9

LT1371

APPLICATIO S I FOR ATIO

saturate abruptly and other core materials fall in be-

tween. The following formula assumes continuous

mode operation but it errs only slightly on the high side

for discontinuous mode, so it can be used for all

conditions.

f = 500kHz Switching Frequency

+

)

)

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. One would not want an

open core next to a magnetic storage media, for in-

stance! 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. 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 LTC 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.

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 needed for typical LT1371 applications is 0.025

Ω

to 0.2

Ω. A typical output capacitor is an AVX type TPS,

22

µF at 25V (2 each), with a guaranteed ESR less than

0.2

Ω. 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 capaci-

tors can be used in parallel. The value in microfarads is

not particularly critical, and values from 22

µF to greater

than 500

µF work well, but you cannot cheat mother

nature on ESR. If you find a tiny 22

µF solid tantalum

capacitor, it will have high ESR and output ripple voltage

will be terrible. Table 1 shows some typical solid tantalum

surface mount capacitors.

Table 1. Surface Mount Solid Tantalum Capacitor

ESR and Ripple Current

E CASE SIZE

ESR (MAX

Ω)

RIPPLE CURRENT (A)

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

AVX TAJ

0.7 to 0.9

0.4

D CASE SIZE

AVX TPS, Sprague 593D

0.1 to 0.3

0.7 to 1.1

AVX TAJ

0.9 to 2.0

0.36 to 0.24

C CASE SIZE

AVX TPS

0.2 (Typ)

0.5 (Typ)

AVX TAJ

1.8 to 3.0

0.22 to 0.17

B CASE SIZE

AVX TAJ

2.5 to 10

0.16 to 0.08

Many engineers have heard that solid tantalum capacitors

are prone to failure if they undergo high surge currents.

This is historically true and AVX type TPS capacitors are

specially tested for surge capability, but surge ruggedness

is not a critical issue with the

output capacitor. Solid

tantalum capacitors fail during very high

turn-on surges,

which do not occur at the output of regulators. High

discharge surges, such as when the regulator output is

dead-shorted, do not harm the capacitors.

Single inductor boost regulators have large RMS ripple

current in the output capacitor, which must be rated to

handle the current. The formula to calculate this is: