LT1507CS8 datasheet(10/20 Pages) LINER

10 / 20 page

LT1507

APPLICATIONS INFORMATION

be due to a radiated magnetic field coupling into PC

board traces. But why were some boards bad and

others good? In a moment of desperation (or divine

inspiration) I unsoldered a “bad” inductor, rotated it

180

° and resoldered it. Problem fixed!!

It turns out that the inductor was symmetrical in all

regards except that the polarity of the magnetic field

reversed when the unit was rotated 180

° because

current flowed in the opposite direction in the coil. In

one direction, the magnetically induced ripple in the

board traces

added to output ripple. Rotating the induc-

tor caused the induced field to

reduce output ripple.

Unfortunately the inductor had no physical package

assymmetry to indicate rotation, including part mark-

ing, so we had to visually examine the winding in each

unit before soldering it to the boards. This little horror

story should not preclude the use of open core induc-

tors, but it emphasizes the need to carefully check the

effect these seductively small, low cost inductors may

have on regulator or system performances.

4. Look for an inductor (see Table 1) which meets the

requirements of core shape, peak current (to avoid

saturation), average current (to limit heat) 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, surface mounting,

low profile and high temperature operation will increase

cost, sometimes dramatically.

5. After making an initial choice, consider secondary things

like output voltage ripple, second sourcing, etc. Use the

experts in the Linear Technology Applications Depart-

ment 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 1. Representative Surface Mount Units

VALUE

CORE SERIES

HEIGHT

MANUFACTURER

(

µH)

(A)

TYPE

(

Ω)

CORE

(mm)

Coiltronics

CTX5-1

2.3

Tor

0.027

4.2

CTX10-1

1.9

Tor

0.039

4.2

CTX5-1P

1.8

Tor

0.021

4.2

CTX10-1P

1.6

Tor

0.030

4.2

Sumida

CDRH64

1.7

0.084

Fer

4.5

CDRH73

1.7

0.055

Fer

3.4

CD73

1.4

Open

0.062

Fer

3.5

CD104

2.4

Open

0.041

Fer

4.0

Gowanda

SM20-102K

1.3

Open

0.038

Fer

Dale

IHSM-4825

3.1

Open

0.071

Fer

5.6

IHSM-5832

4.3

Open

0.053

Fer

7.1

SC = Semi-closed geometry

Fer = Ferrite core material

52 = Type 52 powdered iron core material

µ = Kool Mµ

OUTPUT CAPACITOR

The output capacitor is normally chosen by its effective

series resistance (ESR), because that is what determines

output ripple voltage. At 500kHz any polarized capacitor is

essentially resistive. To get low ESR takes

volume ; physi-

cally larger capacitors have lower ESR. The ESR range

needed for typical LT1507 applications is 0.05

Ω to 0.5Ω.

A typical output capacitor is an AVX type TPS, 100

µF at

10V, with a guaranteed ESR less than 0.1

Ω. This is a “D”

size surface mount solid tantalum capacitor. TPS capaci-

tors are specially constructed and tested for low ESR so

they give the lowest ESR for a given volume. The value in

microfarads is not particularly critical and values from

µF to greater than 500µF 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. The chart in Table 2 shows some

typical solid tantalum surface mount capacitors.

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