LED2001PHR datasheet(20/42 Pages) STMICROELECTRONICS

20 / 42 page

Application notes

LED2001

20/42

DocID024346 Rev 1

Figure 12. LED current falling edge operation

6.6.1

Dimming frequency vs. dimming depth

As seen in Section 6.6, the LEDs current rising and falling edge time mainly depends on the

system bandwidth (TRISE) and the selected output capacitor value (TRISE and TFALL).

The dimming performance depends on the minimum current pulse shape specification of

the final application. The ideal minimum current pulse has rectangular shape, however, it

degenerates into a trapezoid or, at worst, into a triangle, depending on the ratio (TRISE +

TFALL)/ TDIM.

Equation 19

The small signal response in Figure 11 and Figure 12 is considered as an example.

Equation 20

Assuming the minimum current pulse shape specification as:

Equation 21:

it is possible to calculate the maximum dimming depth given the dimming frequency or vice

versa.

AM12903v1

rec

gle

tan

RISE

FALL

DIM

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

→

trapezoid

RISE

FALL

DIM

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

→

triangle

RISE

FALL

DIM

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

RISE

μs

≅

FALL

μs

≅







RISE

FALL

0.5 T

MIN_PULSE

⋅

0.5 D

MIN

DIMMING

⋅⋅

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