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TPS2662: ILIM Characteristics

Glen Schmidt1
Prodigy 160 points
Part Number: TPS2662
Other Parts Discussed in Thread: TPS1663,

I am working on a safety circuit where I need to essentially limit power, not just current.
I have experimented with adding a small voltage via a large resistor to the ILIM pin (along with the ILIM resistor) to cause the current to go down with higher input voltages so that you get the effect of a power reduction.  The small voltage is scaled from the input voltage in order to stay within the 1V range of the ILIM pin.
If I do this just right, it looks like I can almost get to a constant power out (limit) regardless of input, which is what I desire.

In order to predict this effect reliably, and more importantly, involve tolerance that is more than just my ILIM resistance, it would seem that maybe I need to know the tolerance of the internal 1V that is controlled on this pin.
Or if you can supply a simple spice model that involves the tolerance of the set current that would also help.
Or an overview of where the TMR2662 current set tolerance comes from...like is it the scale factor (gain) variance on the 6632 term in the I=6632/Rlim term, or is it more related to the tolerance on the 1V held across Rlim or some of both...etc.?

Thanks

Glen

  • 0
    TI__Guru* 95661 points

    Hi Glen,

    Thanks for reaching out!

    I will check on the ILIM voltage tolerance. We have power limiting device TPS1663, Can you check whether it fits your need.

    Best Regards, 

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 160 points

    Hello Rakesh,

    Thank you for your reply.  I had no idea you had a power limiting feature on one of your devices!

    This would be perfect, except the 0.6A minimum is still too high.  I need to limit to around 0.2A at 12V or 2.5W and hold that power limit roughly up to 30V and down to 6V.
    So very close!
    If you see a way to adapt the TPS1663 that would be great, but I do need a fair amount of accuracy on the current.
    The power limit can be looser.
    Looks like with the TPS2662, three zener diodes and five resistors I can form break points that are voltage dependent and roughly hold the power.  This is an Intrinsic safety circuit (IEC 60079-11) so simpler is better for certification.

    Thanks

    Glen

  • 0 in reply to Glen Schmidt1
    TI__Guru* 95661 points

    Hi Glen,

    For that power level, TPS2662 is a better choice.

    Let me check for the details of ILIM pin. By the way, is TPS2662 suitable for your application ?

    Best Regards, 

    Rakesh

  • 0
    Prodigy 160 points

    Hello Rakesh,

    Yes, the TPS2662 is in the current range so that is my plan to use it.

    Thanks

    Glen

  • 0 in reply to Glen Schmidt1
    TI__Guru* 95661 points

    ok Glen.

    I will get back by 12th with those details

  • 0 in reply to Rakesh
    TI__Guru* 95661 points

    Hi Glen,

    As per design, the ILIM pin voltage has +/-20% variation with respect to 1V

    Best Regards, 

    Rakesh

  • 0 in reply to Rakesh
    TI__Guru* 95661 points

    Please refer attachment for Power limiting with TPS2662

    4426.PLIM_TPS2662.pdf

  • 0 in reply to Rakesh
    TI__Guru* 95661 points

    Hi Glen,

    Can you help with your safety circuit requirements. Do you have any specific standard to comply to ?

    Best Regards, 

    Rakesh

  • 0 in reply to Rakesh
    Prodigy 160 points

    Hello Rakesh,

    Can you possibly make this a private thread or switch to email at glen@analogis.net?

    I am a consultant for designing specialized analog solutions for Intrinsic Safety to IEC 60079-11
    I would rather not share much detail openly on a forum because it may be competitive in nature.

    I took a look at your power limiting attachment.  Thank you for the info.  However, the problem I see is that this only limits power at specific voltages.  Unless you have a voltage that follows a square law, the simple op amp circuit added on the ILIM will not be constant power across the voltage range.  I need to work from 5V to 30V, but my main area of interest is 5-15V where is am trying to hold around 3.5-3W and need that to be sustained to at least 2W all the way to 30V.  I can describe the reasons more if you make our conversation private.

    I have already solved this with a square law approximation circuit (see attachment) which results in simulated data (dashed lines) and benchtop data (see attachment)

    However, the accuracy is a bit unpredictable below 0.15A and above 15V.  I suspect this is because the TPS2662 accuracy falls way off below 0.15A (ref Fig 20 of the data sheet).

    For reasons of safety, I need to keep the circuit very simple, and yet I need to be able to predict the accuracy well enough to perform a safety analysis with tolerances.  Any insight would be helpful.  Clearly, the tolerance on the ILIM voltage is not all there is to contribute.

    Again, please make this thread private or send the discussion to email.

    Best regards,

    Glen2148.TPS2662 Power Limiter Info for TI.pdf

  • 0 in reply to Rakesh
    Prodigy 160 points

    IEC 60079-11 Intrinsic Safety

    But just giving you that answer is quite incomplete.  I cannot share additional details on a public forum.

      Glen E Schmidt

     
    2513 Centre Rd
    Bartlesville, OK  74003
    918 440 3298
    glen@analogIS.net
  • 0 in reply to Glen Schmidt1
    TI__Guru* 95661 points

    Hi Glen,

    I will follow-up on email as it involves your project specifics. Also closing the thread here.

    Best Regards, Rakesh