Backlight Power Measurement

[Aaron Carroll]

Hi,

I'm setting up a GTA02v6 for fine-grained power
measurements, and I'm having trouble getting sense out of
the backlight readings.

To determine backlight power, I've got a DAQ sampling the
voltage drop across R1763 (Rsense for the PCF50663 LED
supply) which gives me the current, and I'm sampling LED+
at C1764 for the input voltage.  I was hoping the product
of these two would yield the power consumed by the
backlight and Rsense.

However, if I vary the backlight brightness (in software)
and measure the power difference, I'm seeing significant
disparity between the power measured at the B/L compared
with total input power measured at the battery connector.

Here's where it gets weird.  If I multiply the measured
B/L power by ~1.5, suddenly everything falls into place
and the measurements match up.  This value was determined
numerically; I have no idea if it has any physical
significance or if it's just an interesting coincidence.

Looking at the signals on a CRO, the input voltage is
nice and DC (though it varies with brightness).  However
the current though Rsense has interesting transients which
I'm assuming is inherent to the converter design.  Even so
the RMS of this signal is very close to the average so I
think it shouldn't affect the results appreciably.

Any ideas in solving this would be greatly appreciated.


Thanks!
             -- Aaron

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[Paul Fertser]

Aaron Carroll <[hidden email]> writes:
> To determine backlight power, I've got a DAQ sampling the
> voltage drop across R1763 (Rsense for the PCF50663 LED
> supply) which gives me the current, and I'm sampling LED+
> at C1764 for the input voltage.  I was hoping the product
> of these two would yield the power consumed by the
> backlight and Rsense.
...

You've got quite puzzling and unexpected results indeed. Could you
please provide us with the actual measurements data?

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[Paul Fertser]

Paul Fertser <[hidden email]> writes:

> Aaron Carroll <[hidden email]> writes:
>> To determine backlight power, I've got a DAQ sampling the
>> voltage drop across R1763 (Rsense for the PCF50663 LED
>> supply) which gives me the current, and I'm sampling LED+
>> at C1764 for the input voltage.  I was hoping the product
>> of these two would yield the power consumed by the
>> backlight and Rsense.
> ...
>
> You've got quite puzzling and unexpected results indeed. Could you
> please provide us with the actual measurements data?

OTOH 66.6% efficiency for a boost converter doesn't sound like
something really exceptionally low. And moreover we lack the datasheet
for L1704 to be able to give proper estimates.

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[Aaron Carroll]

2009/10/10 Paul Fertser <[hidden email]>:

> Aaron Carroll <[hidden email]> writes:
>> To determine backlight power, I've got a DAQ sampling the
>> voltage drop across R1763 (Rsense for the PCF50663 LED
>> supply) which gives me the current, and I'm sampling LED+
>> at C1764 for the input voltage.  I was hoping the product
>> of these two would yield the power consumed by the
>> backlight and Rsense.
> ...
>
> You've got quite puzzling and unexpected results indeed. Could you
> please provide us with the actual measurements data?

Here's some data.  Each point is the average of 1000 samples @ 100 kHz,
alternating between voltage and current samples.

The percentages are backlight brightness according to the position of
the brightness slider (the phone runs Android).

BL is the backlight, with I measured across R1763, and V at C1764.
IN is measured at the battery with no other external connections,
with the current measured via a 100mohm resistor in series with (+).
The power is just the product of the quoted V/I.

To be completely accurate, the backlight figures are not raw measurements,
because they are scaled via an opamp-based attentuator before feeding
into the DAQ.  The values are scaled back up in software, and I have verified
this all happens correctly with an accurate meter.

Let me know if I can provide more details or measurements.


Thanks,
   -- Aaron




===========x8============


100%:
  BL = 14.38 V, 28.53 mA,  410.26 mW
  IN = 4.01 V,  252.89 mA, 1014.09 mW

75%:
  BL = 13.10 V, 15.91 mA,  208.42 mW
  IN = 4.04 V,  178.86 mA, 722.60 mW

50%:
  BL = 11.97 V, 6.45 mA,   77.21 mW
  IN = 4.05 V,  129.57 mA, 520.71 mW

25%:
  BL = 11.13 V, 1.82 mA,   20.26 mW
  IN = 4.06 V,  110.96 mA, 450.50 mW

0%:
  BL = 10.70 V, 0.49 mA,   5.24 mW
  IN = 4.06 V,  104.06 mA, 422.49 mW

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[Joerg Reisenweber]

[Aaron Carroll Di  13. Oktober 2009]:

