Datasheet

Application notes LED2000
20/40 DocID023432 Rev 4
Figure 12. LED current falling edge operation
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 (T
RISE
) and the selected output capacitor value (T
RISE
and T
FALL
).
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
(T
RISE
+ T
FALL
) / T
DIM
.
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 gletan
T
RISE
T
FALL
+
T
DIM
--------------------------------------------- 1«

trapezoid
T
RISE
T
FALL
+
T
DIM
--------------------------------------------- 1

triangle
T
RISE
T
FALL
+
T
DIM
--------------------------------------------- 1=
T
RISE
20s
T
FALL
5s



T
RISE
T
FALL
+ 0,5 T
MIN_PULSE
 0,5 D
MIN
T
DIMMING
==