• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 62 subscribers
  • Views 1384 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

TPS63030: Feedback Pin Resistor Values

Brian Cox31
Prodigy 20 points
Part Number: TPS63030

Your data sheet says "At the adjustable output voltage versions, an external resistor divider is used to adjust the output voltage.  The resistor divider must be connected between VOUT, FB and GND.  When the output voltage is regulated properly, the typical value of the voltage at the FB pin is 500 mV. The maximum recommended value for the output voltage is 5.5 V.  The current through the resistive divider should be about 100 times greater than the current into the FB pin.  The typical current into the FB pin is 0.01μA, and the voltage across the resistor between FB and GND, R2, is typically 500 mV.  Based on those two values, the recommended value for R2 should be lower than 500kΩ, in order to set the divider current at 1μA or higher. TI recommends to keep the value for this resistor in the range of 200kΩ."

Why do you recommend such a high value for such a sensitive and crucial node?  Why can't a 10k or lower value work there?  High impedance nodes are prone to noise, and noise on the regulator feedback pin can have devastating effects on the power supply rail.  I have used many similar regulators in the past from several manufacturers ,and they all use resistors between a few hundred ohms and a few k, and all designs has met or exceeded requirements.

  • 0
    TI__Guru 65880 points
    Hello Brian,

    The recommended value is a compromise between noise and efficiency for systems that are for example supplied by a battery.

    Especially at low load currents, the current through the resistor divider is reducing the efficiency of the converter.
    Let me give you an example: The system is most of the time operating with a load of 1mA or less, but needs several 100mA of load for short period of times. This means that the efficiency at 1mA is very important. For a voltage of 3.3V and a resistor divider of in total 10 kohm, the current through the resistor divider is 0.33mA. This means that the resistor divider pulls the efficiency down by roughly 25% for the whole time the system needs only 1mA load.

    Especially when during the layout process it is taken care that the resistor divider is placed close to the IC and it's GND node is connected to a quiet GND area noise injection into the feedback node is small enough to not disturb the regulation too much.

    Nevertheless, if efficiency at low load is not important, lower resistor divider values are even better.
  • 0 in reply to Brigitte
    Prodigy 20 points

    Thanks, Brigitte

    That makes sense.  Basically, it's purely for efficiency.  In our case, the load is mostly around 100mA and can reach 500mA for brief periods.  And it's in a system that is mains powered.  A fraction of a milliamp is of no consequence at all in our application.  If it will work as others do with lower resistor values, I'll use those lower values.

    Perhaps the next release of the data sheet could explain this.  No need for the example, just a short note in the data sheet to say something like "To retain good efficiency at light loads, TI recommends to keep R2 around 200k, but where light load efficiency is a lower priority, lower resistor values are preferred for better noise immunity.".

    Thanks for your explanation.

    Regards

    Brian