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TL431: Low battery led problem using a TL431 with a DC motor

Asier Mintegia Garzon
Prodigy 185 points
Part Number: TL431

Tool/software:

Hello everyone,

I have designed a circuit which is powered by a 3.7V LIPO battery. This supply voltage passes through a LDO (IC1 in the schematic) which regulates the voltage to 3V to feed a small DC motor (+3V_MOTOR).

In addition, it integrates a low battery detection circuit based on the TL431 (IC2), which activates a low battery LED (D1-LOW_BATT signal) when the battery voltage reaches 3V.

For activation of the DC motor, a button has to be pressed which causes the battery voltage to reach the LDO (BAT+_PULS signal) and then the motor. Therefore, only when this button is pressed, the battery voltage is measured and the LED is activated if necessary.

The general operation of the motor activation is correct. However, in the voltage range where the low battery LED should not activate I see a small flashing of the LED mainly when pressing and releasing.

I have thought that this effect could be due to the flyback effect during the shutdown of our dc motor. But as I mentioned before, this effect also occurs on activation.

Another idea that has occurred to me is that the battery has peak drops when this current is required to power the motor and that is when the low battery circuit detects these drops and activates the red LED for a few ms.

I'm attaching the schematic of the PCB.

  • 0
    TI__Expert 5680 points

    Hello,

    Thanks for the detailed description of your circuit operation.

    First, let me make sure I understand your circuit and your problem. From what I am seeing, you are using TL431 to detect when BAT+PULS has reached slightly below 3V. Once this happens, the TL431 should stop conducting current, causing the current through the LED to increase, and turning it on. The issue is the red LED is flickering when you would expect it to stay off, is that correct?

    It seems to me that you are within the operating currents and voltages for TL431, so I don't see any issue there. 

    I would like to provide some information regarding the leakage current of TL431 when used in comparator mode, as you have done above. When moving the voltage from 0V to 2.5V on the REF pin (labeled as VIN in the plot below), there will be a very small amount of leakage current that you should expect before the cathode starts to conduct current as expected:

    More information can be found at this link: https://www.ti.com/lit/an/snvaa58/snvaa58.pdf 

    While I think it is unlikely, it is possible that this might be contributing to the behavior you are seeing, it would be helpful if you could provide the the following:

    1. The voltage range of BAT+PULS where the LED begins flickering. My understanding is that anything above 3V, the LED should remain off. Is it starting to flicker at 3.2V? 3.5V? This will help determine if the occurrence is repeatable at a consistent voltage, or not, which will help in determining if there is an aspect of TL431 that is affecting the behavior.

    2. Can you provide a scope capture of the voltage on BAT+PULS, the cathode, and the REF pin when this phenomenon occurs? It would also be helpful to know the voltage drop across R5 when this behavior occurs, so if you could provide that as well, that would be helpful.

    Thanks,

    Jackson

  • 0 in reply to Jackson Wightman
    Prodigy 185 points

    Hello Jackson,

    Firstly, thank you for your fast and extensive response.

    The description you comment about the LOW BATTERY LED activation circuit is exactly how it works.

    The phenomenon of possible leakage current in the LOW BATT signal occurs in the entire range from 4.1V to 3V. If it is true, it is visible that the lower the voltage, that is, the closer it is to those 3V, the greater the current leakage.

    I'm attaching the scope captures of every point you told me. The voltage drop on R5 is 1.75V for a BAT+PULS of 3.6V.

    Any other information required please let me know.

    Thanks a lot for your help.

    BAT+PULS (Each pulse is a press of the button that enables the voltage in BAT+PULS)

    CATHODE

    REF

  • 0 in reply to Asier Mintegia Garzon
    TI__Expert 5680 points

    Hello,

    What I am looking for is to see if there is any abnormal leakage going through the cathode of TL431 as BAT+PULS goes on and off. I'm not seeing this from your scope captures. In my lab, I am also not seeing any abnormal leakage current.

    I have one follow up observation: As the BAT+PULS turns on, it will pass through the voltage range (0V to 3V) where the LED would indicate a low battery. If this is the case, I would imagine that the LED could turn on for a very short (ms) period of time as the BAT+PULS signal passes through the low battery voltage level, until 3V is reached when it would turn off. This same phenomenon would appear when turning off the BAT+PULS voltage, as the voltage will drop below 3V and then pass through the low voltage levels, allowing the LED to turn on, until BAT+PULS reaches 0V and the circuit is finally turned off. 

    Basically, the point I am trying to make here is that I believe TL431 is behaving as we would expect it to behave. Since there is a rise time on the order of maybe 10 ms to 50ms (observing from your scope captures), there will be a window where the LED could turn on, due to that gradual rise. Does that make sense? Here is a diagram to show what I think is happening:

    Asier Mintegia Garzon said:
    However, in the voltage range where the low battery LED should not activate I see a small flashing of the LED mainly when pressing and releasing.

    This would explain why the LED is only flashing when pressing and releasing. If you disagree with my analysis, please let me know, but in my opinion, TL431 is behaving as expected. 

    Thanks,

    Jackson

  • 0 in reply to Jackson Wightman
    Prodigy 185 points

    Hello Jackson,

    The phenomenon that you describe of that momentary activation when going from 0V to the BAT+PULS voltage passing through the activation zone of the LOW BAT LED, may make sense if it does. However, there are two other points that I would like to comment.

    The flash practically always occurs on activation, i.e. when pressing the button that starts the motor and activates BAT+PULS, it rarely occurs when the button is released.

    Second point, the flash is more and more intense when we are approaching the 3V voltage. That is to say, at 4.1V the intensity of the flash is much lower than when we are at 3.1V.

    And finally, while BAT+PULS is active, in addition to the initial flash, the LED is always kept minimally lit.

    I attach a video showing these effects. I hope you can appreciate something.

  • +1 in reply to Asier Mintegia Garzon
    TI__Expert 5680 points

    Hello,

    Here is what I believe is happening:

    When BAT+PULS is above 3V, TL431 is conducting current. While most of the current should be traveling through TL431, there will still be some current traveling through R7 in your schematic, so it will probably be dimly lit even when BAT+PULS is above 3V. As you get close to 3V, as you showed in your video, TL431 will start to decrease the amount of current that is conducted, as shown in this figure:

    Due to resistor tolerances and accuracy specifications, this is likely why you are seeing a slightly brighter LED at 3.1 V. 

    In terms of the flash during power on, you can refer to the figure I shared in my last response. How much flash you are able to see depends on how quickly the rise and fall time are during the button press, so it would also make sense to me that you see less of a flash during button release.

    One suggestion I would have is to provide more overdrive voltage than the exact threshold, as is explained in section 9.2.1.2.1.1 of the TL431 datasheet:

    Thanks,

    Jackson