STMicroelectronics STLDC08 Demo Board STEVAL-ILL039V1/2 STEVAL-ILL039V2 STEVAL-ILL039V2 Fiche De Données

Doc ID 18476 Rev 1

17/29

As a first approximation we choose the inductor ripple current, I

L

, equal to approximately 40 

% 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.

I

L

 is determined by the input and output voltage, the value of the inductance, and T

OFF

.

The minimum value of inductance which guarantees the fixed inductor ripple current can be 
determined using the following equation:

where V

d

 is the forward drop of the Schottky diode, I

L

 is the fixed inductor ripple current, and 

T

OFF

 is the constant OFF time.

The following equation shows the average inductor current as a function of the output 
current and duty cycle.

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:

         

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

−

=