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TLV61046A: Phase Margin Consideration when FB resistor divider has MOSFET being change output voltage

Hideki Hayashi1
Expert 6610 points
Part Number: TLV61046A

Hi team,

Regarding the FB resistors (R1=1MΩ, R2=71.5kΩ at the datasheet), I understand that if the resistance ratio is not changed, there is no problem even if the resistance value is reduced by one digit,
However, as stated in the application note: https://www.ti.com/lit/an/slyt469/slyt469.pdf, we also understand that in general, if the resistance value is reduced by one order of magnitude, the phase compensation capacitance must be increased by one order of magnitude.

For the TLV61046A, is it unnecessary to add phase compensation capacitance even if the resistance is reduced by one digit?

For this application, I would like to insert a resistor in parallel with R2 and turn it on/off with a FET to make the output variable between 7~18V I would like to make the output variable between 7~18V by inserting a resistor in parallel to R2 and turning it on and off with a FET.
Example
　At 7V output: R1=100kΩ, 12.7kΩ
　At 12V output: R1=100kΩ, 7.15kΩ
　At 18V output: R1=100kΩ, 4.64kΩ

Is it still unnecessary to add phase compensation capacitance for this type of usage?

Best regards,

Hayashi

  • 0
    TI__Genius 10090 points

    Hi Hayashi,

    Making the feedback resistors of R1 and R2 reduced by one digit than the recommended resistances will make the quiescent current increased. For example, when the Vout is 18V, there will be a 172uA load current at the output even there is no load. If the application doesn't care about this current, then there should be no problem to use smaller R1 and R2 as you listed.

    As for the compensation capacitor, it dependents on the used Cout. If the Cout is not very large, then it's OK to not add the compensation capacitor when smaller R1 and R2 are used.

    Regards

    Lei