DocID025366 Rev 2

19/25

AN4374

LED supply voltage

25

6

LED supply voltage

parameters:

•

(see dedicated section in the datasheet).

•

•

The maximum power can be dissipated by the package under the application ambient

conditions.

•

value should be considered).

Therefore the minimum LED supply voltage can be calculated as:

Equation 1

involved parameters) but not too high in order to keep low the power dissipation:

Equation 2

respectively the voltage drop across, the current generator i and the channel current i (the

dissipation, defined as:

Equation 3

the junction-to-ambient thermal resistance. Junction temperature should be maintained

below 125 °C for industrial devices and below 150 °C for automotive grade devices. To

summarize, the choice of the proper power supply must be a trade-off between the correct

value assuring the desired LED current and the low power dissipation. In RGB applications,

there can be a significant variability of the LED forward voltage (e.g. red LEDs have a lower

forward voltage compared to green and blue ones).

In this case, the supply voltage must be chosen high enough to correctly switch on the LEDs

with the highest forward voltage (green or blue). However this supply voltage is higher than

the voltage required by red LEDs. Thus, the excess of voltage in the lines with red LEDs

drops on the current generators, brings an increase of the power dissipation and loss of

efficiency.

V

LED,min

V

o,typ

V

F,max

+

=

P

D

V

DD

I

DD

V

0i

,

I

CHi

⋅

il

=

16



+

⋅

=

P

D,max

T

j

T

a

–

()

θ

ja

----------------------

=