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LM34936: No proper Output for 12V and 15V input

Anushree H
Prodigy 70 points
Part Number: LM34936
Other Parts Discussed in Thread: TPS65988, LM5176

Hello,

We are using LM34936 to generate 5V output in our application. We generated Webench design for the below requirements

Vin(min)=4.5V

Vin(max)=24V

Vout=5V Iout=4A

Attached the Webench design.

For Vin=12V and Vin=15V, Vout=7V and PG=0V

For Vin=9V we are getting 5V output (PG=High) but the inductor is getting heated alot (60'C)

Can you please review the schematics.

Thanks,LM34936_sch.pdfLM34936_DESN.pdf

Anushree

  • 0
    TI__Guru 58581 points

    Hi Anushree,

    thank you for using the E2E forum.

    Can you confirm that you have used the same inductor then in the Webench BOM, of not please share the part number and/or datasheet for that.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 70 points

    Hello Stefan, 

    Inductor used is XAL4020-102MEB from Coilcraft

  • 0 in reply to Anushree H
    Prodigy 70 points

    We also tried using LM5176 calculator tool, attached the same for your reference. 

    We tried replacing 

    L= 2.2uH (XAL8080-222MEB)

    Rsense = 20mE

    Cslope = 47pF

    Rc= 12.2K

    Cc1 = 2.7nF

    Cc2 = 39pF

    UVLO pin lower resistor = 82.4K

    Even after making all these changes, still observing the same issue. 

    For 5V input, output is proper 5V.

    For 9V input, output is 5V but the inductor is heating alot. (Currently there is no much Load connected to the output rail, Vout is less than 1A)

    For 12V input, output is 5.76V and PG output is low.

     

  • 0 in reply to Anushree H
    TI__Guru 58581 points

    Hi Anushree,

    for the initial design the inductor will go into saturation - check the peak inductor current value in calculator tool.
    The saturation value of the inductor should be well below this value, otherwise the inductor will loose highly the inductance and the current increases dramatically.

    But this also depends on the load current.

    So if this also happens with much lower load current or with the XAL8080-222MEB, something else needs to be wrong.

    In that case it would be good to probe first the two Switch nodes (SW1 and SW2).

    It would be also good if you could probe the output voltage to see if this is stable.

    Not sure if you have the possibility to probe the inductor current with a current probe?

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    Prodigy 70 points

    Hello Stefan,

    Can you please suggest the required compensation network values, Cslope values for the below conditions

    Vin(min)=4.8V Vin(max)=12.4V 

    Vout=5V Iout =1.5A

    L(we are planning to use)=4.7uH with current rating less than 5A

    Rsense=48mE

  • 0 in reply to Anushree H
    TI__Guru 58581 points

    Hi Anushree,

    i also suggest to start with the value provided by the Excel Quickstart Tool.

    I it also recommended to check the results on the real PCB and adjust this based on the parasitic which may have been added.

    Best regards,

     Stefan

  • 0 in reply to Stefan Schauer
    TI__Guru 58581 points

    Hi Anushree,

    I have not seen an update on this thread for the last 24 days.

    So I assume the issue is solved.

    In this case it would be great to press the is solved button.

     

    If there is still something open, just let me know with the new details.

     

    Best regards,

      Stefan