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TPS79301-EP: Feedback Resistor Question

Bastian Mieck
Prodigy 730 points
Part Number: TPS79301-EP

Hi everybody,

one of my customers wants to use the TPS79301-EP for a 1.2V output voltage. But if he uses the equation from page 12 of the datasheet with the Vref of 1.2246V the resistor will become negative..or if you use 1.2246 for Vout the resistor becomes 0!

What is the problem here? 

thank you

  • 0
    TI__Expert 7445 points
    Hi Bastian,

    On page 12 , equation 4 of TPS79301-EP datasheet. There is nothing wrong with the voltage divider equation.
    TPS79301 is the adjustable version, as highlighted in the datasheet Vref is 1.2246V nominal.
    Min Vout will be the internal reference voltage, which is specified in the electrical table to be 1.22 with its variations on pg. 4 i.e. 0.98Vout to 1.02Vo i.e. 1.20V to 1.249V.

    There is no problem with the equation. What it is telling you is that as the nominal Vref voltage is 1.224V then you need R1 to be 0 ohm i.e. shorting FB pin to Vout , thus Vout will be internal Vref voltage.
    Reason you are getting a negative number for R1 resistor since you are using nominal value of Vref and different value for Vo. Whereas Vo = Vref.

    If this addresses your question, please verify the answer.
  • 0 in reply to Ramesh Khanna
    TI__Prodigy 730 points

    Hi Ramesh,

    ok thats what I thought. Many thanks for the help!

    The datasheet is just a little bit confusing sometimes. for example on page 2 it shows 1.2V, on page 4 it shows 1.22V. So with these values the equation doesnt work. Maybe we should fix that. Or maybe just a hint on the first pages.. (Sorry if there is one already and I didnt see it)

    regards

    Bastian