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LTC1874

Slope Compensation and Inductor’s Peak Current

The inductor’s peak current is determined by:

ITH

SENSE

= ()

–.

when the controller is operating below 40% duty cycle.

However, once the duty cycle exceeds 40%, slope com-

pensation begins and effectively reduces the peak inductor

current. The amount of reduction is given by the curves in

Figure 2.

Figure 2. Percentage of Maximum Output Current vs Duty Cycle

DUTY CYCLE (%)

110

100

1874 F02

70 80 90 100

20 30

40 50

IRIPPLE = 0.4IPK

AT 5% DUTY CYCLE

IRIPPLE = 0.2IPK

AT 5% DUTY CYCLE

VIN = 4.2V

The basic LTC1874 application circuit is shown in

Figure 1. External component selection for each control-

ler is driven by the load requirement and begins with the

selection of L1 and RSENSE (= R1). Next, the power

MOSFET (M1) and the output diode (D1) are selected

followed by CIN and COUT (= C1).

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 controller can provide is given by:

OUT

SENSE

RIPPLE

=−

012

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

where SF is the “slope factor.”

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

APPLICATIO S I FOR ATIO

OPERATIO

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