7

LTC1772

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.

Power MOSFET Selection

An external P-channel power MOSFET must be selected

for use with the LTC1772. The main selection criteria for

the “on” resistance RDS(ON), reverse transfer capaci-

Since the LTC1772 is designed for operation down to low

work close to this voltage. When these MOSFETs are used,

make sure that the input supply to the LTC1772 is less than

erned by its allowable power dissipation. For applications

that may operate the LTC1772 in dropout, i.e., 100% duty

R

P

Ip

DS ON

P

OUT MAX

DC

()

()

%

=

100

2

1

δ

temperature curve, but

δp = 0.005/°C can be used as an

approximation for low voltage MOSFETs.

In applications where the maximum duty cycle is less than

is governed by:

R

P

DC I

p

DS ON

P

OUT

()

+

()

where DC is the maximum operating duty cycle of the

LTC1772.

Output 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

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

it is important to adequately specify the diode peak current

and average power dissipation so as not to exceed the

diode ratings.

Under normal load conditions, the average current con-

ducted by the diode is:

I

VV

VV

I

D

IN

OUT

IN

D

OUT

=

−

+









The allowable forward voltage drop in the diode is calcu-

lated from the maximum short-circuit current as:

V

P

I

F

D

SC MAX

≈

()

determined by efficiency and/or thermal requirements.

Kool Mu is a registered trademark of Magnetics, Inc.