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TPS548B28EVM: Resistor value for 3.3V output of TPS548B28EVM

Bill Young
Prodigy 20 points
Part Number: TPS548B28EVM

Hello,

I'm trying to adjust the output of the TPS548B28EVM-023 evaluation module to 3.30V. According to section 1.3.1 on page 3 of the TPS548B28EVM-023 User Guide, the Output Voltage Setpoint is: (R6+R7) = [(Vout-Vintref)/Vintref] x 10000.

Assuming:

Vintref = 0.60
R7 = 0 ohms
R6 = 45300 ohms

Vout should work out to 3.318V but I'm measuring a much lower value, around 3.01V.

What am I doing wrong?

Can you recommend the correct R6 value?

Thanks.

  • +1
    TI__Mastermind 31260 points

    Hello Bill,

    try using 10K on top and 2.5K on the bottom or any other combination that can give 3.0V

    thanks 

    Tahar

  • 0
    TI__Mastermind 22130 points

    Hi Bill,

    Were you able to get 3.3V output on the TPS548B28EVM

  • 0 in reply to David_Daniels
    Prodigy 20 points

    Hi David,

    I don't know if you can see it but this is what I replied to Tahar:

    Hello Tahar,

    1. I need the output to be 3.3V, not 3.0V.

    2. To avoid any confusion, please specify which resistor is on top. Is it R6 or R9? If I define R6 as 2.5K and R9 as 10K, the equation produces an output value of 0.75V.

    I have attached the schematic for your reference.

    Thanks,

    Bill

  • 0 in reply to Tahar Allag
    Prodigy 20 points

    Hello Tahar,

    1. I need the output to be 3.3V, not 3.0V.

    2. To avoid any confusion, please specify which resistor is on top. Is it R6 or R9? If I define R6 as 2.5K and R9 as 10K, the equation produces an output value of 0.75V.

    I have attached the schematic for your reference.

    Thanks,

    Bill

  • 0 in reply to Bill Young
    TI__Mastermind 22130 points

    R9 = 10kOhm (low side feedback resistor)and R6+R9 = 45.3kOhm should yield an output voltage of 3.3V. 

    If you are using 10k and 45.3k resistors and getting a 3.0V on the Vout+ and Vout- test point, can you check the SW node test point with respect to PGND using an oscilloscope.   The SW should be a periodic pulse train with pulses that have consistent pulse widths.    

    Also, check the SS/REFIN pin voltage with wrt to AGND and make sure voltage is greater than 0.6V.

    During your test  what is the input voltage and output current

  • 0 in reply to Bill Young
    Prodigy 20 points

    Hello Tahar,

    I'm now getting the correct voltage output of 3.324V. I have no idea why I didn't get the same result earlier. Very strange. Anyway, thanks for your assistance. We can close this case now.

    Thanks,

    Bill

  • +1 in reply to David_Daniels
    Prodigy 20 points

    Hi David,

    I'm now getting the correct voltage output of 3.324V. I have no idea why I didn't get the same result earlier. Very strange. Anyway, thanks for your assistance. We can close this case now.

    Thanks,

    Bill

  • 0 in reply to Bill Young
    TI__Mastermind 22130 points

    Thanks for the update and I will close.    If the incident repeats, post the issue again and let's root cause 

    Best Regards,

    David