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LM317M: Recommended ESR range to prevent oscillation

Takashi Onawa
Genius 13200 points
Part Number: LM317M
Other Parts Discussed in Thread: LM1117

Hi Team,

My customer is looking for LDO supporting low ESR capacitor as Cout such as ceramic capacitor.

According to LM317M datasheet, it supports ceramic capacitor but there is no description about ESR range. 

Do you have any data about the range of LM317M to not get oscillation?

Regards,

Takashi Onawa

  • 0
    TI__Expert 5231 points

    Hi Takashi,

    You do not need to worry about the ESR because the point of the output capacitor for this device is to improve transient response and not effect stability. As long as you keep within the ESR range of a ceramic capacitor you shouldn't experience oscillation. Please let me know if you have any more questions. 

    Thank you,
    Josh Nachassi

  • 0 in reply to Josh Nachassi
    TI__Genius 13200 points

    Hi Nachassi-san,

    I understand that point.. I'm asking because there are no description about "the ESR range" in the datasheet.
    Fundamentally, old LDO that is optimized with tantalum and Electrolytic capacitor needs additional series resister on Cout to get well stability with ceramic capacitor.

    My customer want to clarify that LM317M needs such the additional resister for good stability since they don't want to change their board layout(they forgot to put the pad of the resister on current board and facing oscillation issue with LM1117 now)

    Regards,

    Takashi Onawa

  • 0 in reply to Takashi Onawa
    TI__Expert 5231 points

    Hi Onawa-san,

    This LDO was developed back in 2000 so we don't have the full design data (including ESR range). But according to the datasheet, this regulator will be stable with ceramic output capacitors as referenced in section 8.2.2.2 of the datasheet. Having too low of ESR can potentially cause this regulator to have less stability (and hence, more ringing during transients) but it should remain stable.

    Thank you,

    Josh Nachassi