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TPS65131: TPS65131 unstable wave form and incorrect negative output

Jiandong Liu
Intellectual 1801 points
Part Number: TPS65131

Hi expert,

Here is a technical support need of TPS65131 from customer: The negative output of TPS65131 is not correct. Customer set negative output to -13.3V, but the actual output is -14.1V. 

Below is waveform of OUTN pin. As you can see, during the first soft start, it shutdown, and then soft start again after 24ms.

It seems normal during the first soft start from below figure.

But it seems unstable waveform during the second soft start, and the final output is not correct.

I reviewed customer’s schematic, and recommend them to change C248(feedforward capacitor) to 7.5pF, also add a 51.1K resistor in series with C248,but it still can’t work correctly.

The unstable waveform during the second soft start change to like this:

Could you please give some suggestions to this situation? Many Thanks!

  • 0
    TI__Mastermind 18550 points

    Hi Jiandong,

    This post appears to be a duplicate of the e-mail that you sent me yesterday and I have already responded to your e-mail. So I will close this thread.

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    TI__Intellectual 1801 points

    Hi Liaqat,

    I'm sorry but this support need is urgency and I noticed that you are on leave so I post the question on E2E forum. Customer add a 51.1K resistor in series with C248,but it still can’t work correctly. I also send an email to you to explain about this. Could you please give some other suggestions to this situation? Many thanks!

  • 0 in reply to Jiandong Liu
    TI__Mastermind 18550 points

    Hello Jiandong,

    The switch node waveform you show is not a problem in itself. This is likely happening due to light load on the negative converter and is normal behavior of the device at light load because negative converter goes to dis-continuous conduction mode at light load. Positive converter stays in continuous conduction mode even at lighter loads. You can verify this by increasing load current on the inverting converter and you will see that with increasing load current, the ringing will reduce successively and at approx. 80mA or so, the ringing will completely go away and the device will operate in continuous conduction mode.

    Kind Regards,
    Liaqat

  • 0 in reply to Liaqat Khan
    TI__Intellectual 1801 points

    Hi Liaqat,

    Thanks so much for your great support even when you are on leave. But there is still 3 questions:

    1. What kind of reason could cause 2 times soft start as shown in the figure? 

    2. If the ringing is normal, how do you explain there is no ringing in the first soft start, and there is a ringing in the second soft start?

    3. What kind of reason could cause the output voltage incorrect? Customer set negative output to -13.3V, but the actual output is -14.1V. 

    Best Regards,

    Jiandong

  • 0 in reply to Jiandong Liu
    TI__Mastermind 18550 points

    Hello,

    1.   I think what is happening is that at power up, the output capacitors are charging and because of that, there is enough load current on the inverting converter to keep it in the continuous conduction mode. Once the capacitors are charged, there is no load on the inverting converter for ~23ms and than as the  load comes on, switching starts again. So it is not really two soft-start phases.

    2.   As I explained in answer 1 above, there is likely enough current out of the inverting converter when output capacitors are being charged to keep it in continuous conduction mode. Once the output capacitor are charged, current out of the inverting converter is only what is consumed by the load, which is light enough so as to keep the converter in the dis-continuous mode.

    3.   For this, I would suggest to check the resistor values to make sure that FB network is indeed set for correct output voltage. Even with correct feedback resistor values, the resistor tolerance + accuracy of the converter (3%) could yield an overall tolerance on the output voltage of little over +/- 5%.  There may also be some voltage drop across the dc resistance of L38. So the output voltage is not that much off from this tolerance.

    Kind Regards,

    Liaqat