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TLV3511-Q1: TLV3511 behaviour over the rails

Aaron Bruty
Prodigy 30 points
Part Number: TLV3511-Q1
Other Parts Discussed in Thread: TLV3511

I'm creating a circuit to detect diode conduction across a FET for synchronous rectificaiton. I've noticed some strange behaviour when simulating the TLV3511 in LT spice using the TI provided PSpice model. I wanted to confirm what the real behaviour of the device would be in this situation. I've attached a screenshot, I'll explain what's going on there:

  1. The voltage at Vds (pink) rises, causing the comparator output to go low initially as Vds goes above 0V (the set trip point).
  2. BSS123 starts to turn off but isn't quite fast enough, allowing the voltage at IN- to rise above 5V.
  3. During the time where the TLV3511 IN- input is above 5V (specifically above 5.2V), the output sits at a half rail value

Is the behaviour described in point 3 real? I would find it strange that activating the internal protection structure would cause the output state to change. Considering the comparator inputs would still be putting the output in a low state. It's also strange that the voltage is able to increase up to 5.7V, rather than being clamped at 5.3V. 298a993f-0c56-49ad-85c2-ea5cb1a53206.png

Thanks!

Aaron

  • +1
    TI__Guru**** 168336 points

    Hello Aaron,

    Output at half voltage is a model alert telling you that VCM is out of range. It is absolutely not real behavior. 

    I would expect real output to stay low. However Paul would want you to know that output state is not certain if any input is above VCM range. 

    Aaron Bruty said:
    being clamped at 5.3V

    Why should voltage be clamped at 5.3V. How large is voltage at M1 source?

  • 0 in reply to Ron Michallick
    Prodigy 30 points

    Thanks very much for the info. The alert functionality makes sense! For the 5.3V I thought the internal clamp diode of the inputs would limit the input voltage to around V_supply + 0.3V (with an input current of < 10mA). Voltage at the source of M1 is very similar to that at IN-, with the ratio of the potential divider between the voltage at M1 and 5V. Screenshot attached. I've also included the current into the IN- pin. Zooming in, it seems that this current is mostly coming from the pin capacitance, as the current already appears before the voltage exceeds 5.3V. Are the internal diodes modelled in this model?

  • 0 in reply to Aaron Bruty
    TI__Guru**** 168336 points

    Aaron,

    Aaron Bruty said:
    Are the internal diodes modelled in this model?

    No, they are not. I see a fixed 1nA regardless of the input voltage. Even for thousands of volts. 

  • 0 in reply to Ron Michallick
    Prodigy 30 points

    Thank you for the help! That's all the information that I need.