LTC1872 datasheet(6/12 Pages) LINER

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LTC1872

APPLICATIONS INFORMATION

The basic LTC1872 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 LTC1872 can provide is given by:

OUT

SENSE

RIPPLE

OUT

=−









012

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

(see Inductor Value Calculation section) and VD is the

forward drop of the output diode at the full rated output

current.

A reasonable starting point for setting ripple current is:

RIPPLE

OUT

()( ) +

Rearranging the above equation, it becomes:

SENSE

OUT

= ()( )









for Duty Cycle < 40%

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

compensation’s effect has to be taken into consideration

to select the appropriate value to provide the required

amount of current. Using the scaling factor (SF, in %) in

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 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:

RIPPLE

OUT MAX

OUT

















()

In Burst Mode operation, 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

≤

003

This implies a minimum inductance of:

MIN

SENSE

OUT









+−









003

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

however, the inductor current will not be continuous

during burst periods.

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