LTC1474/LTC1475
APPLICATIONS INFORMATION
If the L
MIN
calculated is not practical, a larger I
PEAK
should
be used. Although the above equation provides the mini-
mum, better performance (efficiency, line/load regulation,
noise) is usually gained with higher values. At higher
inductances, peak current and frequency decrease (im-
proving efficiency) and inductor ripple current decreases
(improving noise and line/load regulation). For a given
inductor type, however, as inductance is increased, DC
resistance (DCR) increases, increasing copper losses,
and current rating decreases, both effects placing an
upper limit on the inductance. The recommended range of
inductances for small surface mount inductors as a func-
tion of peak current is shown in Figure 3. The values in this
range are a good compromise between the trade-offs
discussed above. If space is not a premium, inductors with
larger cores can be used, which extends the recom-
mended range of Figure 3 to larger values.
1000
INDUCTOR VALUE (µH)
500
100
50
10
100
PEAK INDUCTOR CURRENT (mA)
1000
1474/75 F03
Figure 3. Recommended Inductor Values
Inductor Core Selection
Once the value of L is known, the type of inductor must be
selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy
or Kool Mµ
®
cores. Actual core loss is independent of core
size for a fixed inductor value, but is very dependent on
inductance selected. As inductance increases, core losses
go down. Unfortunately, as discussed in the previous
8
U
W
U
U
section, increased inductance requires more turns of wire
and therefore copper losses will increase.
Ferrite and Kool Mµ designs have very low core loss and
are preferred at high switching frequencies, so design
goals can concentrate on copper loss and preventing
saturation. Ferrite core material saturates “hard,” which
means that inductance collapses abruptly when the peak
design current is exceeded. This results in an abrupt
increase in inductor current above I
PEAK
and consequent
increase in voltage ripple.
Do not allow the core to satu-
rate!
Coiltronics, Coilcraft, Dale and Sumida make high
performance inductors in small surface mount packages
with low loss ferrite and Kool Mµ cores and work well in
LTC1474/LTC1475 regulators.
Catch Diode Selection
The catch diode carries load current during the off-time.
The average diode current is therefore dependent on the
P-channel switch duty cycle. At high input voltages the
diode conducts most of the time. As V
IN
approaches V
OUT
the diode conducts only a small fraction of the time. The
most stressful condition for the diode is when the output
is short-circuited. Under this condition, the diode must
safely handle I
PEAK
at close to 100% duty cycle.
To maximize both low and high current efficiency, a fast
switching diode with low forward drop and low reverse
leakage should be used. Low reverse leakage current is
critical to maximize low current efficiency since the leak-
age can potentially approach the magnitude of the LTC1474/
LTC1475 supply current. Low forward drop is critical for
high current efficiency since loss is proportional to for-
ward drop. These are conflicting parameters (see Table 1),
but a good compromise is the MBR0530 0.5A Schottky
diode specified in the application circuits.
Table 1. Effect of Catch Diode on Performance
DIODE (D1)
BAS85
MBR0530
MBRS130
LEAKAGE
200nA
1µA
20µA
FORWARD
NO LOAD
DROP
SUPPLY CURRENT EFFICIENCY*
0.6V
0.4V
0.3V
9.7µA
10µA
16µA
77.9%
83.3%
84.6%
*Figure 1 circuit with V
IN
= 15V, I
OUT
= 0.1A
Kool Mµ is a registered trademark of Magnetics, Inc.
LINER [ LINEAR TECHNOLOGY ]
