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I am trying to calculate the new output voltage when the temperature changes the resistance of the feedback network which sets the output voltage.

circuit

I have this circuit using the MCP16311/2 buck regulator. In order to choose the feedback resistors (the voltage divider connected to Vfb), the following equation is used:

feedback resistor equation

The feedback voltage needs to be 0.8V, I need an output of 8V and I use 10kOhms for the bottom resistance, this results in a 90kOhm resistance required in the top (I used 2x 45kOhm).

Resistance calculation

The 45kOhm resistors have a tolerance of +/-0.1% and a Resistance Temperature Coefficient (RTC) of +/-15uOhms/°C.

The 10kOhm resistor has a tolerance of +/-5% with a RTC of 200ppm/°C.

I don't want to make the question too long so I have left the calculation of how much the resistance can vary. If required I can add this in after.

I am assuming an ambient temperature of 21°C and an operating temperature of +10°C to 30°C.

The 45kOhm resistors can vary between 45045.000135Ohms and 44954.999835Ohms. The 10kOhm resistor can vary between 10518Ohms and 9478Ohms.

Rearranging the below expression in order to calculate the new output voltage:

Resistor calculaiton

Rearranged equation

But this is not enough information, we do not know what the new feedback voltage will be. The feedback voltage changes because the voltage divider/ resistance ratio has changed and the output voltage changing will also change the feedback voltage. The output voltage changes because the feedback voltage has changed.

So both the output and feedback voltages are changing and affect one another. I know they will reach some equilibrium and won't both constantly be changing, but how can we calculate what these voltages will be?

I attempted to substitute another expression into the first one to reduce the number of unknowns:

Substitution attempt

But this yields a weird result and I am not quite sure why this result is obtained.

Is there something else I should be trying?

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    \$\begingroup\$ The feedback voltage will be constant (well, somewhat constant, it too will vary some with temperature, see figure 2-8 in datasheet ). The feedback resistors will not change the feedback voltage, they will only change the output voltage in such a way that the feedback voltage remains constant. \$\endgroup\$
    – Klas-Kenny
    Nov 25 at 10:38
  • \$\begingroup\$ So am I correct to work based on the assumption that the feedback voltage will remain (around about) 0.8V and the output voltage will change in order to keep the feedback voltage 0.8V? \$\endgroup\$
    – MRB
    Nov 25 at 10:53
  • \$\begingroup\$ @MRB exactly. It is a good practise that you estimate the maximum output voltage variation over temperature. \$\endgroup\$
    – tobalt
    Nov 25 at 11:49

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