Backlight Power Measurement
Werner Almesberger
werner at openmoko.org
Mon Oct 19 18:37:04 CEST 2009
I wrote:
> Now something funny: I tried the same setup with a GTA02v4, and I found
> that it capped Vledfb at only 60 mV:
I tried a different device and got something a lot saner:
LEDOUT Vbat Ibat Pbat Vledout Vledfb (V) Isense Vled Pled
(V) (mA) (mW) (V) nominal meas. (mA) (V) (mW)
0x01 4.174 92 383.966 10.041 0.003 0.004 0.097 10.037 0.975
0x02 4.173 92 383.946 10.151 0.006 0.007 0.161 10.144 1.633
0x03 4.173 92 383.909 10.267 0.009 0.012 0.276 10.255 2.827
0x04 4.173 93 388.076 10.308 0.012 0.014 0.334 10.294 3.441
0x05 4.173 93 388.094 10.343 0.016 0.017 0.394 10.327 4.067
0x06 4.173 93 388.080 10.400 0.020 0.021 0.509 10.379 5.280
0x07 4.173 94 392.270 10.451 0.023 0.027 0.630 10.424 6.568
0x08 4.173 94 392.258 10.475 0.028 0.029 0.691 10.446 7.219
0x09 4.173 94 392.244 10.516 0.032 0.034 0.808 10.482 8.467
0x0a 4.173 95 396.409 10.554 0.036 0.039 0.925 10.515 9.728
0x0b 4.172 95 396.362 10.590 0.041 0.044 1.046 10.546 11.028
0x0c 4.172 96 400.521 10.623 0.046 0.049 1.161 10.574 12.272
0x0d 4.172 96 400.488 10.655 0.051 0.054 1.278 10.601 13.548
0x0e 4.172 97 404.645 10.685 0.057 0.059 1.396 10.627 14.836
0x0f 4.172 97 404.637 10.730 0.063 0.066 1.573 10.664 16.772
0x10 4.172 97 404.652 10.757 0.069 0.071 1.688 10.686 18.042
0x11 4.171 98 408.793 10.798 0.075 0.079 1.861 10.719 19.943
0x12 4.171 98 408.788 10.824 0.082 0.083 1.978 10.741 21.241
0x13 4.171 99 412.932 10.862 0.089 0.091 2.149 10.771 23.150
0x14 4.171 99 412.888 10.899 0.096 0.098 2.322 10.801 25.078
0x15 4.170 100 417.034 10.936 0.104 0.105 2.496 10.830 27.034
0x16 4.170 101 421.166 10.983 0.112 0.115 2.727 10.868 29.638
0x17 4.169 102 425.287 11.019 0.121 0.123 2.904 10.896 31.641
0x18 4.169 103 429.450 11.064 0.130 0.132 3.137 10.932 34.289
0x19 4.169 104 433.594 11.109 0.139 0.142 3.370 10.967 36.965
0x1a 4.169 105 437.712 11.154 0.149 0.152 3.605 11.002 39.661
0x1b 4.168 105 437.690 11.197 0.160 0.162 3.834 11.035 42.307
0x1c 4.168 107 445.982 11.249 0.171 0.174 4.123 11.075 45.664
0x1d 4.167 108 450.042 11.301 0.183 0.186 4.413 11.115 49.045
0x1e 4.167 109 454.189 11.353 0.195 0.199 4.704 11.154 52.470
0x1f 4.167 111 462.504 11.404 0.208 0.211 5.000 11.193 55.971
0x20 4.166 111 462.473 11.454 0.221 0.223 5.288 11.231 59.387
0x21 4.166 113 470.774 11.512 0.235 0.238 5.631 11.274 63.488
0x22 4.165 114 474.823 11.569 0.250 0.252 5.970 11.317 67.562
0x23 4.165 116 483.103 11.637 0.266 0.269 6.382 11.367 72.543
0x24 4.164 118 491.397 11.701 0.283 0.286 6.779 11.415 77.385
0x25 4.163 120 499.611 11.767 0.300 0.303 7.186 11.463 82.370
0x26 4.162 122 507.825 11.833 0.318 0.321 7.603 11.512 87.521
0x27 4.162 124 516.116 11.906 0.337 0.341 8.069 11.566 93.323
0x28 4.161 126 524.345 11.979 0.358 0.360 8.536 11.618 99.178
0x29 4.161 129 536.738 12.058 0.379 0.382 9.055 11.676 105.733
0x2a 4.159 131 544.864 12.138 0.401 0.404 9.576 11.734 112.365
0x2b 4.159 134 557.254 12.224 0.424 0.428 10.152 11.796 119.746
0x2c 4.157 137 569.534 12.309 0.449 0.453 10.725 11.857 127.163
0x2d 4.156 139 577.748 12.396 0.475 0.478 11.316 11.919 134.871
0x2e 4.155 143 594.219 12.488 0.502 0.504 11.945 11.984 143.149
0x2f 4.154 146 606.494 12.588 0.531 0.534 12.644 12.055 152.423
0x30 4.153 150 622.942 12.689 0.561 0.563 13.347 12.126 161.843
0x31 4.152 153 635.222 12.794 0.593 0.595 14.094 12.199 171.940
0x32 4.150 158 655.698 12.907 0.626 0.629 14.898 12.278 182.916
0x33 4.149 162 672.094 13.018 0.661 0.663 15.702 12.356 194.006
0x34 4.146 166 688.291 13.136 0.698 0.699 16.560 12.437 205.958
0x35 4.145 171 708.828 13.263 0.736 0.738 17.496 12.525 219.135
0x36 4.143 176 729.192 13.395 0.777 0.780 18.472 12.616 233.031
0x37 4.141 181 749.535 13.527 0.819 0.821 19.461 12.706 247.268
0x38 4.139 187 773.950 13.665 0.864 0.865 20.504 12.800 262.457
0x39 4.136 194 802.465 13.818 0.912 0.914 21.661 12.904 279.502
0x3a 4.134 201 830.921 13.966 0.961 0.962 22.803 13.004 296.526
0x3b 4.131 208 859.321 14.125 1.013 1.014 24.031 13.110 315.054
0x3c 4.128 217 895.840 14.294 1.068 1.070 25.354 13.224 335.284
0x3d 4.125 225 928.140 14.462 1.126 1.126 26.675 13.337 355.761
0x3e 4.121 234 964.339 14.644 1.186 1.187 28.118 13.458 378.409
0x3f 4.117 244 1004.608 14.834 1.250 1.250 29.631 13.584 402.503
So this confirms Aaron's observation. The script that did the measurements
is in svn.openmoko.org/developers/werner/bl-pwr/led.py
The results above (minus slight editing for better readability) are in
svn.openmoko.org/developers/werner/bl-pwr/bl
- Werner
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