Cirrus Logic AN374 - Manual (Page 8)

AN374
8 AN374REV2
I
Red
, NTC, and dim variables, the curve fitter will have to compute gains across a larger number of points,
which creates the risk of increased error. Failure to compensate for these points may cause a color shift
during startup until the LED strings reach thermal equilibrium.
Constraint 4: Red currents increase with an increase in ambient temperature
The current I
Red
in the red channel should increase monotonically with ambient temperature at a given dim
value.
For example, if the current at ambient temperature T
amb
= 50 C is less than the current at ambient temperature
T
amb
= 25 C, then the currents are non-monotonic with ambient temperature.
Constraint 5: Maximum allowable color gain is 4.0
The recommended maximum gain value that the color system generates is 4.0. For example, the red current
at 25

C cannot be four times larger or smaller than any other current at the same dim value.
4.2 Constraints Imposed by Second Stage
Constraints in this section are imposed by the second stage. Calculations of these parameters are detailed in
application note AN368 Design Guide for a CS1630/31 2-Channel TRIAC Dimmable SSL Circuit. All variables
referenced in this document are computed in application note AN368.
Constraint 1: Currents in either channel should not be zero at non-zero dim values
Calculate peak current I
PK1(FB)
during Mode 1 using Equation 4:
Calculate the average current I
MODE1(avg)
during Mode 1 using Equation 5:
Equations 4 and 5 show that both channel currents share the same peak currents. It is difficult to have zero
current in one channel and have a non-zero current in the other channel. As a result, currents must be greater
than zero at all non-zero dim values.
Constraint 2: No abrupt change in current ratios at a dim point
It is recommended to avoid any abrupt change in the ratio of two-channel currents because it may cause color
flicker. The ratio of the channel current and the actual value of the string current should be a smooth function
which does not have any abrupt changes that become visible.
Ambient Temperature
Red Current Flowing in
Current Compensated Channel
0

C 300mA
25

C 330mA
50

C 360mA
Table 1. Maximum Measurable Switching Period
I
PK1 FB  
V
BST
T1
CH1
L
P
----------------
 
 
 200V
5.3 s
3543 H
---------------------
 
 
 299mA = = = [Eq. 4]
I
MODE1 avg  
I
PK1 FB  
N
T2
CH1
2 TT 
----------------
 
 
  299mA 5.57
9.0 s
2 35.05 s 
------------------------------
 
 
  214mA = = = [Eq. 5]