> 2009/10/10 Paul Fertser <[hidden email]>:
> > Aaron Carroll <[hidden email]> writes:
> >> To determine backlight power, I've got a DAQ sampling the
> >> voltage drop across R1763 (Rsense for the PCF50663 LED
> >> supply) which gives me the current, and I'm sampling LED+
> >> at C1764 for the input voltage.  I was hoping the product
> >> of these two would yield the power consumed by the
> >> backlight and Rsense.
> > ...
> >
> > You've got quite puzzling and unexpected results indeed. Could you
> > please provide us with the actual measurements data?
>
> Here's some data.  Each point is the average of 1000 samples @ 100 kHz,
> alternating between voltage and current samples.
>
> The percentages are backlight brightness according to the position of
> the brightness slider (the phone runs Android).
>
> BL is the backlight, with I measured across R1763, and V at C1764.
> IN is measured at the battery with no other external connections,
> with the current measured via a 100mohm resistor in series with (+).
> The power is just the product of the quoted V/I.
>
> To be completely accurate, the backlight figures are not raw measurements,
> because they are scaled via an opamp-based attentuator before feeding
> into the DAQ.  The values are scaled back up in software, and I have

verified

> this all happens correctly with an accurate meter.
>
> Let me know if I can provide more details or measurements.
>
>
> Thanks,
>    -- Aaron
>
>
>
>
> ===========x8============
>
>
> 100%:
>   BL = 14.38 V, 28.53 mA,  410.26 mW
>   IN = 4.01 V,  252.89 mA, 1014.09 mW
>
> 75%:
>   BL = 13.10 V, 15.91 mA,  208.42 mW
>   IN = 4.04 V,  178.86 mA, 722.60 mW
>
> 50%:
>   BL = 11.97 V, 6.45 mA,   77.21 mW
>   IN = 4.05 V,  129.57 mA, 520.71 mW
>
> 25%:
>   BL = 11.13 V, 1.82 mA,   20.26 mW
>   IN = 4.06 V,  110.96 mA, 450.50 mW
>
> 0%:
>   BL = 10.70 V, 0.49 mA,   5.24 mW
>   IN = 4.06 V,  104.06 mA, 422.49 mW

Seems our converter is really operating with 66% efficiency :-/
I suspect L1704 and C1764 see some loss due to ESR.
You might want to test what happens if you connect a couple of low-ESR
capacitors (e.g. 5 pcs 4u7) parallel to C1764.
If bat current drops significantly on doing this, then this component really
wasn't a good choice for the purpose.

OTOH 200mW loss should be detectable anyway, just check where the temperature
rises ;-) Then you know which component is the hog

cheers
jOERG

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[Aaron Carroll]

2009/10/13 Joerg Reisenweber <[hidden email]>:
> Seems our converter is really operating with 66% efficiency :-/
> I suspect L1704 and C1764 see some loss due to ESR.
> You might want to test what happens if you connect a couple of low-ESR
> capacitors (e.g. 5 pcs 4u7) parallel to C1764.
> If bat current drops significantly on doing this, then this component really
> wasn't a good choice for the purpose.
>
> OTOH 200mW loss should be detectable anyway, just check where the temperature
> rises ;-) Then you know which component is the hog

Hi,

I wired up L1704, C1764, and the PCF50633 with a thermocouple.
Here are the results for max and min backlight, expressed as
degrees K above ambient.

100%:
  L1704 = 18.3
  C1764 = 6.6
  PCF  =    9.0

0%:
  L1704 = 6.0
  C1764 = 6.5
  PCF  =    7.2

I think this rules out C1764 as the problem.  However since the
PCF550633 has a large surface area compared with L1704, I'm
not sure this helps work out where most of the power is going...

The original question has been answered though... the power is
lost :(   What I still don't understand is why the efficiency so linear
over the entire backlight power range.


Thanks!
            -- Aaron

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[Werner Almesberger]

Aaron Carroll wrote:
> BL is the backlight, with I measured across R1763, and V at C1764.
[...]
>   BL = 14.38 V, 28.53 mA,  410.26 mW

Is the BL voltage what you measure at LED+ relative to GND or have
you already subtracted LED- ? In the former case, your result would
be about 10% high with maximum backlight.

> IN is measured at the battery with no other external connections,
> with the current measured via a 100mohm resistor in series with (+).
> The power is just the product of the quoted V/I.
[...]
>   IN = 4.01 V,  252.89 mA, 1014.09 mW

Which side is IN - battery side of the shunt resistor or Neo side ?
In the former case, you'd be another 6% high.

