LTC1701
APPLICATIO S I FOR ATIO
where V
D
is the output Schottky diode forward drop.
Accepting larger values of
∆I
L
allows the use of low
inductances, but results in higher output voltage ripple
and greater core losses. A reasonable starting point for
setting ripple current is
∆I
L
= 0.4A.
The inductor value also has an effect on low current
operation. Lower inductor values (higher
∆I
L
) will cause
Burst Mode operation to begin at higher load currents,
which can cause a dip in efficiency in the upper range of
low current operation. In Burst Mode operation, lower
inductance values will cause the burst frequency to de-
crease.
Inductor Core Selection
Once the value for L is selected, the type of inductor must
be chosen. Basically, there are two kinds of losses in an
inductor —core and copper losses.
Core losses are dependent on the peak-to-peak ripple
current and core material. However, it is independent of
the physical size of the core. By increasing inductance, the
peak-to-peak inductor ripple current will decrease, there-
fore reducing core loss. Unfortunately, increased induc-
tance requires more turns of wire and, therefore, copper
losses will increase. When space is not a premium, larger
wire can be used to reduce the wire resistance. This also
prevents excessive heat dissipation in the inductor.
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. These low core loss materials allow the user to
concentrate on reducing copper loss and preventing satu-
ration.
Ferrite designs have very low core loss and are preferred
at high switching frequencies. Ferrite core material satu-
rates “hard,” which means that inductance collapses
abruptly when the peak design current is exceeded. This
results in an abrupt increase in inductor ripple current and
consequent output voltage ripple. Do not allow the core to
saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
U
facturer is Kool Mµ core material. Toroids are very space
efficient, expecially when you can use several layers of
wire. Because they generally lack a bobbin, mounting is
more difficult. However, surface mount designs that do
not increase the height significantly are available
Catch Diode Selection
The diode D1 shown in Figure 1 conducts during the off-
time. It is important to adequately specify the diode peak
current and average power dissipation so as not to exceed
the diode ratings.
Losses in the catch diode depend on forward drop and
switching times. Therefore, Schottky diodes are a good
choice for low drop and fast switching times.
Since the catch diode carries the load current during the
off-time, the average diode current is dependent on the
switch duty cycle. At high input voltages, the diode con-
ducts 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 regulator
output is shorted to ground.
Under short-circuit conditions (V
OUT
= 0V), the diode
must safely handle I
SC(PK)
at close to 100% duty cycle.
Under normal load conditions, the average current con-
ducted by the diode is simply:
W
U
U
V
−
V
I
DIODE(avg)
=
I
LOAD(avg)
IN OUT
V
IN
+
V
D
Remember to keep lead lengths short and observe proper
grounding (see Board Layout Considerations) to avoid
ringing and increased dissipation.
The forward voltage drop allowed in the diode is calculated
from the maximum short-circuit current as:
P
V
+
V
V
D
≈
D
IN D
I
SC(avg)
V
IN
where P
D
is the allowable diode power dissipation and will
be determined by efficiency and/or thermal requirements
(see Efficiency Considerations).
Kool Mµ is a registered trademark of Magnetics, Inc.
5
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