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

16/29

Doc ID 18476 Rev 1

The LED current is set by an external resistor connected between the FB pin and GND. The 
following equation can be used to calculate the value of the R

FB

 resistor which guarantees 

the desired output current:

The feedback signal V

FB

 is compared with the internal precision 100 mV voltage reference 

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.

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 
continuous conduction mode (CCM) the controller adapts the T

ON

 time in order to obtain the 

duty cycle given by the following relationship:

where V

O

 is the output voltage given by: 

and VD is the forward voltage of the Schottky diode. 

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

+

×

=