitor

is essentially resistive. To get low ESR takes

volume, so

physically smaller capacitors have high ESR. The ESR

range for typical LT1372 and LT1377 applications is

0.05

Ω to 0.5Ω. A typical output capacitor is an AVX type

TPS, 22

µF at 25V, with a guaranteed ESR less than 0.2Ω.

This is a “D” size surface mount solid tantalum capacitor.

TPS capacitors are specially constructed and tested for

low ESR, so they give the lowest ESR for a given volume.

To further reduce ESR, multiple output capacitors can be

used in parallel. The value in microfarads is not particu-

larly critical, and values from 22

µF to greater than 500µF

work well, but you cannot cheat mother nature on ESR.

If you find a tiny 22

µFsolidtantalumcapacitor,itwillhave

high ESR, and output ripple voltage will be terrible. Table

1 shows some typical solid tantalum surface mount

capacitors.

APPLICATIO S I FOR ATIO