This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS7A49 Vfb versus Vref

The dataheet rev E (May 2015) does not give worst case values for Vfb, while the formula for Vout (equation 2) does now contain Vfb instead of Vref. This means that we cannot calculate the worst case values for Vout anymore.

Question 1): what are the worst case values for Vfb?

We measure values for Vref lower tan the given minimum value, while Vfb has almost its nominal value. We have Vout set to 19.5V.

Question 2): is the measurement of Vref at the NR/SS pin dependent on the Vout setting? 

  • Hi,

    1) The datasheet was changed as the Vref measurement does not account for the offset voltage of the error amplifier. For Vfb please refer to the overall accuracy spec of 2.5% over line, load, and temperature - Vfb will be within 2.5% of it's nominal value over these conditions.

    2) When measuring Vref keep in mind that it is a high impedance node (there is an internal several hundred kOhm resistor from the internal bandgap to the NR/SS pin for noise filtering). When measuring this pin make sure that your measurement device is set to a really high input impedance (for this measurement I personally use an HP34401A with the input resistance set to > 10 Gohm), elsewise the measurement instrument will drag down the internal reference.
    Another common issue when making this measurement is that as a high impedance pin the NR/SS pin is sensitive to board contamination from things such as solder flux. Typically a quick board clean will resolve this issue

    Regards,
    David
  • Thanks for the answers.

    We measured with a multimeter with R-=10MOhm and measured a dropped voltage of about 20-30mV at the NR/SS pin. An internal resistance of a few hundred kOhm does explain this.