8

LTC1622

preferred at high switching frequencies, so design goals

can concentrate on copper loss and preventing saturation.

Ferrite core materials saturate “hard,” which means that

the inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequently, 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 surface mountable designs 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 LTC1622. The main selection criteria for

Since the LTC1622 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 LTC1622 is less than

erned by its allowable power dissipation. For applications

that may operate the LTC1622 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

+

()

2

1

δ

where DC is the maximum operating duty cycle of the

LTC1622.

When the LTC1622 is operating in continuous mode, the

MOSFET power dissipation is:

P

VV

VV

Ip R

KV

I

C

f

MOSFET

OUT

D

IN

D

OUT

DS ON

IN

OUT

RSS

=

+

+

()

+

() ( )( )( )

2

2

1

δ

()

where K is a constant inversely related to gate drive

current. Because of the high switching frequency, the

second term relating to switching loss is important not to

overlook. The constant K = 3 can be used to estimate the

contributions of the two terms in the MOSFET dissipation

equation.

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

=

−

+







