Cirrus Logic AN374 - Manual (Page 26)

AN374
26 AN374REV2
If resistance is measured, then
The data below shows the white/blue currents across the dim. The format in Figure 18 is useful because it is
required when the data is imported into the curve fitter.
The blue/white current at 100% will be considered the nominal current at full sine wave. The reference currents
are set based on this value.
The red currents and the corresponding temperature data are shown below.
The target currents will be the currents at 100% dim and 25

C ambient temperature. The NTC code data in
Figure 19 shows the computed NTC resistance according to Equation 22. The NTC code is the internal
computed value used by the CS1630/31.
NTC
4M 
R
NTC
R
Series
+
-------------------------------------- = [Eq. 23]
enter enter
љ љ
BwDim I_bw (A)
0.1 0.068
0.2 0.131
0.3 0.190
0.4 0.246
0.5 0.297
0.6 0.344
0.7 0.386
0.8 0.425
0.9 0.460
1 0.490
Enter dim values
in ascending order
At 50% dim,
I
White
= 0.297 A
Figure 18. Example of Formatted Data for the White/Blue Current Across Dim
ї enter 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Tamb(room) computed ќ
5.00 0.042 0.078 0.110 0.138 0.161 0.180 0.196 0.207 0.216 0.221
15.00 0.044 0.083 0.116 0.145 0.170 0.190 0.206 0.218 0.227 0.233
25.00 0.046 0.087 0.122 0.153 0.179 0.200 0.217 0.230 0.239 0.245
45.00 0.049 0.091 0.128 0.160 0.187 0.210 0.228 0.241 0.251 0.257
65.00 0.051 0.096 0.135 0.168 0.197 0.220 0.239 0.253 0.264 0.270
85.00 0.053 0.101 0.142 0.177 0.207 0.231 0.251 0.266 0.277 0.284
Tamb(room) enter ќ
5.00 5.20 7.40 9.60 11.80 14.00 16.20 18.40 20.60 22.80 25.00
15.00 15.20 17.40 19.60 21.80 24.00 26.20 28.40 30.60 32.80 35.00
25.00 25.20 27.40 29.60 31.80 34.00 36.20 38.40 40.60 42.80 45.00
45.00 45.20 47.40 49.60 51.80 54.00 56.20 58.40 60.60 62.80 65.00
65.00 65.20 67.40 69.60 71.80 74.00 76.20 78.40 80.60 82.80 85.00
85.00 85.20 87.40 89.60 91.80 94.00 96.20 98.40 100.60 102.80 105.00
computed ќ
13.5 15.2 17.0 19.0 21.2 23.6 26.1 28.9 31.9 35.1
22.5 25.0 27.6 30.5 33.6 36.9 40.5 44.2 48.2 52.4
35.4 38.8 42.5 46.3 50.4 54.7 59.2 64.0 68.9 73.9
74.4 79.7 85.1 90.6 96.2 102.0 107.7 113.6 119.5 125.3
125.9 131.7 137.5 143.3 149.0 154.6 160.0 165.4 170.7 175.8
176.2 181.1 185.9 190.6 195.0 199.3 203.4 207.4 211.2 214.8
RedDim
Tntc[C]
NTC code
I_red
At 30% dim,
I
Red
= 0.122 A and
T
NTC
= 29.60°C
Enter dim values in ascending order
Enter
temperature
values in
ascending
order
Figure 19. Example of Formatted Data for Red Current and NTC Resistance/Code Across Dim