Application information

STLDC08

16/29

Doc ID 18476 Rev 1

8

Application information

8.1

LED current programming

The LED current is set by an external resistor connected between the FB pin and GND. The

the desired output current:

Equation 1

by the error amplifier. The internal reference has a guaranteed tolerance of 10 %. Tolerance

of the sense resistor adds additional error to the output voltage. 1 % resistors are

recommended.

8.2

Duty cycle

The controlled off-time architecture is a hysteretic mode control. Hysteretic operation

eliminates the need for small signal control loop compensation. When the converter runs in

duty cycle given by the following relationship:

Equation 2

Equation 3

and VD is the forward voltage of the Schottky diode.

8.3

Inductor selection

As the hysteretic control scheme is inherently stable, the inductor value does not affect the

stability of the regulator. The switching frequency, peak inductor current, and allowable

ripple of the output current determine the value of the inductor.

LED manufacturers generally recommend a value for LED current ripple ranging from 5 % to

20 % of LED average current.

LED

FB

I

1

.

0

R

=

D

OUT

IN

V

V

V

1

D

+

−

=

FB

)

LED

(

F

O

V

V

n

V

+

×

=