LTC1772CS6 datasheet(6/16 Pages) LINER

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LTC1772

APPLICATIONS INFORMATION

The basic LTC1772 application circuit is shown in Figure

1. External component selection is driven by the load

requirement and begins with the selection of L1 and

RSENSE (= R1). Next, the power MOSFET and the output

diode D1 is selected followed by CIN (= C1)and COUT(= C2).

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 LTC1772 can provide is given by:

OUT

SENSE

RIPPLE

=−

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 effect 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.

In Burst Mode operation on the LTC1772, the ripple

current is normally set such that the inductor current is

continuous during the burst periods. Therefore, the peak-

to-peak ripple current must not exceed:

RIPPLE

SENSE

≤

0 0288

This implies a minimum inductance of:

MIN

OUT

SENSE

OUT

−

















0 0288

(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

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