Cirrus Logic AN374 - Manual (Page 21)

AN374
AN374REV2 21
6 Data Improvements for the Curve Fitting Process
The curve-fitting process consists of translating a typical LED system specification into polynomial gain equations
that the curve fitter can then calculate the required current for a given dim and NTC reading. The gains can be used
in conjunction with the design of the power stage to complete a system design for a CS1630/31-based LED driver.
6.1 Typical Light Engine System Specifications
A typical light engine system specification is concisely captured in Figure 11. It is essentially a plot of the target
lumen output at a given target color correlated temperature (CCT). Points 1, 2, and 3 are some data points of
interest. These points have been used to define system behavior. The graph shows that the behavior is
expected to be constant across the entire operating temperature of the driver. If the behavior over temperature
were different, it can be represented as separate parameterized curves using the same axis.
6.2 Translation into Input Specifications for Calculating Color Gains
The light engine specifications above need to be translated into typical driver specifications that can be used
by the second stage of the CS1630/31 based driver.
Step 1) Introduction of an independent dim axis
The curve fitter polynomial equation is an equation across dim and NTC temperature. The independent dim
axis can be introduced as shown in Figure 12. At this point, the specifications can be redefined as needed to
find a trajectory the light engine needs to traverse in terms of CCT and brightness when the input dim level to
the system changes.
1600
650
1
2
3
2700
Brightness (lm)
CCT (K)
Figure 11. Typical LED Light Engine Specifications
0.02
650
Brightness (lm)
1.0
1
0.02
2700
1600
CCT
(K)
Dim
1.0
2
3
Figure 12. Light Engine Specifications Translated to an Imaginary Dim Axis