LTC1775IGN datasheet(12/24 Pages) LINER

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LTC1775

Normally, the top and bottom MOSFETs will be of the same

type. A bottom MOSFET with lower RDS(ON) than the top

may be chosen if the resulting increase in short-circuit

current is tolerable. However, the bottom MOSFET should

never be chosen to have a higher nominal RDS(ON) than the

top MOSFET.

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 M

µ® cores. Actual core loss is independent of core

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

the 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 loss and are preferred

at high switching frequencies, so design goals can con-

centrate on copper loss and preventing saturation. Ferrite

core material saturates “hard,” which means that induc-

tance collapses rapidly 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. 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, designs for surface mount are available

which do not increase the height significantly.

Schottky Diode Selection

The Schottky diode D1 shown in Figure 1 conducts during

the dead time between the conduction of the power

MOSFETs. This prevents the body diode of the bottom

MOSFET from turning on and storing charge during the

dead time, which could cost as much as 1% in efficiency.

A 1A Schottky diode is generally a good size for 3A to 5A

regulators. The diode may be omitted if the efficiency loss

can be tolerated.

Parasitic Lead Inductance Effects

Because the LTC1775 is designed to operate with rela-

tively large currents through single (or multiple) MOSFET

switches, the lead inductance of these power switches can

become a significant concern. The table below shows

typical values of lead inductance for some common pack-

ages:

MOSFET Package

Lead Inductance

TO-220

4nH to 12nH

DDPAK

4nH

DPAK

1.5nH

SO-8

1nH

Of particular concern are switches in TO-220 packages

which can have a series inductance of between 4nH and

12nH depending upon the depth of insertion into the

circuit board. When the main (top) switch is turned on, the

lead inductance LP forms a voltage divider with the power

inductor L1. The voltage VLP across this parasitic adds to

the voltage from the switch on-resistance and increases

the current sense voltage.

VLP = (VIN – VOUT)LP/L1

The result is lower value of current limit than would have

been expected otherwise. For example, a 10nH lead induc-

tance with a 5

µH power inductor has 50mV across it when

VIN = 30V and VOUT = 5V. Thus, the 300mV current limit

will be reached when the switch voltage due to on-

resistance is only 250mV, a 17% reduction. This effect is

most noticeable at higher input voltages.

Lead inductance also reduces the benefit of the Schottky

diode D1 by delaying commutation of the inductor current

from the diode over to the synchronous (bottom) switch.

With the diode forward biased when the synchronous

switch turns on, there is only about 500mV applied across

the lead and trace inductance between the switch and the

diode. It takes about 400ns to commutate a 20A current in

this case. This delay reduces efficiency and can also

increase the foldback current limit of the LTC1775. The

Kool M

µ is a registered trademark of Magnetics, Inc.

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