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LM337: Input Capacitor

Jejomar Ildefonso
Genius 9490 points
Part Number: LM337
Other Parts Discussed in Thread: LM317,

Hi Team,

Our customer is currently working with both the LM337 and LM317 LDOs. Currently using with a 1µF tantalum input capacitor with a 3Ω ESR (some 7Ω in some versions). The capacitors are installed on the output of an isolated DC-DC converter (+Vout=+12V, -Vout=-12V) which is used as the input to the LDOs (Vout=+5V and -5V respectively).

They're currently having issues with the DC/DC converter apparently due to its short circuit protection so they'd like to change into one without SCP.

What would be the recommended input capacitor at this case? It seems that a capacitor having higher ESR would be better (not quite sure). How important would be the ESR value at this case?

Thanks in advance!


Kind Regards,

Jejomar

  • 0
    TI__Mastermind 37005 points

    Hi Jejomar,

    It is likely that the impedance of the DC/DC converter will dominate here.  If you use a Cin on the LDO with ESR = 3 ohms, the output impedance of the DC/DC will be lower than that, so the effective impedance will be much lower as a result.  It is better to have a low ESR capacitor (and low impedance) on the input.  Ceramics or polymers on the input will be fine.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    TI__Genius 9490 points

    Hello Stephen,

    Thank you for looking into this. In the case of LDOs input capacitors with very low ESR, isn't the problem even greater that they are then seen as a short for the DC-DC converter?

    The customer thinks that 3 or 7 Ohm ESR could be unsafe in case of DC-DC converter without continuous protection. But since the DC-DC can have 330uF in output, we're thinking it could be ok.

    We've also read that often for old LDOs, a small series resistor is added to the input capacitor when low ESR capacitors are used. In the current layout is not present, but is this true?


    Kind Regards,

    Jejomar

  • +1 in reply to Jejomar Ildefonso
    TI__Mastermind 37005 points

    Hi Jejomar,

    The capacitor is a high impedance at low frequency so there is no concern about a short.  The first order model of a capacitor is a series RLC circuit, with the R = ESR and L = ESL.  Even with a very small ESR you will still have the series capacitor in the model.  If you review impedance curves of capacitors you can see that they have large impedance at low frequency.

    For old LDO's, a small series resistor is added to the output capacitor, not the input capacitor.  This is when the LDO datasheet specifies an ESR range and the capacitor ESR is too small.  Then a small series capacitor is needed to maintain stability.  But on the input this is different.  The input wants a low ESR capacitor to react quickly to transients, so that the input voltage will not droop.  If the input voltage droops, this will cause the LDO to see a line transient which (due to the high gain of the internal feedback system) will cause a large perturbation on Vout.  So using plenty of low ESR capacitance on the input prevents this issue.

    Thanks,

    - Stephen