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LM25576: Problem in inverting buck boost design (simulation and pcb fail)

Yiannis Kazas
Prodigy 85 points

Part Number: LM25576
Other Parts Discussed in Thread: TINA-TI

Hello,

I'm trying to design a power supply with 12V input and -5V output (@2A). Using the webench power designer, the following circuit was generated

which I imported in my design 

 

However, when testing the fabricated PCB, the output is measured to be just a few mV. I'm now trying to setup a simulation in Tina-TI, but it doesn't seem to work in simulation either.

I downloaded the unencrypted PSpice transient model and I included it in my simulation project but even in simulation the output is practically 0V.

Any help on what might be wrong in the simulation and then how to debug the actual circuit, would be much appreciated.

Thank you.

  • 0
    TI__Guru* 79935 points

    Hello

    I think it is more important to work with the actual circuit and get that working.

    I think you have an error in one of your schematics:

    The FB pin must be connected to the junction of R23 and R26.

    Also, if you can show us your PCB layout that may be helpful.

    For the simulation, you might want to try using a step voltage source on the input, rather

    than a battery.

    Thanks

  • 0 in reply to Frank De Stasi
    Prodigy 85 points

    Hello Frank, 

    Thank you very much for your reply. I thought about the FB pin as well, that's why I wanted to setup the simulation.
    In the generated schematics by texas instruments it's not quite clear because there is no dot (which should indicate a junction) in the wire between the FB pin and the Ccomp capacitor.
    Nevertheless, I came back to lab and I tried to connect those 2 on the pcb (they are pretty close, C40-C41) but still no luck.

    I attach a snapshot of the layout in case someone can spot anything wrong. Does anyone have any suggestions of some parts of the circuits that I should probe on the scope which might help debug this issue?

    Also, I tried using a step voltage source on the simulation, instead of a battery, but again nothing changed.

    Thank you.

  • 0 in reply to Yiannis Kazas
    TI__Guru* 79935 points

    Hello

    With the FB connection correct, try to measure the following nodes:

    pin#1 VCC, pin#11 SS, pin#9 with scope, pin#18/17 SW with scope

    Be sure that pin#1 is connected to the junction of R22/C38 on the PCB.

    You can temporarily jumper pin#2 to VIN through a 100k to see if something is pulling the SD pin low.

    Did Webench specify a value for the RAMP resistor ?

    With zero volts on the output, either the device is not starting or you have a short or very heavy load on the output.

    The waveforms should tell what is going on.

    You can also get the EVM for this device and test on the bench as an IBB configuration.

    Thanks

  • 0 in reply to Frank De Stasi
    TI__Guru* 79935 points

    Hello

    I will close this post.

    If you need further help, please open a new post and the engineer assigned to E2E 

    will be glad to help.

    Thanks