Datasheet

Application notes - buck conversion LED5000
22/51 Doc ID 023951 Rev 1
5.8.1 Dimming frequency vs. dimming depth
As seen in Chapter 5.8 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, in any case it
degenerates into a trapezoid or, at worst, into a triangle, depending on the ratio (T
RISE
+
T
FALL
)/ T
DIM
Equation 23
The small signal response in
Figure 14 is considered as an example.
Equation 24
Assuming the minimum current pulse (T
MIN_PULSE
) shape specification as:
Equation 25:
where T
DIMMING
represents the dimming period and D
MIN
the minimum duty cycle which
gives the T
MIN_PULSE
charge. In the given example T
MIN_PULSE
=9µs
Figure 15. dimming signal
Given T
MIN_PULSE
it is possible to calculate the maximum dimming depth given the dimming
frequency or vice versa.
For example, assuming a 10 KHz dimming frequency the maximum dimming depth is 9% or
given a 5% dimming depth it follows a 5.5 KHz maximum f
DIM
.
The LED5000 dimming performance is strictly dependent on the system small signal
response. As a consequence, an optimized compensation network (good phase margin and
bandwidth maximized) and minimized C
OUT
value are crucial for best performance. Once
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
5μs≅
T
FALL
2μs≅
⎩
⎨
⎧
T
RISE
T
FALL
+ 0.75 T
MIN_PULSE
⋅ 0.75 D
MIN
T
DIMMING
⋅⋅==
AM13499v1