5

LTC1701

APPLICATIO S I FOR ATIO

Accepting larger values of

inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

The inductor value also has an effect on low current

operation. Lower inductor values (higher

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 M

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-

facturer is Kool M

µ 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-

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 normal load conditions, the average current con-

ducted by the diode is simply:

II

VV

VV

DIODE avg

LOAD avg

IN

OUT

IN

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:

V

P

I

VV

V

D

D

SC avg

IN

D

IN

≈









+









()

be determined by efficiency and/or thermal requirements

(see Efficiency Considerations).

Kool M

µ is a registered trademark of Magnetics, Inc.