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OPA4990: OPA4990 output cap max recommend value

Red Yen
Expert 4330 points
Part Number: OPA4990

Hi Support team 

since the noise coupling, my customer would like to increase the output cap to maybe 0.1uF to filter the noise, 

the AC_L1 & AC_N1 is 50-60HZ AC sin wave AC input

the application is as following, pls kindly let me know the side effect if increasing the cap and the max recommend value 

thanks a lot 

  • 0
    Guru 140200 points

    Hi Red,

    increasing CC34 to 100nF would be very bad design practise and it would not help at all. The consequence would be increased instability because of ruining the phase margin of OPA4990.

    Why not adding a cap in parallel to RC41?

    By the way, CC38 is critical as well, because it also reduces the phase margin.

    Kai

  • 0
    TI__Guru**** 150516 points

    Red,

    May I assume that there is a lot of series resistance (made with multiple resistors) between node AC_L1 and AC_N1 and the actual line voltage?

    Pin 12 should not be directly connected to ground. give it a 10k resistor or connect it to pin 10 and share that resistor.

    To reduce high frequency noise, remove CC38, and change CC39, CC42 to 5.6nF (for BW of 1.35 kHz)

    Do not add capacitance directly to pin 8, add a series RC filter instead.