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Thread: Why is the output filter capacitance so strange here?

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    Default Why is the output filter capacitance so strange here?

    Hi, I'm Yangki. I met some problems when I tried to use 79L05 as the negative electrical level.

    I found that the output filter capacitance is very large at first, just like the figure below:



    It is -12V. Under this condition, the chip is very hot,and the output voltage is just about -2V.

    I changed the 220uf to 47uf, then the chip worked well and the output voltage was at -5V without heating.

    I wonder why? Can anyone answer me? Thanks very much!

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    Well, in short, it is due to the capacity of C48. L7905 has a max. Output current of 100mA. You charge C48 & C49 without charging resistor with 100mA. The charging current that would be adjusted would be ESR (C48 + C49) / ESR (C48 * C49). Assuming C48 = R & C49 = R / 2 -> Rc = 3R | 5V / 3R = 1.6A where the max current from L7905 is 100mA. The circuit causes a load that overloads the source 16 times. Therefore, the chip is warm and thanks to the internal current limitation it still works.

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    Jon Gerow (05-17-2017)

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    As C49 is ceramic, it would have a very low ESR, much lower than the electrolytic capacitor's ESR. C49 would probably have less than 10mOhm esr while the 220uF/10v capacitor would be around 250-600 mOhm.

    Still, what he said is valid... you're pushing too much current on the output. You could fix it by using a smaller capacitor which would have higher ESR (you technically don't need mode than 10uF or something like that) or you could just put a 0.5/1ohm resistor in series with the capacitor in order to "fake" a higher capacitor ESR.

    Also note, typically ceramic capacitors are for decoupling and should be placed as close as possible to the regulator pins, where they're most effective.

    While the datasheets may not say it, the regulator would also benefit from a tiny capacitor on the input, for example a 1uF ceramic capacitor with voltage rating 2-3x the input voltage, or a 10-100uF electrolytic capacitor.

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