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LM78L: Noise at inductor connected to regulator output

Mark Posner
Prodigy 20 points
Part Number: LM78L

Hello,

I have the following circuit layout and I am getting about 60Hz ripple with a 200mV magnitude at the output of the LDO after the inductor (+5v rail).  I found that when I short the inductor, the noise goes away.  The designer of this circuit is no longer available, so I am not completely sure of the intent of the inductor.  Can you offer any insights, thoughts, or improvements?

Thank you,

Mark

  • 0
    TI__Mastermind 32550 points

    Hi Mark,

    That inductor would only be used if you needed additional filtering after the LDO, and you also had a shunt capacitor in parallel with D4 and R14.  This additional filtering would complement the PSRR specification of the LDO.  60Hz ripple is not the frequency you would target here, as the values of L and C will become very large.  If 60Hz frequency were the issue then you would just use a second LDO (this is not common and I can't recall ever seeing 2 series LDO's for 60Hz filtering).  I could go on, but I would not recommend using the inductor by itself.  You should take the inductor out of the circuit.

    Thanks,

    Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 20 points

    Hi Stephen,

    Thanks for the in-depth explanation.  Can I learn more about this particular LDO's stability with respect to the impedance connected to its output? Is there any maximum capacitance value?  How about minimum load requirement?

  • 0 in reply to Mark Posner
    TI__Mastermind 32550 points

    Hi Mark,

    No capacitance is required around the LM78L for stability. The capacitance on the output of the LDO is recommended to be at least 0.01 uF, which your design exceeds. Ceramic capacitors are fine.  There isn't a maximum capacitance limitation on the output (so you can increase your Cout if you need to).  There is no minimum load requirement listed in the datasheet.

    Thanks,

    Stephen