7

LTC1779

however, the inductor current will not be continuous

during burst periods.

limit, the maximum possible output current before the

maximum specified output current in a design, must be

comparator determines the inductor’s peak current. The

given by:

IM

SF

V

R

I

CL

SENSE

RIPPLE

= 







+Ω









100

012

22

.

–

where SF and M are as defined in the previous section,

and 20

Ω. Current limit is at a minimum at minimum input

voltage and maximum at maximum input voltage. Both

conditions should be considered in a design where current

limit is important.

To calculate several current limit conditions and choose

the best sense resistor for your design, first use minimum

input voltage. Calculate the duty cycle at minimum input

voltage.

DC

V

V

OUT

IN MIN

=

()

Choose the slope factor, SF, from Figure 2 based on the

duty cycle. The ripple current calculated at minimum input

voltage and the chosen L should be used in the current

limit equation (see Inductor Value Calculation). Figure 3

M values at different input voltages. Select the minimum

input voltage and calculate the resulting minimum current

limit settings.

The process must be repeated for maximum current limit

using duty cycle, slope factor, ripple current and mirror

ratio based on

maximum input voltage in order to choose

the best sense resistor for a particular design and to

understand how it is going to work over the entire input

voltage range.

Inductor Core Selection

Once the value for 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 Mu

size for a fixed inductor value, but it is very dependent on

inductance selected. As inductance increases, core losses

go down. Unfortunately, increased inductance requires

more turns of wire and therefore copper losses will

increase. Ferrite designs have very low core losses 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 cur-

rent 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 Mu. Toroids are very space efficient,

especially when you can use several layers of wire.

Because they generally lack a bobbin, mounting is more

difficult. However, new designs for surface mount that do

not increase the height significantly are available.

Output Diode Selection

The catch diode carries load current during the off-time.

The average diode current is therefore dependent on the

internal P-channel switch duty cycle. At high input volt-

the time. The most stressful condition for the diode is

when the output is short-circuited. Under this condition

cycle. Therefore, it is important to adequately specify the

diode peak current and average power dissipation so as

not to exceed the diode ratings.

Kool Mu is a registered trademark of Magnetics, Inc.

APPLICATIO S I FOR ATIO