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Oscillation at the output of DC-DC buck converter

Sakthivishnu Rangaswamy1
Prodigy 60 points
Part Number: TPS65283

Hi,

In our application, TPS65283 was used for our stepping down voltage levels from 14V to 5V and 14V to 12V.

14V to 5V circuit functions properly and no issues with it.

Whereas the 14V to 12V circuit has the below-mentioned problem.

The image attached with this query captures the circuit implementation for 14V to 12V

1. The output of the 14 to 12V DC-DC converter oscillates from 5V to 10.5V constantly.

We are not able to see constant output like 5V.

2. When one the Buck channel in the converter oscillates the other Buck does not even turn on even after its enabled.

3. While trying to enable converter it gets heated up after some time.

Later we found that the operating range of the DC-DC is from 0.6 to 9V (voltage output) and LX and LX2 output is 20V Max.

When datasheet shows the above-mentioned details the quick search page of the TI website shows the operating output voltage of this part no is 16V.

We find dissimilarity in the details of the PMIC between quick search page and datasheet.

Could you please help with this.

Thanks and warm regards,

Sakthivishnu14V to 12V.pdf

  • 0
    TI__Genius 15135 points

    Hi, Sir

    The TPS65283 can support 14V to 12V application.

    Questions:

    1. What is the max loading of Buck1 and Buck2?

    2. The schematic looks ok, could you send the PCB layout screenshot to me for review?   

    Further test comments:

    1. Suggest to change L6 and L7 to 10uH, then test again and probe the LX1, Vout1 voltage waveform.

    2. If still unstable, continue to change R63=100kohm which will set Fsw=500kohm, then test again and probe the LX1, Vout1 voltage waveform.

  • 0 in reply to Zhao Ma
    Prodigy 60 points

    Dear Ma,

    Thanks for your response.

    Answers to your questions;

    1. Maximum load: 0.5A (requirement)

    But we tested it at no load condition.

    Also, the feedback network was changed to generate 6.6V output by changing R55 and R56 to 10K. Still, the oscillation remains the same.

    Buck 2 circuit no response.

    2. Please find the PCB layout image.

    Further test comments:

    1. Since we have not connected the load, is it necessary to change the present inductor value?

    2. Will change R63 and test the output condition and update you soon.

    Thanks and warm regards,

    Sakthivishnu

  • 0 in reply to Sakthivishnu Rangaswamy1
    TI__Genius 15135 points

    Hi, Sir

    I find the root cause:

    The C85 and C86 is too small.

    TI suggests to change C85=C86=10uF, then test again.

    BTW,

    Please ignore the comments in last post.

    And you can refer the EVM's schematic, the input cap is 2*10uF.

    http://www.ti.com/general/docs/lit/getliterature.tsp?baseLiteratureNumber=slvua70&fileType=pdf

  • 0 in reply to Zhao Ma
    Prodigy 60 points

    Dear Ma,

    We have 10uF D-Cap placed before the ferrite bead.

    If required I can remove the FB and short the D-Cap to 1uF.

    Now we tried generating 12V from the 5V circuit with the change in feedback resistors only. This circuit is working fine.

    When this circuit can work why not the 12V circuit. Both the circuits are the same no differences. Double checked.

    Is there any chance of malfunctioning of IC?

    Thanks and warm regards,

    Sakthivishnu

  • 0 in reply to Sakthivishnu Rangaswamy1
    TI__Genius 15135 points

    Hi, Sir

    Some comments:

    1. Shorting ferrite bead has benefit for this issue, but might still not fix the problem.

    The input cap for TPS65283 must be placed closely to VIN1/2 pin, so still suggest to change C85=C86=10uF, then test again.  

    2. The duty cycle of 14Vto12V is differnt from 12Vto5V, the inductor current is different, the input cap requirement is different, so you can't judge the stability of 14Vto12V by 12Vto5V.

    3. No, I think this issue is not caused by IC.   

    BTW, what is the D-cap? not MLCC?

  • 0 in reply to Zhao Ma
    Prodigy 60 points

    Dear Sir,

    Thanks for your comment.

    1. Will try shorting the FB. Will update you once tested.

    2. Okay.

    3. It is MLCC we have 10uF, 2*4.7uF and 2*0.47uF close to the ferrite bead.

    Thanks,

    Sakthivishnu