Regarding expected efficiency, the 50633 manual is silent about it,
the 50606 manual has the following data:

Current Conversion
(mA) 3.6 V -> 1.8 V 3.6 -> 3.3 V
  1 75% 92%
 10 85% 95%
100 85% 96%

Up conversion seems to have similar characteristics as down
conversion, but the voltages given don't make sense (Vi = 5.0 V
and Vp = 1.8 V. That's a down conversion !)

So we should expect an overall conversion loss of not much more
than 25%.

- Werner

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[Aaron Carroll]

2009/10/14 Werner Almesberger <[hidden email]>:
> Aaron Carroll wrote:
>> BL is the backlight, with I measured across R1763, and V at C1764.
> [...]
>>   BL = 14.38 V, 28.53 mA,  410.26 mW
>
> Is the BL voltage what you measure at LED+ relative to GND or have
> you already subtracted LED- ? In the former case, your result would
> be about 10% high with maximum backlight.

LED+ to GND, because I want to include the power lost in R1763.

>> IN is measured at the battery with no other external connections,
>> with the current measured via a 100mohm resistor in series with (+).
>> The power is just the product of the quoted V/I.
> [...]
>>   IN = 4.01 V,  252.89 mA, 1014.09 mW
>
> Which side is IN - battery side of the shunt resistor or Neo side ?
> In the former case, you'd be another 6% high.

I can't see which shunt resistor you are refering to, but the
measurement is taken at the battery connector (i.e. CON1701).

> Regarding expected efficiency, the 50633 manual is silent about it,
> the 50606 manual has the following data:
>
> Current         Conversion
> (mA)            3.6 V -> 1.8 V  3.6 -> 3.3 V
>  1             75%             92%
>  10             85%             95%
> 100             85%             96%
>
> Up conversion seems to have similar characteristics as down
> conversion, but the voltages given don't make sense (Vi = 5.0 V
> and Vp = 1.8 V. That's a down conversion !)
>
> So we should expect an overall conversion loss of not much more
> than 25%.

The 50606 converter has a much lower peak voltage/higher peak
current... can we make a meaningful extrapolation here?


Cheers,
          -- Aaron

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[Werner Almesberger]

Aaron Carroll wrote:
> LED+ to GND, because I want to include the power lost in R1763.

Okay, this would explain about 10% of the loss.

>> Which side is IN - battery side of the shunt resistor or Neo side ?
>> In the former case, you'd be another 6% high.
>
> I can't see which shunt resistor you are refering to, but the
> measurement is taken at the battery connector (i.e. CON1701).

I mean the 0.1 Ohm resistor you used to measure the current:

               0R1
Battery -----/\/\/\----- CON1701
          ^          ^
          |          |
          A          B

Your "IN" voltage is taken at point B, so it doesn't add a (false)
extra loss.

> The 50606 converter has a much lower peak voltage/higher peak
> current... can we make a meaningful extrapolation here?

It's all ballpark figures ;-) I was just looking for a reference for
the typical performance of the sort of DC-DC converters NXP use in
these chips. They all seem to have their peak efficiency of 85% in
the middle of their operating range and drop to 75% at the edges.

- Werner

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[Joerg Reisenweber]

[Werner Almesberger Do  15. Oktober 2009]:
> Aaron Carroll wrote:
> > LED+ to GND, because I want to include the power lost in R1763.
>
> Okay, this would explain about 10% of the loss.

Nope it won't, as the converter is powering the LED plus R. So the loss on R
isn't of any relevance to estimate the efficiency of the converter. It *is*
relevant for the efficiency of the whole LCM-backlight system though. But
that's not exactly what we're concerned about here.

/j

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[Werner Almesberger]

Joerg Reisenweber wrote:
> Nope it won't, as the converter is powering the LED plus R.

Argh, of course. I got that the wrong way around :-(

Now something funny: I tried the same setup with a GTA02v4, and I found
that it capped Vledfb at only 60 mV:

