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
MANUFACTURER
Coiltronics
CTX5-1
CTX10-1
CTX5-1P
CTX10-1P
Sumida
CDRH64
CDRH73
CD73
CD104
Gowanda
SM20-102K
Dale
IHSM-4825
IHSM-5832
VALUE
(µH)
5
10
5
10
10
10
10
10
10
10
10
DC
(A)
2.3
1.9
1.8
1.6
1.7
1.7
1.4
2.4
1.3
3.1
4.3
CORE SERIES
TYPE
(Ω)
CORE
Tor
Tor
Tor
Tor
SC
SC
Open
Open
Open
Open
Open
0.027
0.039
0.021
0.030
0.084
0.055
0.062
0.041
0.038
0.071
0.053
KMµ
KMµ
52
52
Fer
Fer
Fer
Fer
Fer
Fer
Fer
HEIGHT
(mm)
4.2
4.2
4.2
4.2
4.5
3.4
3.5
4.0
7
5.6
7.1
10
U
W
U
U
SC = Semi-closed geometry
Fer = Ferrite core material
52 = Type 52 powdered iron core material
KMµ = 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 0.05Ω to 0.5Ω.
A typical output capacitor is an AVX type TPS, 100µF at
10V, with a guaranteed ESR less than 0.1Ω. 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
22µF to greater than 500µF work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22µF 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.
LINER [ LINEAR TECHNOLOGY ]
