Thursday, August 27, 2015

Cheap TL431 voltage references

Until a year ago I had never heard of the TL431.  Then I read Ken Shirriff's blog post, as well as other mentions of the TL431 on and  I found out the 431 is useful not only as a voltage reference, but also as a constant current control, and even a voltage controlled oscillator.

I had started suspecting my cheap (~$20) auto-ranging multimeter was reporting voltages a bit on the high side, and when I found 100 TL431s selling for less than 150c, I ordered them.  While waiting for them to arrive I tried to find out more information about the manufacturer, Wing Shing Computer Components of Hong Kong.   I could not find an active web site (at least in English), and although I found an old datasheet for the WS-TL431, I could not find anything current.  I did find another Aliexpress seller that posted a photo of a box full of WS-TL431A showing a 0.3% accuracy rating, which, considering the low price, is quite good.  Even 1% rated genuine TI TL431 parts are difficult to find for less than 2c each.

Once I received the package, I checked out the chip markings, which were all the same:

I suspect the 155 is a date code for 2015, 5th week, indicating these are new parts.  The old datasheet from Wing Shing shows the TL431A part as only 1%, and a TL431AA as 0.5%, and nothing listed for a 0.3% part.  I don't think I'm perpetuating an unfair stereotype to say that the Chinese are notorious for bad or non-existent documentation.  I think that the parts I received are actually rated to within 0.3% at 25C, and the manufacturer has not undertaken to produce an updated datasheet (or English website, for that matter).  Other compatible parts such as Linear's LT1431 is rated at a 0.4% initial tolerance, and the  price is in line with similar Chinese TL431 parts such as the ALJ TL431A and the CJ431.  After checking the WS TL431 chip markings, I setup a simple circuit on my breadboard with a 270 Ohm input resistor (which should give about 9.5mA) from a ~5V USB power supply to test the parts.

I tested a total of 25 parts with ambient temperature of 24C.  The average voltage reading was 2.513, and the range was from 2.506 to 2.517, or 2.5115V +-0.23%  The measurements are consistent with the parts being 0.3% rated, as well as suggesting my meter is reading about 0.6-0.7% high.

The next thing I tried was to crack open the TO-92 package with a pair of pliers in an attempt to expose the die.  Like Ken, I was able to expose the copper anode (seen in the very first picture), but was not able to expose the die.  The die appears to be around 0.6mm x 1mm, so even if I was able to expose the die, with only a magnifying glass, I doubt I would have been able to see much.

My intention in trying to expose the die was to see if the Wing Shing parts are fuse trimmed like the TI part depicted by Ken.  Two fuses give four different combinations of trimming options, which should show up as more than one peak in the distribution of the voltages.  Without a die, I could still analyze my measurements and look for peaks.   A simple shell command was all I needed:
sort voltages.txt | uniq -c
      1 2.506
      1 2.508
      2 2.509
      3 2.510
      2 2.511
      3 2.512
      1 2.513
      3 2.514
      3 2.515
      3 2.516
      3 2.517

Even with only a quarter of the parts tested, it is evident the voltages are concentrated around 2.510V, 2.512V, and 2.515/2.516V.  While more data points would be helpful, the testing is consistent with fuse-trimmed 0.3% parts.

The first practical circuit I made with the TL431 uses it as a 2.5V zener for battery reconidtioning.  I had been using a 270 Ohm resistor to discharge the batteries.  With the TL431 acting as a 2.5V zener, a high current red LED and 160 Ohm resistor add up to an addition 2.5V drop, very close to the 4.8V total when discharging a 12-cell battery to 0.4V/cell.

I'd like to have a discharge closer to 0.1C, which would be around 130mA, but the red LED is rated for 50mA maximum continuous current.  The TL431 datasheet has a simple constant current circuit, and by making a couple small modifications to that circuit I think I can make a 130mA constant current discharge circuit with a cut-off voltage just below 5V.

2015/09/22 Update

I did a quick test of the dynamic impedance, or the change in reference voltage vs change in shunt current.  Increasing the current from 3mA to 15mA resulted in a 2mV increase in reference voltage, indicating a dynamic impedance in the range of 0.15 to 0.2 Ohms.

2015/12/20 Update

Based on my testing with an AD584, I now believe these are 2.5V references, not 2.495.

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