LEDOUT  Vbat    Ibat    Pbat    Vledout Vledfb  (V)     Isense  Vled    Pled  
        (V)     (mA)    (mW)    (V)     nominal meas.   (mA)    (V)     (mW)  
0x01      4.195     149 625.054  10.539   0.003   0.004   0.100  10.534   1.056
0x02      4.194     153 641.753  10.806   0.006   0.008   0.191  10.798   2.057
0x03      4.194     160 670.990  11.097   0.009   0.014   0.326  11.083   3.616
0x04      4.194     164 687.765  11.241   0.012   0.017   0.402  11.224   4.510
0x05      4.193     166 696.007  11.332   0.016   0.019   0.452  11.313   5.112
0x06      4.192     176 737.854  11.615   0.020   0.026   0.619  11.589   7.177
0x07      4.192     183 767.080  11.856   0.023   0.033   0.773  11.824   9.141
0x08      4.192     187 783.887  11.959   0.028   0.036   0.842  11.923  10.033
0x09      4.191     196 821.494  12.177   0.032   0.042   0.990  12.135  12.012
0x0a      4.190     204 854.823  12.375   0.036   0.048   1.130  12.328  13.930
0x0b      4.190     214 896.604  12.577   0.041   0.054   1.277  12.523  15.994
0x0c      4.189     222 929.948  12.754   0.046   0.060   1.410  12.695  17.903
0x0d      4.189     222 929.966  12.754   0.051   0.060   1.414  12.694  17.948
0x0e      4.189     222 930.000  12.753   0.057   0.060   1.414  12.693  17.951
0x0f      4.189     224 938.342  12.752   0.063   0.060   1.414  12.692  17.945
[...]

Note sure what happened to the poor device.

- Werner

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[Werner Almesberger]

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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[Aaron Carroll]

2009/10/20 Werner Almesberger <[hidden email]>:
> 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

Thanks!  I'm happy that my device isn't faulty.

Based on the heating I've noted in L1704, I've ordered some low-R
inductors to see if that has much effect.  I may have also smoked
the existing one while measuring its voltage waveform :)



Cheers,
              -- Aaron

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[Aaron Carroll]

Hi all,

2009/10/20 Aaron Carroll <[hidden email]>:
> Based on the heating I've noted in L1704, I've ordered some low-R
> inductors to see if that has much effect.

I replaced L1704 with a TDK VLF3012AT @ 2.2uH (data sheet:
http://www.tdk.co.jp/tefe02/e531_vlf3012a.pdf) and repeated the
experiments.  With this new part the efficiency of the conversion
jumped to 80%  :-)



Cheers,
       -- Aaron

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[Paul Fertser]

Aaron Carroll <[hidden email]> writes:
> 2009/10/20 Aaron Carroll <[hidden email]>:
>> Based on the heating I've noted in L1704, I've ordered some low-R
>> inductors to see if that has much effect.
>
> I replaced L1704 with a TDK VLF3012AT @ 2.2uH (data sheet:
> http://www.tdk.co.jp/tefe02/e531_vlf3012a.pdf) and repeated the
> experiments.  With this new part the efficiency of the conversion
> jumped to 80%  :-)

Just "wow" :)

Seriously, Aaron, great thanks for your investigation and
persistence. The result is something we all can learn from:

1. Don't be as insane as (some) TPE EEs ;)
2. When choosing a part think of what purpose you're going to employ
it for.

:D

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Be free, use free (http://www.gnu.org/philosophy/free-sw.html) software!
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[Werner Almesberger]

Aaron Carroll wrote:
> I replaced L1704 with a TDK VLF3012AT @ 2.2uH (data sheet:
> http://www.tdk.co.jp/tefe02/e531_vlf3012a.pdf) and repeated the
> experiments.  With this new part the efficiency of the conversion
> jumped to 80%  :-)

Wow ! Impressive result, congratuilations !

There are quite a lot of these Subaru inductors in GTA02. I wonder
how many more silent contributors to global warming of this kind
we have, e.g., IO_3V3 or USB power.

Has anyone been able to locate a data sheet for the Subaru parts
so that one could see if the specifications give any hint ?

- Werner

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[Paul Fertser]

Werner Almesberger <[hidden email]> writes:

> Aaron Carroll wrote:
>> I replaced L1704 with a TDK VLF3012AT @ 2.2uH (data sheet:
>> http://www.tdk.co.jp/tefe02/e531_vlf3012a.pdf) and repeated the
>> experiments.  With this new part the efficiency of the conversion
>> jumped to 80%  :-)
>
> Wow ! Impressive result, congratuilations !
>
> There are quite a lot of these Subaru inductors in GTA02. I wonder
> how many more silent contributors to global warming of this kind
> we have, e.g., IO_3V3 or USB power.
>
> Has anyone been able to locate a data sheet for the Subaru parts
> so that one could see if the specifications give any hint ?

Even my friend who lives in Japan atm and knows japanese wasn't able
to :(

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[Joerg Reisenweber]

[Paul Fertser Mi  28. Oktober 2009]:
> > Aaron Carroll wrote:
> >> I replaced L1704
> >>
> > Has anyone been able to locate a data sheet for the Subaru parts
> > so that one could see if the specifications give any hint ?
>
> Even my friend who lives in Japan atm and knows japanese wasn't able
> to :(

Also no datasheet in varaha (OM internal doc-server) it seems :-(

/j

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