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LT1959

APPLICATIONS INFORMATION

CHOOSING THE INDUCTOR AND OUTPUT CAPACITOR

For most applications the output inductor will fall in the

range of 3

µH to 20µH. 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 LT1959 switch, which has a 4.5A limit. Higher values

also reduce output ripple voltage, and reduce core loss.

Graphs in the Typical Performance Characteristics section

show maximum output load current versus inductor size

and input voltage.

When choosing an inductor you might have to consider

maximum load current, core and copper losses, allowable

component height, output voltage ripple, EMI, fault cur-

rent 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. Choose a value in microhenries from the graphs of

maximum load current and core loss. Choosing a small

inductor with lighter loads may result in discontinuous

mode of operation, but the LT1959 is designed to work

well in either mode. Keep in mind that lower core loss

means higher cost, at least for closed core geometries

like toroids. The core loss graphs show absolute loss

for a 3.3V output, so actual percent losses must be

calculated for each situation.

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 4.5A overload condition.

Dead shorts will actually be more gentle on the induc-

tor because the LT1959 has foldback current limiting.

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

because they saturate softly, whereas ferrite cores

saturate abruptly. Other core materials fall in between

somewhere. The following formula assumes continu-

ous mode of operation, but it errs only slightly on the

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

all conditions.

fL V

PEAK

OUT

−

()

()( )( )

VIN = Maximum input voltage

f = Switching frequency, 500kHz

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 (see representative

surface mount units in Table 2) which meets the

requirements 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. Get a quote on the cheapest unit first to

calibrate yourself on price, then ask for what you really

want.

5. After making an initial choice, consider the secondary

things like output voltage ripple, second sourcing, etc.

Use the experts in the Linear Technology’s applica-

tions 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 devel-

opments in low profile, surface mounting, etc.

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