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LMV431A: Cathode current

Carlo
Expert 7715 points
Part Number: LMV431A
Other Parts Discussed in Thread: LMV431, TLV803E, TLV431, TLVH431

Hello E2E,

Our customer is using voltage reference LMV431A as a comparator. What is the cathode current if the voltage at ref. pin is< 1.24 V (means when cathode voltage is high)?

The datasheet specifies "minimum cathode current for regulation" as 80 uA. However, it seems that this only applies to closed-loop usage (voltage regulation) and not to open-loop usage (comparator).

Regards,
Carlo

  • 0
    TI__Mastermind 19550 points

    Hi Carlo,

    The IKA leakage current will depend on the voltage of VREF.

    If VREF is GND then there should only be leakage close to the Ioff condition. If IKA is close to 1.24V then there will be a increase in leakage. We do not characterize the leakage but you can assume it is close to figure 4 of the datasheet if you assume the X axis is VREF instead of cathode voltage.

  • 0 in reply to Marcoo Z
    TI__Expert 7715 points

    Hello Marcoo,

    Is IKA current shown in Figure 4 of the datasheet dependent on the temperature? And how much? Can this be somehow estimated?

    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Mastermind 19550 points

    Hi Carlo,

    That data is not characterized so we don't have it.

    In general it decreases when cold, increases when hot. I add a margin of 1.5x for high temperature calculations as a save margin. 

  • 0 in reply to Marcoo Z
    TI__Expert 7715 points

    Hello Marcoo,

    One more question related to the voltage reference: what is defined low VKA voltage in comparator mode when VREF > 1.24 V? How is this voltage dependent of IKA current?

    Regards,
    Carlo

  • 0 in reply to Carlo
    TI__Mastermind 19550 points

    Hi Carlo,

    When VREF > 1.24V the cathode will drop to as low as it can while maintaining functionality. This varies with temperature and current. I've seen it vary around 0.9V to 1.1V.

    If you need a low cost comparator, we do have the TLV803E family which has an open drain/low Iq/ low cost benefits over a LMV431.

  • 0 in reply to Marcoo Z
    Prodigy 60 points

    Hello Marcoo,

    I have measured higher VKA voltages than range 0,9 V to 1,1 V. My values were about 1,4 V to 1,5 V at IKA < 1 mA. That is around 25 °C temperature. I do not know why my values are higher. Do you have any idea?

    Regards,
    Martin

  • 0 in reply to Martin Bohnjat
    TI__Mastermind 19550 points

    Is the current on your REF limited to values below the datasheet table?

    REF pin is more of a current based pin instead of voltage. 

  • 0 in reply to Marcoo Z
    Prodigy 60 points

    I see that the absolute maximum reference input current is 3 mA.

    Current into REF pin is limited to lower values in my circuits by quite large resistance.

  • 0 in reply to Martin Bohnjat
    TI__Mastermind 19550 points

    Hi Martin,

    This situation seemed a bit odd. I went to test it and validated with others.

    There is 3 states for the LMV431.

    1. Vref < 1.24V        -> Vka = Vcc
    2. Vref = 1.4V          -> Vka = 1.24V
    3. Vref > 1.24V        -> Vka = 1V and increases linearly to Vref and exceeds 1.24V. For example when Vref = 1.5V, Iref ~ 1uA -> Vka = 1.3V. If Vref keeps increasing then Vka keeps increasing and the fact it goes above 1.24V means it does not work correctly even when current limited.

    My assumption was that the LMV431 had the same behavior as the TLVH431 family since these device are P2P drop in in other applications. It seems this variant called LMV431 does not work well for comparator mode. Please use TLVH431 or TLV431 with a preference on TLVH431. I did the same test for TLVH431 and it worked perfectly fine.