Cirrus Logic AN374 - Manual (Page 22)

AN374
22 AN374REV2
Step 2) Translate into LED current profile at room temperature
The translation is as shown in Figure 13.
Variables K
1
and K
2
represent the luminous efficacy of the LED, and variables m
1
and m
2
are arbitrary
constants. The plot in Figure 13 illustrates the relationship between CCT and output currents, lumens, and
output power.
The data points 1, 2, and 3 are recreated under thermal equilibrium with the target LED light engine at room
temperature to obtain the LED current profiles for each channel across the entire range of the target lumen
output. The NTC temperature recorded on the final measurement is used on the driver. At the end of this
procedure, the following are obtained: the current versus dim, forward voltage versus dim, NTC resistance
versus dim, current ratio versus dim, and the LED light engine power profile data at room temperature.
Step 3) Validate data across various ambient temperatures
Finally, the same step needs to be completed across different ambient temperatures. All the measurements
should be performed when the system has reached thermal equilibrium. Once the data is taken, the sample
data is shown in Figures 14 and 15.
m
1
, I
Red
, m
2
, I
White
I
Red
/ I
White
Ratio of the 2
Channel’s Lumens
Sum of Each
Channel’s Lumens
Approximated
Using Current
Approximated
Using Current
0.02
650
1.0
1
0.02
2700
1600
Dim Dim Dim
1.0
2
3
0.02 1.0
1
0.02
Ratio
1.0
2
3
0.02 1.0
1
0.02
Ratio
1.0
2
3
K
1
, P
Red
+ K
2
, P
White
K
1
, P
Red
K
2
, P
White
Brightness (lm) Brightness (lm)
Relative Brightness (lm)
CCT
(K)
Figure 13. Translate Data and CCT Lumen Dependence on Output Current
0.02
I Red
(A)
Dim
1.0
T
amb
= 5°C
T
amb
= 25°C
T
amb
= 40°C
T
amb
= 55°C
Figure 14. Red Current Across Dim at Various Ambient Temperatures