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LM61495: Buck converter passing voltage with converting

Todd Keaffaber
Prodigy 20 points
Part Number: LM61495

Hello - 

I'm using the LM61495Q3 in a design as implemented in the schematic is attached below.  The input voltage range is 24V to 33V and the output voltage is 24V.  I have the enable pin (EN, pin 13) connected to a microcontroller to control power on/off.  The symptoms I see are:

  1. Voltage out is the same as voltage in.  Tested input range is from 25V to 31.  Regardless of Vin, Vout is the same as Vin.
  2. The voltage at the feedback pin (FB, pin 7) is 1.27V.  I thought for adjustable output, the reference voltage is 1.0V.
  3. The EN pin is controlled by the microcontroller:  When Ven = 0V, the output of the converter is 0V as expected.  When Ven = 3.3V, the output of the converter is Vin.
  4. RESET is always low whether EN is high or low.  So 

I tried increasing the voltage on the EN to 6Vand 9V with the same results.  It was not clear to me Ven,min is from the datasheet but I interpreted it as anything above 1.365V should be good.  I have not applied a load to the output yet.  That's the next step but I thought it would convert the voltage even at no load.

I'm sure I'm missing something but have failed to find it. Any input would be greatly appreciated.

Thanks,

Todd

PDU_Schematics.PDF-part-12.pdf

  • 0
    Prodigy 20 points

    I apologize the title should be "Buck converter passing voltage without converting."

  • 0 in reply to Todd Keaffaber
    TI__Mastermind 47060 points

    Hello Todd,

    Let me review your schematic and see what could be causing this issue.

    When I finish I will provide updates. Please stay tuned.

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 20 points

    Thanks, Jimmy.  I'm looking forward to your thoughts.

    Thanks,

    Todd

  • +1 in reply to Todd Keaffaber
    TI__Mastermind 47060 points

    Hello Todd,

    If you have a PCB layout that you can share with all the layers that would also help with the review.

    The first thing I noticed was that the schematic IC has more pins than the device. 

    LM61495 should only have 16 pins, yet your schematic footprint (U700) has more than 16 pins. Are these pins redundant (17,18,19)? 

    Also you mentioned that you are using the LM61495Q3 which is a 3.3V internally fixed device however the schematic shows LM61495QRPHRQ1 which is an output voltage adjustable device. Please reconfirm if you are using the LM61495Q3RPHRQ1 or the LM61495RPHRQ1. 

    For the schematic, I would suggest increasing the inductor from 5.5uH to 10uH if you are using 400kHz following datasheet Equation 6. The design of this inductor is to prevent the device from being in subharmonic oscillation. Other than that, all the components in the schematic look okay. 

    When using the fixed output variants, the FB is connected directly to the output (second terminal of the inductor). The device will regulate to whatever output voltage that results in the feedback being 1V. For the adjustable variant, feedback top 100kOhm and feedback bottom of 4.32k should result in ~24V output. 

    Also is this failure reproducible, as in if you took out the U700 IC and swapped it for new IC, will the output voltage not regulate to 24V?

    Regards,

    Jimmy

  • 0 in reply to Jimmy Hua
    Prodigy 20 points

    Hi Jimmy,

    Thank you for the quick response.  I will work on answering you questions today.

    Todd

  • 0 in reply to Jimmy Hua
    Prodigy 20 points

    HI Jimmy,

    Here are the answers to your questions.

    The big revelation from your input is that I do, indeed, have a 3.3V fixed part.  I'm embarrassed to admit it as its such a rookie mistake.  But in my defense I ordered the adjustable version and received the -q3 version.  I should have put that together.  Regardless, when tying the FB pin to Vout, the converter works as expected.

    The extra pins on symbol are redundant.  This is the footprint being used:

     Pins 17, 18, 19, and 20 are extra but don't add extra nets.

    I will change the inductor to 10uH.  That's a great catch and can easily be implemented on the current board as well as the next rev.

    I'm sorry the issue was so trivial.  I was spending too much time trying to figure this out and your input resolved the issue.

    Thanks,

    Todd