ADP1864

Rev. 0 | Page 10 of 16

APPLICATION INFORMATION

DUTY CYCLE

To determine the worst case inductor ripple current, output

voltage ripple, and slope compensation factor, determine the

system maximum and minimum duty cycle. The duty cycle is

calculated by the equation

()

D

IN

D

OUT

V

V

V

V

DC

CYCLE

DUTY

+

+

=

has a forward voltage drop of 0.5 V.

RIPPLE CURRENT

Choose the peak-to-peak inductor ripple current between 20%

and 40% of the maximum load current at the system’s highest

input voltage. A good starting point for a design is to pick the

peak-to-peak ripple current at 30% of the load current.

SENSE RESISTOR

Choose the sense resistor value to provide the desired current

limit. The internal current comparator measures the peak

current (sum of load current and positive inductor ripple

current) and compares it against the current limit threshold.

The current sense resistor value is calculated by the equation

()

()

()

2

PEAK

MAX

LOAD

MIN

SENSE

I

I

PCSV

R

Δ

+

=

where PCSV is the peak current sense voltage, typically 0.125 V.

To ensure the design provides the required output load current

over all system conditions, consider the variation in PCSV over

temperature (see the Specifications section) as well as increases

in ripple current due to inductor tolerance.

If the system is being operated with >40% duty cycle, incor-

porate the slope compensation factor into the calculation.

()

()

()

2

PEAK

MAX

LOAD

MIN

SENSE

I

I

PCSV

SF

R

Δ

+

×

=

where SF is the slope factor correction ratio, taken from

Figure 14, at the system maximum duty cycle (minimum input

voltage).

0

1.0

DUTY CYCLE

1.05

0.35

0.95

0.85

0.75

0.65

0.55

0.45

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

Figure 14. Slope Factor (SF) vs. Duty Ratio

INDUCTOR VALUE

The inductor value choice is important because it dictates the

inductor ripple and, therefore, the voltage ripple at the output.

When operating the part at greater than 40% duty cycle, keep

the inductor value low enough for the slope compensation to

remain effective.

The inductor ripple current is inversely related to the inductor

value.

()

(

)

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

+

×

×

−

=

Δ

D

IN

D

OUT

OUT

IN

PEAK

V

V

V

V

f

L

V

V

I

Smaller inductor values are typically smaller in size and usually

less expensive, but increase the ripple current and the output

voltage ripple. Too large an inductor value results in added

expense and may impede effective load transient responses at

>40% duty cycle because it reduces the effect of slope

compensation.

Start with the highest input voltage, and assume ripple current

is 30% of the maximum load current:

(

)

()

⎟

⎟

⎠

⎞

⎜

⎜

⎝

⎛

+

+

×

×

×

−

=

D

IN

D

OUT

MAX

LOAD

OUT

IN

V

V

V

V

f

I

V

V

L

3

.

0

From this starting point, modify the inductance to obtain the

right balance of size, cost, and output voltage ripple while

maintaining the inductor ripple current between 20% and 40%

of the maximum load current.