LT1507 Linear Technology, LT1507 Datasheet - Page 10

• Manufacturer Part Number: LT1507
• Description: 500kHz Monolithic Buck Mode Switching Regulator
• Manufacturer: Linear Technology

4. Look for an inductor (see Table 1) which meets the
5. After making an initial choice, consider secondary things
LT1507
APPLICATIONS
10
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.
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.
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.
U
INFORMATION
U
W
U
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
SC = Semi-closed geometry
Fer = Ferrite core material
52 = Type 52 powdered iron core material
KM = Kool M
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 to 0.5 .
A typical output capacitor is an AVX type TPS, 100 F at
10V, with a guaranteed ESR less than 0.1 . 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
22 F to greater than 500 F work well, but you cannot
cheat mother nature on ESR. If you find a tiny 22 F 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.
VALUE
( H)
10
10
10
10
10
10
10
10
10
5
5
(A)
2.3
1.9
1.8
1.6
1.7
1.7
1.4
2.4
1.3
3.1
4.3
DC
CORE SERIES
TYPE
Open
Open
Open
Open
Open
Tor
Tor
Tor
Tor
SC
SC
0.027
0.039
0.021
0.030
0.084
0.055
0.062
0.041
0.038
0.071
0.053
( )
CORE
KM
KM
Fer
Fer
Fer
Fer
Fer
Fer
Fer
52
52
HEIGHT
(mm)
4.2
4.2
4.2
4.2
4.5
3.4
3.5
4.0
5.6
7.1
7