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LM34927: RON power loss calculation

Roy Hsu
Intellectual 2880 points
Part Number: LM34927

Hi Team,

May I know what's the recommended resistor size for RON?

Because in EVM it use 0805 size, could you let me know how I can know the current flow through RON to calculate the voltage across it and power consumption? If you can take datasheet value below have a example I will very appreciate.

Thanks.

Best Regards,

Roy

  • +1
    TI__Guru* 76035 points

    Hi Roy,

    This is not disclosed but RON to VIN is usually regulated at about 1V to 2V, so the current into RON can be used to program the on time.  Given RON resistor usually in 100kOhm range, there is basically negligible power losses in it.

    EVM used 0805 side for RON but it is not necessary IMHO. 

    Hope this answers your question.

    Best Regards,

    Youhao

  • 0 in reply to Youhao Xi
    TI__Intellectual 2880 points

    Hi Youhao,

    But my customer measure the point below, then the voltage is only about 0.5V, may I know the 1~2V means voltage across RON? Is it possible to share the EVM test result for voltage across RON? 

    Thanks.

    Best Regards,

    Roy

  • 0 in reply to Roy Hsu
    TI__Guru* 76035 points

    Hi Roy, 

    The 1 to 2V was what we usually do for such COT part, but let me double check with our design team.  Sorry I cannot measure at this moment because I don't have an EVM in hand.  I will get back to you ASAP.

    Thanks,

    Youhao

  • +1 in reply to Youhao Xi
    TI__Guru* 76035 points

    Hi Roy,

    Very sorry my misunderstanding.  I mistook RON for the RT of our other buck circuit, in witch the low side RT pin is usually regulated at 1 to 2V and the IC reads the current to program the frequency.  For COT, the RON is basically tied to low impedance node to GND, and the IC reads the current from VIN through RON and this current increases to VIN to adjust the ON time according. This is different than the RT pin.  Again, sorry for confusing you. 

    The RON pin voltage is usually is very low compare to VIN, and we can simplify it to be zero voltage to get a quick estimate of its worst case max voltage, which is VIN itself.  Namely RON resistor would almost see the entire VIN voltage during operation, and this explains the large sized RON resistor used in our EVM and example circuit. 

    Let me correct my previous answer:  the losses can be safely estimated as VINmax^2 / Ron.

    Again apologize for the misunderstanding and thank you for being persistent on this issue which allows me to realize the mistake.  Also sorry for the confusion and any inconvenience this may cause to you and our customer.

    Best Regards,

    Youhao