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TLV8541: protections needed for the input?

Antonio Faggio
Expert 7540 points
Part Number: TLV8541
Other Parts Discussed in Thread: CC1310, TPS22860,

Hi,

My customer is developing a wireless system where they use micro CC1310 and CC2642 + TLV71318, TPS22860, TPS619291 and TLV8541.

For cost optimization, I was considering whether to add protections to the non-inverting input of the TLV8541DBVR.

We are currently using it in a Non-inverting configuration with input impedance of several MegaOhm. I wanted to ask if the operational input has diodes that can act as a clamp in case the input voltage exceeds the operating range provided by the integrated for a few moments ... or should the input be protected with external clamp diodes?
Ex: TLV8541DBVR powered at 3V. Non-inverting input with 2MOhm series resistance: it is essential to put clamp diodes on the input (e.g. BAV199) to protect the operational input from overvoltages (e.g. -50V or + 50V) or I can consider that, given the high input impedances, the operational is not damaged?
Please let me know,

Many Thanks,

Antonio

  • 0
    Guru 140200 points

    Hi Antonio,

    I would add external protection diodes (e.g. BAV199, if you can live with the additional leakage current). You could split the 2M into two 1M resistors. The first limits the current through the BAV199 and the second limits the current into the TLV8541.

    The add of an external protection diode is safer when the overvoltage event has a considerable HF component (-> Surge, Burst, ESD, etc.)

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 6625 points

    Hi Antonio, 

    Kai is correct. For additional protection from over-voltage please consider adding external protection diodes. It is also better to split the impedance into smaller resistor values, as this can help with power dissipation. Make sure to limit the input current to less than 1mA. 

    For your case of 50V surge, I = V/R = 50V/(2Mohm) = 0.025mA. So you're well within the current limit when surge happens. Please let us know if you've any further questions. 

    Best Regards,

    Bala Ravi

  • 0 in reply to Bala Ravi
    TI__Expert 7540 points

    Hi All,

    Thanks for your prompt responses.

    We would like to eliminate external protection diode for a matter of costs (using diodes inside the amplifier) ... obviously evaluating that the Amplifier can't be damage from over or under voltage.

    Could you also confirm that in case of Input voltage of -50V ( and input resistance og 2Mohm) we are still protected against amplifier damage?

    When the input diodes inside the amplifier enter in conduction (i.e. over or under input voltage) what voltage value should I expected at the amplifier output if we are in voltage-follower configuration?

    Many Thanks,

    Antonio

  • 0 in reply to Antonio Faggio
    TI__Expert 6625 points

    Hi Antonio, 

    Yes, you can protect the op amp from damage by limiting the current to less than 1mA. And this can be accomplished by having a 2Mohm in series with the inputs. TLV8541 doesn't have back to back input diodes but the inputs have diodes clamped to the power-supply rails. 

    During an over-voltage condition of +/-50V, your op amp will be protected but please note that this is exceeding the input common-mode abs max rating. If this happens rarely, then you will be okay, but if this condition happens for an extended period of time it may affect device lifespan. Even though, it may not necessarily destroy the device, over-voltage should be avoided if at all possible. Also please be aware that adding a resistor in series with the input pins degrades the noise performance and introduces additional offset error to your circuit.

    If the internal ESD diodes start conducting, your output will probably be saturated to either of the supply rails or you could experience an output voltage phase reversal.

    Best Regards,

    Bala Ravi

  • 0 in reply to Antonio Faggio
    Guru 140200 points

    Hi Antonio,

    I think what mainly speaks against the 2M resistor at the input of TLV8541 is the use in a wireless application. The TLV8541 has extremely high ohmic inputs and in connection with the 2M protection resistor the input circuitry of TLV8541 becomes very very high ohmic and highly susceptible to EMI. With an added external protection diode you can split the protection resistor into two parts and massiveley reduce its value while at the same time providing the same level of protection.

    You are right, costs are an important factor but correct functionality is even more important...

    Kai

  • 0 in reply to kai klaas69
    Guru 140200 points

    Hi Antonio,

    what decision did you take?

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 6625 points

    Hi Antonio, 

    I will go ahead and close this thread. Please let us know if you've any further questions. 

    Regards,

    Bala Ravi

  • 0 in reply to Bala Ravi
    Intellectual 811 points

    Be careful about the protective diode. The leakage current of the protective diode could create large offset at the output of Op Amp, especially with high input impedance.

  • 0 in reply to user4090349
    TI__Expert 6625 points

    Hi User,

    Thanks for sharing your thoughts. 

    Regards,

    Bala Ravi