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LM27761: Spice simulation model issue

Joselito Go
Expert 5080 points
Part Number: LM27761

Tool/software:

Hi Experts,

I'm having trouble with the Spice model of the LM27761, simulating the device startup. In my circuit the EN is tied to VIN. I'm expecting a negative voltage at VOUT, but it stays close to 0.
Looking at the simulation setup provided through https://www.ti.com/lit/zip/snvmak0 it does seem to be able to generate a negative output voltage, but not when EN is directly tied to VIN, instead of to a separate voltage source.
There's something wrong with the Spice model, or, when the model Spice model does simulate actual device behaviour, the EN input cannot be simply tied to VIN.

Regards,

Josel

  • 0
    TI__Mastermind 19660 points

    Hello Joselito,

    thanks for reaching out in E2E.

    Some models require some 0V startup initialization.

    What you could do as a workaround is to use the delayed VEN voltage source for both, VIN and EN pins. This way these pins can be tied together and the simulation is starting up as expected.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    TI__Expert 5080 points

    Hi Andreas,

    Yes, that circuit functions. The problem with my setup is that Vin (tied to EN) does come up at some time, but with a slow edge. I tried simulating you circuit and it manages to create the negative voltage when TR=0.26ms, but at TR=0.27ms convergence problems occur, and I need something like TR=1.25 ms...

    Regards,

    Josel

  • 0
    TI__Mastermind 19660 points

    Hello Joselito,

    thanks for your feedback.

    I'm not sure if we can update the simulation model quickly at this time.

    Would it be an option to evaluate the real part?

    There is an EVM available which I think you can order as a free sample under your myTI account.

    Let me know if you have any questions.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    TI__Expert 5080 points

    Hi Andreas,

    Thanks. The EVM is too expensive. I'll build the circuit and find out if it works.
    However it would be nice if the simulation model (and I'm still assuming it is flawed) would model the actual behaviour. The EN is supposed to see a Logic High when its voltage is above 1.2 V. However the IC is specified for working from Vin starting at 2.7 V, and I read it will shut when Vin drops below 2.4 V. In my circuit, with the slowly rising Vin, I see the pump attempting to start some time after EN rises above 1.2 V, but then the fly cap doesn't see the full Vin even when it's above 2.7 V. It keeps on switching a small voltage and the output never reaches its target. I hope that's not actual behaviour of the IC.

    Regards,

    Josel
  • 0
    TI__Mastermind 19660 points

    Hi Joselito,

    thanks for the update.

    The request for updating the simulation model is noted and will be considered in the work process.

    I will contact you via e-mail regarding the EVM.

    I don't expect any issues with the IC at the mentioned Vin ramp. Please let me know if you see any issues.

    Best regards,

    Andreas.

  • 0 in reply to Andreas Mueller
    TI__Expert 5080 points

    Hi Andreas,

    Regarding the EVM, you may contact Luc Meson luc.meson@proton.me. He is the customer I am supporting.

    Regards,

    Josel

  • 0
    TI__Mastermind 19660 points

    Hi Joselito,

    I can also check next week if we have an EVM in our lab and quickly try the slow Vin ramp and see if it powers up properly.

    Best regards,

    Andreas.

  • 0
    TI__Mastermind 19660 points

    Hello Joselito,

    I tried to start up the LM27761EVM slowly as described in the thread. Vin of 5V was ramped up within 1.25ms. The output voltage was generated without any problems.

    Best regards,

    Andreas.