Cirrus Logic AN374 - Manual (Page 24)

AN374
24 AN374REV2
6.3.3 Experiment and Approximation
In many cases, collecting data at room ambient is not difficult. However, getting data across various ambient
temperatures, particularly at low temperatures, is not easy. In an event where getting data is not feasible at
ambient temperatures outside of room temperature, existing 25

C data can be extended to other temperatures
based on data sheet numbers of the LED and measured temperature rises. If the requested operating
temperature is very large (

T
amb
> 50

C), then the errors due to this simple linear approximation may be too
high. Remember that the method is a simple approximation and subject to errors that may vary depending on
the system under investigation.
The following method is suggested to extend data to other ambient temperatures. In this case, two ambient
temperatures of 0

C and 40

C are chosen. These are some of the assumptions that are made. The
temperature rise from ambient is assumed to be constant irrespective of the actual ambient temperature level.
Step 1) Collect red current I
Red
, white current I
White
, and NTC data and junction temperature T
J
of the LED at
25

C ambient temperature T
amb(25  C)
from 0% dim to 100% dim.
Step 2) Calculate the following:
Step 3) Calculate the various NTC temperature T
NTC
and junction temperature T
J
of the LED at various
ambient temperatures across the entire dimming range as follows:
Step 4) Calculate the NTC resistance using the equation of the system at 25

C ambient and the expression
At this point, a complete sample space has been recreated using the temperature rise. The remaining steps
are focused on recreating the red currents I
Red
across temperature.
Step 5) Refer to the LED data sheet for the Relative Luminous Flux Curve and note the relative luminous flux
at ambient temperature T
amb
= 25

C, 0

C, and 40

C. If

1
,

2
, and

3
are the relative luminous
fluxes at 25

C, 0

C, and 40

C ambient, then the red currents can be calculated using Equations 20
and 21:
Thus by performing this at every dim level, the entire dimming data required can be constructed.
T
amb
T
J
T
25 C
– = [Eq. 15]
T
NTC
T
J
T
NTC
– = [Eq. 16]
T
J
T
amb 0 C  
T
amb
+ = [Eq. 17]
T
NTC
T
amb 0 C  
T
NTC
+ = [Eq. 18]
R
T1
R
T0
e

1
T1
-------
1
T0
------- –
 
 

 =
[Eq. 19]
I
Red 0 C  
I
Red 25 C  

1

2
-------  = [Eq. 20]
I
Red 40 C  
I
Red 25 C  

1

3
-------  = [Eq. 21]