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Floating feedback pin on LM285-ADJ reference diode

John Morin
Prodigy 10 points

We use LM285BXZ (National Semiconductor) as a reference in a power measurement application. The design intention was to use a fixed voltage reference at 1.2V. In the circuit pin 3 (feedback) was left floating (it is a NC in the fixed parts). We have several hundered products in the field with this flaw and have now seen a small shift in our power measurement. In house we were able to mitigate this by connecting the FB pin to the anode as per the data sheet. The problem is seen infrequently and is difficult to repeat. Can you tell me what the expected behavior is in the event the FB pin is left floating? It is a reasonably noisy RF environment. Any advice is greatly appreciated.]

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    TI__Intellectual 1745 points

    Hi John,

    Sorry for the delay.

    As you stated, the feedback pin really needs to be tied to the anode.  The feedback pin is essentially the negative input of an opamp (seen in the block diagram on page 2 of the LM285 datasheet).  Any fluctuation of that pin is going to show up on the output voltage.  The environment you describe is likely causing noise to couple onto the FB pin, which is creating fluctuations in Vout, in turn affecting your measurements.  

    You said you're having a hard time replicating results, are you simply leaving FB floating? Try driving FB slightly above ground, or perhaps with a function generator with a noise waveform output and see if you can replicate the issue more reliably.

    The best solution is grounding the feedback pin, assuming the anode is grounded.  If you are creating floating references, then tie feedback to the anode specifically. 

    Hope that helps you.

    -Chris