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LM27313-Q1: High Peak-to-Peak Sawtooth Signal on Output

Rik Verberne
Prodigy 10 points
Part Number: LM27313-Q1


Tool/software:

Dear [TI Support / Team],

I’ve been testing the LM27313-Q1 on a breadboard to validate whether the design meets the intended specifications. However, I’ve noticed that the output consistently exhibits a high peak-to-peak sawtooth ripple.

The expected output voltage is 12 V (different loads, 10 - 200mA), but the measured waveform shows significant ripple, as illustrated in the attached oscilloscope capture. I’ve also included the test schematic for reference.

Side-notes: Adding C13 makes the ripple even higher, adding more input and output capacitors only vary the timing in ripple but the peak-to-peak stays the same.

Could you please advise whether this behavior could be caused by issues in the schematic, or if it is likely a result of testing the design on a breadboard rather than a proper PCB?

Thank you in advance for your support.



Oscillosciope Capture:                                                                    Test Schematic:     
        

  • 0
    Prodigy 10 points

    Additionally, I noticed that the input voltage (Vin) experiences periodic drops approximately every 2 ms, coinciding with the timing of the output ripple.

    The Vin signal after connecting the LM27313-Q1:                      The SW pin:
         

  • 0 in reply to Rik Verberne
    TI__Mastermind 18520 points

    Hi Rik,

    Test on breadboard will cause high noise and lead to issue. Please use PCB. And the 47uH inductor is too large, recommend 10uH. Thanks.

  • 0 in reply to Shipeng Cheng
    Prodigy 10 points

    Dear Shipeng Cheng,

    I have transitioned my boost converter circuit from a breadboard to a PCB to mitigate noise issues. Additionally, I replaced the previously used 47 µH inductor with a 22 µH inductor, as a 10 µH component was unavailable.

    Despite these modifications, the output voltage continues to exhibit the same sawtooth waveform previously observed. I have verified that the feedback resistor network is correctly configured, and the input/output capacitors are positioned in close proximity to the IC, adhering to the layout guidelines specified in the datasheet.

    Could you provide insights into potential causes for this persistent behavior?

    PCB Design:





  • 0 in reply to Rik Verberne
    TI__Mastermind 18520 points

    Hi Rik,

    Do you use correct ceramic cap (22uF, 25V for C4 and 22uF, 10V for C5)? 

    If it's OK, could you show me Vin, SW, Vout waveform at 5Vin, 12Vout, 0A load and 5Vin, 12Vout, 0.2A load?

    Thanks.

  • 0 in reply to Shipeng Cheng
    Prodigy 10 points

    Dear Shipeng Cheng,

    I used 22uF, 25V for both C4 and C5.

    0A Load (10kOhm) at 5Vin:

    Vin Pin:                                                                     Vout Pin:                                                                  SW Pin:
           



    0.15A Load (80Ohm) (had no lower load avaiable) at 5Vin:
     
    Vin Pin:                                                                      Vout Pin:                                                                      SW Pin:
         


    Sorry for the low quality, I had no usb avaiable at the time.

    The sawtooth waveform seems to have disappeared at lower loads after implementing the boost converter on a PCB.

  • +1 in reply to Rik Verberne
    TI__Mastermind 18520 points

    Hi,

    The 0A load performance looks OK. But the 0.15A load Vout has a 3.5k low frequency ripple. I think the loop is unstable. Could you try remove C13?

  • 0 in reply to Shipeng Cheng
    Prodigy 10 points

    Dear Shipeng Cheng,

    Thank you for your fast replies, the loop was indeed unstable.
    Removing C13 made the loop stable again, solving the Sawtooth Signal output.