LTC1622CS8 datasheet(7/16 Pages) LINER

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LTC1622

APPLICATIONS INFORMATION

Kool Mu is a registered trademark of Magnetics, Inc.

The basic LTC1622 application circuit is shown in Figure

1. External component selection is driven by the load

requirement and begins with the selection of L and RSENSE.

Next, the Power MOSFET and the output diode D1 are

selected followed by CIN and COUT.

RSENSE Selection for Output Current

RSENSE is chosen based on the required output current.

With the current comparator monitoring the voltage devel-

oped across RSENSE, the threshold of the comparator

determines the inductor’s peak current. The output cur-

rent the LTC1622 can provide is given by:

OUT

SENSE

RIPPLE

=−

008

where IRIPPLE is the inductor peak-to-peak ripple current

(see Inductor Value Calculation section).

A reasonable starting point for setting ripple current is

IRIPPLE = (0.4)(IOUT). Rearranging the above equation, it

becomes:

SENSE

OUT

= ()( )

for Duty Cycle < 40%

However, for operation that is above 40% duty cycle, slope

compensation has to be taken into consideration to select

the appropriate value to provide the required amount of

current. Using Figure 2, the value of RSENSE is:

SENSE

OUT

= ()( )( )

100

Inductor Value Calculation

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the use

of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses.

The inductance value also has a direct effect on ripple

current. The ripple current, IRIPPLE, decreases with higher

inductance or frequency and increases with higher VIN or

VOUT. The inductor’s peak-to-peak ripple current is given

by:

RIPPLE

OUT

−

()









where f is the operating frequency. Accepting larger values

of IRIPPLE allows the use of low inductances, but results in

higher output voltage ripple and greater core losses. A

reasonable starting point for setting ripple current is

IRIPPLE = 0.4(IOUT(MAX)). Remember, the maximum IRIPPLE

occurs at the maximum input voltage.

With Burst Mode operation selected on the LTC1622, the

ripple current is normally set such that the inductor

current is continuous during the burst periods. Therefore,

the peak-to-peak ripple current should not exceed:

RIPPLE

SENSE

≤

0 036

This implies a minimum inductance of:

MIN

OUT

SENSE

OUT

−

















0 036

(Use VIN(MAX) = VIN)

A smaller value than LMIN could be used in the circuit;

however, the inductor current will not be continuous

during burst periods.

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

® cores. Actual core loss is independent of core

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

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