LM5156EVM-BST: Input voltage greater than 12V

Part Number: LM5156EVM-BST

Is there a way to allow the EVM to support input voltages greater than 12V? My customer needs to evaluate the part up to 42V. Maybe I'm missing something in the user guide for the EVM. I do see that the IC has a 60V Vin max though. 

  • HI TJ,

    you are right the device is designed for up to 60V whereas the EVM has been set for a demo application with lower voltages and tested based on this settings.

    The most critical components when going to higher voltages are the capacitors - checking for them they are graded for 50V.

    So for the capacitors doing some test up to 42 V would be OK but for a production design higher voltage rating should be used.

    For checking the other components the details about input and output voltage and load current would be required.

    As the EVM is a Boost and you say input voltage should be up to 42V, the output voltage would also be >= 42V.

    (So for the output side the Capacitors might need to be replaced by a higher voltage rating.)

    Best regards,

     Stefan

  • For an initial test I went up to 14V, but above 12.5V Vin, the output loses regulation and roughly tracks the input. Is there a reason that's happening? From your statement it seems like the EVM should still work.

  • Hi TJ,

    sorry, I might not have been clear enough with my answer. The device is able to support higher voltages but the EVM is designed for 4-12V input with 12V output I If you need other system parameter you need to check each component and adopt it.

    In your case (I assume you just used the board without modification): this is a Boost design, this means it can convert input voltages to higher output voltages.

    If you feed in 14V this is higher then the 12V output voltage which would be generated by the converter. In this case the voltage just passes through the inductor and diode. So more or less the output voltage is the input voltage minus the diode voltage.

    Note: if you have input voltages above and below the output voltage you need a Buck-Boost solution

    Best regards,

     Stefan

  • Hey Stefan,

    Circling back to this. Can the dev board be modified to support my needs? Or do you have a schematic that I could use to build a reference board off of? 

  • Hi TJ,

    to have the full picture - can you please summarize the full spec of the application:

    Input Voltage Range

    Output Voltage

    Output Current

    Any additinal info which is important for you, e.g. output voltage ripple, ...

    Thanks,

     Stefan

  • input voltage: 4-28V, most times we only go up to 14V

    output voltage: 5V mainly. some times we create a 3.3V rail

    output current: 1-2A 

    looking to avoid using Electrolytics

  • Hi TJ,

    sorry for the late reply, we had a public holiday yesterday.

    I am a little bit confused by the operating voltage range you have given above, as i do not see the requirement for going up to 42V.

    With a input voltage of 4-28V you can the EVM, you just would need to adjust the feedback voltage divider for the 24V.

    Best regards,

     Stefan

  • Even at 12V i see issues. The resistor for RT pin on the EVM is 49.9k ohms. from using formula 5 from section 9.3.4; the switching frequency is 440kHz. This takes me back to the start of my confusion where the top of the EVM document says that it should operate with an input voltage of up to 42V. 

    Please let me know if I'm understanding something incorrectly. 

  • Hi TJ,

    You are right the RT with the 49.9k Ohm sets the switching frequency to 440kHz.

    But I struggle to get the relationship between the switching frequency and the input voltage, you have shown above. This two values are independent from each other.

    Best regards,

     Stefan

  • My confusion as well. Since the input is supposed to support my voltage range, is there something else I'm missing in the EVM user guide? 

  • Hi TJ,

    really sorry but i think we are circling around and do not come to common understanding of the issue you have.

    I also would like to support and help you solve the issues you have.

    Therefore i suggest to more or less start from scratch and ask you to phrase you question(s) from the beginning without referencing to any history.

    So good would be to say what you need (specification of the power supply with input and output parameters) and where you have questions or see any issue for that.

    Thanks,

     Stefan

  • resolved offline