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LMZ23605DEMO: There are self oscillation on my board's outputs when I using this demo

user3453450
Intellectual 280 points
Part Number: LMZ23605DEMO

Hello ,  Sir or Madam

I'm testing my own board with this power demo , LMZ23605DEMO  , which outputs -3.5V  with 12V input . 

However ,  there are self-excited oscillation in my own board's outputs . 

But , If I used a power supply unit (DP832 RIGOL)  to  supply for my board , there is no self-excited oscillation .

Are there any points I  missed when I using it ?

How can I solve it  with using  LMZ23605DEMO ?

Looking forward your advice .

Best regards

Anglic

  • 0
    TI__Guru 50120 points
    Hi,

    It sounds like you have a 12V supply powering the the LMZ23605DEMO board that is configured for inverting application. The LMZ23605DEMO has a 3.3V output voltage setting and if you were to configure this for inverting, then I assume you have -3.3V on the output of the LMZ23605DEMO board that now feeds into your board. Is this correct?

    Can you also provide the schematic of your board? This will help me understand how your board is getting powered and why the output looks unstable.


    Regards,
    Jimmy
  • 0 in reply to Jimmy Hua
    Intellectual 280 points

    Thank you 

    Indeed, I've changed the value of the feedback resistor , such that it can supply -3.5V.

    I'm afraid I can't post the schematic here because of the confidential . 

    Could you please teach me that in which situation , it's easy to get self oscillation because of power supply . 

    Thank you all the time .

    Anglic

  • 0 in reply to user3453450
    TI__Guru 50120 points

    Hi Anglic,

    You can send your schematic directly to me at j-hua@ti.com if you don't want to post it on here. Is the waveform you set the output of your board or is it the output of the LMZ23605DEMO board? 

    I've listed a few follow up questions I'd like to know:

    1. What is the load current you have? If the load is low, the device can be in cycle skipping at light load and result in higher output ripple. 
    2. Is this oscillation causing issues to your device? Looking at the Vpk-pk it is roughly 20mV (4 divs x 5mV/div). This is twice the typical 9mV stated in the datasheet but it not as alarming if you have a stable -3.5V output. 

    Regards,

    Jimmy 

  • 0 in reply to Jimmy Hua
    Intellectual 280 points

    Hello Jimmy

    The waveform is from the outputs of my own board not of the LMZ23605DEMO .

    The load current is about 1.5A.

    The LMZ23605DEMO is working well , without oscillation .

    Sometimes , the waveform can up to 400mVpp .

    Thank you very much for your help.

    Best.

    Anglic

  • 0 in reply to user3453450
    TI__Guru 50120 points

    Hi Anglic,

    I can only comment on the LMZ23605DEMO power module characteristics. Since you mentioned that the LMZ23605DEMO is working well without oscillation, I don't believe this is an issue with the device. It does not explain why the oscillation goes away from using a power supply, but if the LMZ23605DEMO is operational and functioning as intended, I can't pinpoint this as the issue in your system. 

    I can only conjecture that your system is somehow unstable without knowing what your schematic looks like. Perhaps not enough output capacitors. The output voltage of the LMZ23605DEMO is expected to be stabilized and not cause any issues downstream. 

    Regards,

    Jimmy