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LM5008A Output ripple amplitude issue.

Other Parts Discussed in Thread: LM5008A

Hi,

I am using LM5008A in my design Buck converter. Application area is Automotive. Design Specs are as follows.

Vin = 12

Vout = 6.2V

Iout = 300mA

I have attached schematics as well as component placement here for your reference.

My application requirement is output ripple should be less than 100mV Peak to Peak and I am getting it around 600mV Pk-Pk, I am stuck at this point I have attached Output ripple in AC coupled configuration.

 DC DC LM5008A Design.pdfCould you help me to minimize this output ripple ?

Thanks You.

Regards,

Rajesh Matere.

  • Rajesh,
    Thanks for your interest in TI. I've contacted the appropriate product group. You should see a response soon.
    Regards,
    John
  • Hi John,

    Thanks for your reply.

    I am waiting for valuable suggestion from TI.

    Regards,
    Rajesh M.
  • Hello Rajesh,

    Apologies for the delayed response.

    You can reduce the ripple content by increasing the value of the R7 to say 200k and this should help you.

    Do you see the  ripple at the output at full load or light load. Also is your operation during a continuous conduction mode (CCM) or is it during a discontinuous conduction mode (DCM).

    In a non sync COT buck converter, at light load during the DCM the part will enter into a pulse skipping mode (characterized by burst switching waveforms) which in turns causes increased ripple in the output voltage. In that case you can only increase the inductance (to push the part into CCM) or reduce the output capacitance.

    Can you share a waveform of the output voltage and the SW node during this operation.


    Regards,

    Sourav

  • Hi Sourav,

    Thanks for the support.

    I tried by changing R7 to 200K, but it didn't work. In my application output ripple is playing major role, We want ripple within 30-40mV Pk-Pk range.

    I have made some changes in design which are attached here also you can find PCB layout placement and routing of Input/Output Component of Switching regulator.

    Observed waveform at switch node and Output (at No Load and 260mA load) are attached here. Output high frequency noise amplitude at 260mA laod is 470mV,  which is very high.

    Could you help me to reduce this high frequency noise ? 

    DC DC LM5008A Design Updated.vsd.pdfWaveform.zip

    Regards,

    Rajesh Matere.

  • Hello Rajesh,

    It would be good to know how you are probing the output voltage ripple. Are you probing it at the output node with a probe and a 'long' ground plug? If that is the case, I would request you to probe the output ripple right at the output capacitor by using a probe and a small ground clip. Many times the long ground plug of the probe induces a high frequency noise.

    If you are already probing the waveforms with a small ground clip and still see the issue, I would recommend adding a 0.1uF ceramic capacitor across the output parallel to C11. This should help you get rid of the high frequency noise at the output.

    Also, when you are using a ripple injection circuit, you can replace the output capacitor C11 with a ceramic capacitor. Also, the C14 should be placed as close as possible to the VIN pin, closer than C6.

    Let me know if placing the output ceramic capacitor helps or not.

    Regards,

    Sourav