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TINA/Spice/LM5165: Inverting Buck

Ryan.Bishop
Expert 3900 points
Part Number: LM5165
Other Parts Discussed in Thread: TINA-TI,

Tool/software: TINA-TI or Spice Models

Hello,

I am trying to use the LM5165 as an inverting buck because of it's wide voltage range and very low Iq.

Vin=3.3V, Vout=-30V, Load=<10mA, PFM mode.

When simulating this circuit, the device begins switching and the output voltage starts to ramp. However, the device then stops switching and the waveforms behave strangely afterwards.

I'm assuming there are some dumb errors in my schematic? Does COT buck work for inverting applications?

1423.circuit.TSC

Regards,
Ryan B

  • 0
    TI__Mastermind 25590 points
    Ryan,

    Always great to hear from you!

    I think your schematic is fine, it's just the model which can't handle the topology. We have models for some parts that specifically say they can do buck AND inverting buck-boost. This one looks like it's just set up to do buck.

    Let me double check if this is the case for this one as well as why this is the case. I'll get back to you in a bit.

    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Mastermind 25590 points
    Ryan,

    Well if I had searched E2E I could have saved me 20 minutes looking at your schematic. Haha. This is a common issue and I was correct in my previous post. Models which are not designed to allow inverting supplies often have hard GND connections. So the GND symbol outside the IC will connect to the GND pin in the IC regardless of whatever else is connected to that pin.

    We do have models for some parts where the GND pin of the sim IC is floating but unfortunately this is not the case for your part.

    Your best bet would be to get an EVM and modify it as necessary. You could also try posting to the modeling E2E forum.

    -Sam
  • 0 in reply to Samuel Jaffe
    TI__Expert 3900 points
    Much appreciated Sam! Sorry to take up your time on what could have been an easy google search on my end. I trusted the model more than I trusted myself, so I was convinced it was just something off in my circuit.

    Looking forward to our next E2E connection!

    -Ryan B
  • 0 in reply to Ryan.Bishop
    TI__Mastermind 25590 points
    Not a problem. It's always a pleasure bumping into you.

    Have a great day!
    -Sam