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LM317 PSRR

John Walton
Prodigy 50 points
Other Parts Discussed in Thread: LM317, LM317M

I think that the chart on page 5 of the datasheet is mislabeled.  The regulator with Cadj=10uF should have a better PSRR.  Left me asking a question, however.  Why is the PSRR of the LM317 worse than that depicted in prior datasheets, and in the NatSemi databook. 

  • 0
    TI__Mastermind 22610 points

    I'm looking at the LM317M datasheet (SLVS297O–APRIL 2000–REVISED JULY 2006) Page 5 top/left "RIPPLE REJECTION vs FREQUENCY".

    The curve labeled Cadj=10uF is about 70dB from 100Hz to 1kHz, compared to the curve labeled Cadj=0uF which is about 55dB across the same frequency range.

    For Ripple Rejection 70dB (1:3162) is better than 55dB (1:562). The '-' sign is generally optional, since it's known to be an attenuation from the input to the output.

  • 0 in reply to Donald Jones
    Prodigy 50 points

    You're referring to chart 9 in the 2006 version.  In the most recent version SLVSO44U - April 2008 the chart on page 5 is different, showing PSRR of -62dB @ 100Hz for the regulator with ADJ bypassed by 10uF, compared to -65dB for the unbypassed regulator. I accessed the data sheet from the NSM site.

  • 0 in reply to John Walton
    TI__Mastermind 22610 points

    OK, I got it : LM317 SLVSO44U.

    http://www.ti.com/lit/ds/symlink/lm317.pdf

    Even though you found this datasheet on the NSM site, it's for the TI datasheet for the TI silicon, not the rebranded 'National' version.

    I agree, something is not quite right, no doubt. Due to Xc of Cadj the ripple rejection should be closer to the same at very low ripple frequencies and then start to diverge as ripple frequency increases.

    I'll need to defer to my colleagues in Dallas for this issue.

  • 0 in reply to Donald Jones
    TI__Guru**** 156616 points

    The data sheet series have the labels reversed. The blue line should be 10uF. The lines converge at high frequency due to the device gain reduction at high frequency. It is not shown in the graph, but the lines will also converge at very low frequency.

    Regards,
    Ron Michallick

     

  • 0 in reply to Ron Michallick
    Prodigy 60 points

    Datasheet (LM317) is still incorrect.  LM317M datasheet is correct, and it is helpful to have the Zout graph.

    It can also be demonstrated that the LM317 may be unstable if a tantalum capacitor is used on the OUT pin.  For one of the most widely used regulators, it's surprising that this hasn't been discussed more broadly.  See this application note from Omicron:

    http://www.omicron-lab.com/fileadmin/assets/application_notes/App_Note_Noninvasive_Stability_V1_0.pdf

  • 0 in reply to John Walton1
    TI__Guru**** 156616 points

    John,

    I will check into why the data sheet was not updated.

    With a low ESR capacitor (tantalum in the application note), the phase margin will be very low. However it will be positive if there is no third pole.
    There will definitely be ringing on transients, but the ringing will die out and it will be low amplitude. For most applications this will not pose any noticeable problems.
    If ringing is a problem, measure the ringing frequency then add a series resistance to the output capacitor that makes a zero at 1/2 the measured frequency.