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TPS62170: Choosing Feedback Resistors Based on Internal Cff

d_zero
Genius 4815 points
Part Number: TPS62170

Dear *,

we want to use TPS62170 for generating different supply voltages ( e.g. 1.1V, 1.2V, 1.3V, 1.8V, 2.8V, 2.9V, 3.3V ...) from 5V or 12V input.

As the TPS62170 has internal cff we want to calculate feedback resistor for internal cff.

In the document   slva466 on page 3 and 4 there is a method to calculate this but we have to measure the fCO which is different for different Vout ?

In your example the fCO for Vout 3.3V  is 30kHz.

Can I simulate the fCO for other Vout or can I take the fCo=30 kHz also for Vout =1.2V?

Best Regards.

  • 0
    TI__Intellectual 2415 points

    Hello,

    calculation of the feedback resistor is in datasheet: 9.2.2.1 Programming the Output Voltage.

    Further information regarding feedback divider calculation you can find here:

    Cross over frequency is different for different Vout. Documents below shows that crossover frequency for low value of the output cap is significantly higher.

     In the document slva466 is output capacitor 200uF.

    What is worst case transient load and maximum over/undershoot on the output voltage rail?

    Why do you want to use Cff?

    Best regards

    Lubomir

  • 0 in reply to Lubomir
    Genius 4815 points
    Dear Lubomir,

    how can I then calculate a starting point, if there is no simulation to plot out the bode diagram and fCO?

    All the voltages has different load capacitance and different current load.
    Cff, because I want to use smaller resistance for feedback (eg. from 70k to 240k).

    Best Regards.
  • 0 in reply to d_zero
    TI__Intellectual 2415 points
    Hello

    you can use resistance in range of 70k to 240k according to the datasheet:
    It is recommended to choose resistor values which allow a current of at least 2 uA, meaning the value of R2 should not exceed 400kΩ. Lower resistor values are recommended for highest accuracy and most robust design. For applications requiring lowest current consumption, the use of fixed output voltage versions is recommended.

    I would recommend to follow Table 1 Recommended LC Output filter Combinations.

    Stability of the feedback loop you can estimate based on the transient response, document: www.ti.com/.../slva381b.pdf

    Best regards

    Lubomir