STLDC08

Application information

Doc ID 18476 Rev 1

17/29

% of the output current. Higher ripple current allows for smaller inductors, but it also

increases the output capacitance for a given LED current ripple requirement. Conversely,

lower ripple current can be obtained increasing the value of the inductance, and this enables

a reduction of the output capacitor value. This trade-off can be altered once standard

inductance and capacitance values are chosen.

The minimum value of inductance which guarantees the fixed inductor ripple current can be

determined using the following equation:

Equation 4

The following equation shows the average inductor current as a function of the output

current and duty cycle.

Equation 5

An inductor that can carry the maximum input DC current which occurs at the minimum

input voltage should be chosen. The peak-to-peak ripple current is set by the inductance

and a good starting point is to choose a ripple current of at least 40 % of its maximum value

of the:

Figure 18.

Timing diagram

I

RIPPLE

I

L

t

I

PEAK

I

OUT

T

OFF

T

ON

D

1

I

I

OUT

IN

−

=

AM07848v1

I

RIPPLE

I

L

t

I

PEAK

I

OUT

T

OFF

T

ON

D

1

I

I

OUT

IN

−

=

AM07848v1

OFF

L

INMIN

d

OUT

T

)

I

(

)

V

-

V

+

(V

L

×

Δ

>

D

1

I

I

LED

)

AVG

(

L

−

=