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Unexplainable design fault

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Vovk_Z:

--- Quote from: Pentoad on January 16, 2020, 03:47:07 pm --- I have done a bit more fault finding and it seems to be to do with the reference voltage. It is meant to be 1.225 and on the faulty boards it is 1.07 when the battery is at 3.16. As I increase the voltage, the reference voltage slowly increases as I increase the battery voltage.

--- End quote ---
- a capasitor across D1 can help. Have you tried that?

Vovk_Z:
U1A does the same as U1D. Or no?

Pentoad:
Thanks for the suggestions. I checked all of the resistor values on a faulty and good board and they are all exactly the same apart from allowable variation due to their tolerances. Another suggestion is that maybe I have damaged the comparator or reference with heat as I am using a temperature controlled hot air station to assemble them but that is unlikely as I have tried soldering the reference and comparator using a normal soldering iron and still got a faulty board from that.  U1A and U1D are not doing the same thing. U1D is connected to an optoisolator to output a signal when the module is cell balancing. It is up to the customer what they use the signal for but it could maybe be interfaced with a charger somehow so that it reduces current output so the balancers can keep up.

splin:
You need to measure the current through the LM4041 when it's only delivering 1.07V to make sure it's at least 65uA. Also check it with a scope to ensure it's not oscillating.  If not, then replace the reference.

Could you have been supplied with fake parts perhaps?  Could they have suffered from ESD damage?

SiliconWizard:
You can also progressively isolate VREF to find out the culprit: remove R12, R13, R31, R32 one after the other. Monitor Q2_Drain and VREF signals at each step.

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