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LM385 BZ 2.5 not producing advertised voltage

Bob Manekshaw
Prodigy 30 points
Other Parts Discussed in Thread: TL431

Not sure is this is the correct forum to post this query in so apologies in advance if not.

I need a 2.5V reference for my PIC 16F886 project to act as a voltage reference for the Analogue/Digital converters built into the chip.  I found the LM385BZ2.5 which seems to fit the bill for price and spec on the datasheet but it only seems to want to produce 2.2V in my circuit rather than the 2.5V it should (2.425 to 2.575).

The reference is powered from a +5V regulated supply though a 2K5 resistor to the Cathode and the anode is connected to ground. This should give 1mA flowing through the device which operates from 20uA to 20mA.  Thinking the PIC may be causing excessive loading, we added a op-amp voltage follower circuit which had no effect.  The only thing that had any effect was a cap from cathode to ground which took the voltage up from around 2.16 to 2.2V.

Having followed the datasheet recommendations we are not really confused by this so any suggestion about what what we have done wrong will be gratefully received.

Bob

  • 0
    TI__Mastermind 24665 points
    Hello Bob,

    The key is really to make sure that the reference has enough cathode current to ensure regulation. If you decrease the 2.5k resistor, this may help. As you mentioned, I suspect the loading from the PIC may be such that enough current is not flowing through the reference.
    You may want to check the current through cathode of the device, possibly by using a current sense resistor into the cathode of the device.
    Also, is the output of the reference oscillating?
    Have you tried another device in the same circuit to see if its that single device?

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 30 points
    Reducing the 2K5 to 700R does indeed bring the voltage up to acceptable value (2.45). What is odd is that we got the problem with just the components on a breadboard and a DVM for load. Never mind - we have it working and it's stable.

    Thanks - Bob
  • 0 in reply to Bob Manekshaw
    TI__Mastermind 24665 points
    Bob,

    The multimeter should not be considerable when considering loading.
    If you have just a 5V supply, a 2.5kOhm resistor and the reference similar to below, it should most definitely regulate to close to 2.5V. 1mA is certainly more than enough to make sure it is regulating. Typically these devices would be used on the order of say 50 to 100uA, which is the benefit to using these devices.
    If you are ok with using a reference that runs on 1 to 2mA, then something like the TL431 might make more sense. If you need precision, then something like the LM4040B25 might make more sense.

    The key is understanding how much current is being drawn away from the 2.5V reference. You may be able to look into the PIC datasheet for the pin that you are connecting the 2.5V reference to and look at the input bias current for that pin. Then once you know what the necessary bias current is you can make sure that the reference has enough current to regulate. Even as you mention that the voltage is 2.45V, this does not meet the datasheet minimum for cathode voltage of 2.462, and i suspect this is the case because there isn't enough current into the cathode of the device, not because the device isn't functioning properly.

    Does this make sense? Just trying to make sure that the device is working properly.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    Prodigy 30 points
    This is what has been worrying me about the situation in that the original design should work but doesnt. To try and avoid the loading issue, we added an op-Amp voltage follower circuit which had little or no effect. Further examination of the bread board test circuit highlighted an issue with the bench PSU which was far from clean (found when we were checking for oscillation). We have also tried the L4040BIZ2.5 and had the same issues. It's kind of nuts ...

    The project isnt low power however I like to miniise power usage wherever possible so while 5ma is OK, 100uA is much better all round. The Draw on the Vref of the PIC is 50uA max on a conversion cycle so there should be more than enough capacity in the system for that. We are making a valve tester and using the PIc to make measurements of voltage and current so the reference voltage is crucial for stability of readings and precision.

    Thanks for the advice - Bob
  • 0 in reply to Bob Manekshaw
    TI__Mastermind 24665 points
    Bob,

    Very happy to help. Please let me know if you have additional questions or concerns.

    Best,
    Michael