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LM78XXC, UA78XX, UA78MXX - ESR of the output capacitance

Hirotaka Matsumoto
Guru 27565 points

Hi all 

Would you mind if we ask LM78XXC, UA78XX, UA78MXX?

Our customer uses LM78XXC, UA78XX, UA78MXX with following conditions. 
It oscillated because of low ESR, we guess that it causes of beyond from stability region.

<Conditions>
Input capacitance: 0.33uF 50V
Output capacitance: 10uF 16V and 1uF 16V(in parallel) 

So, in order to check stability region, do you have as following figure?

<Figure which our customer needs>
Y axis ：ESR(Output capacitance)
X axis： Output current

Kind regards,


Hirotaka Matsumoto

  • 0
    Guru 27565 points

    Hi team,

    We guess that your days are so busy.
    However, could you let us know our request if you have some information?


    Kind regards,

    Hirotaka Matsumoto

  • 0
    TI__Guru*** 145286 points
    Hirotaka-san.

    These regulators predate modern high capacity ceramic capacitors.
    They also are stable without any output capacitor but input capacitor is helpful and can be low ESR without issue.
    I have found these devices are marginally stable with ceramics . Meaning they ring a lot on transients. This not ideal.

    Do you know the oscillation frequency and is it close to sinusoidal in shape?
    Would it be stable if there was no output cap at the regulator and the capacitors were only at the loads?
    I don't have any ESR charts, but the oscillation should tell what ESR is required.
  • 0 in reply to Ron Michallick
    Guru 27565 points

    Ron san,

    Thank you for your prompt reply!

    Wave form： we will hear whether the wave form is sinusoidal or not.
    Load：70mA(In this case, it occurs oscillation.)
    Oscillation cycle: 4us(frequnecy=250kHz)

    We guess that it causes the output capacitance is larger than the input capacitance.(both are ceramic type.)
    So, we suggest that it should be input 1uFx2(in parallel) and output 0.33uFx2(in parallel).
    Is our recognition correct?

    We need your help.

    Kind regards,

    Hirotaka Matsumoto

  • 0 in reply to Hirotaka Matsumoto
    TI__Guru*** 145286 points
    10uF and 250kHz could zero with resistance of 1/(6.28*F*C) = 0.064 ohms.
    This is a very low resistance so my idea of having capacitor only at loads might add enough resistance.
    If an actual resistor is added, try double the value 0.13 ohms. Exact value is not important. Next higher value is OK.
  • 0 in reply to Ron Michallick
    Guru 27565 points
    Ron san, Thank you for your help.
    Kind regards,
    Hirotaka Matsumoto