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TLV431: start-up as comparator

Davide Tosatto
Prodigy 90 points
Part Number: TLV431
Other Parts Discussed in Thread: LMV431

Hello everyone,

I am facing a problem in the start-up of a TLV431 used as a comparator.

In my application, I use a TLV431BCDBZR as a comparator to sense when the voltage across the resistance R176 is higher than 2.13V. When this happens the voltage at Vref exceeds 1.24V and so U26 starts conducting cathode current, Q16 goes from an interdiction state to a saturation state, driving a logic circuit and commuting the signal BRIDGE_SHORT from high logic state to low logic state.

 


To simulate its operation I generate a voltage pulse with an amplitude of 3V across R176. Everything is working well if the pulse rise time is higher than 10us, when it is lower TLV431 starts conducting cathode current in advance.

rise time:10us

rise time: 4us

In the picture I attached you can see the voltages waveforms I measured in my circuit at different pulse rise times. The light blue curve shows the voltage between the TLV431 cathode and anode. The red curve shows the voltage between TLV431 ref pin and anode. The green curve shows the voltage between the signal BRIDGE_SHORT and GND_P.

I tried to use an LMV431BCM5 also and it is working well if I have a rise time above 1us.

rise time: 4us

rise time:1us

I would like to use a TLV431 because it is cheaper than an LMV431, so is it possible to improve his behaviour in any way? Otherwise could you suggest to me a component with a price similar to the TLV431 and witch has an appropriate respose?

Thank you,

Davide

  • 0
    TI__Prodigy 170 points

    Hi Davide,

    The issue you're experiencing may be counteracted by removing C174 from the circuit. This capacitor may be causing some unwanted delay or frequency response issues. Unfortunately, since there aren't any startup characteristics in the LMV431 data sheet, so I can't comment on that.

    I also want to recommend considering switching this circuit out for an active-low supervisor. This would reduce the component count, simplify the circuit, and allow for additional flexibility. TPS3703A7125DSERQ1 has a threshold voltage of 1.25V, and may be a worthwhile alternative.

    If removing C174 doesn't help, please let me know and I'll continue to look into it.

    Hope this was helpful,

    Alex

  • 0 in reply to Alex Noggle
    Prodigy 90 points

    Hi Alex,

    I will consider using a supervisor in my application.

    I already tried to remove C174 but I did not spot any difference.

    Thank you for your help,

    Davide

  • +1 in reply to Davide Tosatto
    TI__Genius 11401 points

    Hi Davide 

    Cathode terminal will have some leakage even when the voltage at refernce pin is < 1.24V that may be causing problem in the circuit. You can refer to figure 2-3 in the app note

    You can choose a lower value of pullup resistor to eliminate this. 

    Using a voltage supervisor is a good idea. Please note that it's low output voltage will be around 0V. Can you please tell your accuracy requirement and what is the max voltage the device will see? 

    Regards

    Trailokya 

  • 0 in reply to Trailokya Rai
    Prodigy 90 points

    Hi Trailokya,

    reducing the pullup resistor value solved my issue, now the circuit is working well even with lower rise times.

    The max voltage I can have on R176 is about 10V and my accuracy requirements are around 2%.

    Thank you,

    Davide