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Date: Thu, 6 Feb 2020 01:20:12 -0800
Subject: Re: [EE] 78L05 (SO8) overshoot on removal of over-current event?
Thread-Topic: [EE] 78L05 (SO8) overshoot on removal of over-current event?
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Quoting Jason White <whitewaterssoftwareinfo@gmail.com>:

> Hello everyone,
>
> At a previous job I observed a failure mode where the 100mA current
> rating of a 78L05 type regulator (12->5V, SO8) was exceeded causing it
> to saturate and then briefly overshoot to >7V when the load was
> removed causing damage to 5V circuitry. (a micro-controller was
> driving a large number of opto-isolators, all toggled on and off at
> the same time)
>
> Do anyone have any links to literature that documents this behavior?

Hi, Jason:

Just out of curiosity I tested a 78L06 with a AO3400  MOSFET applying
and removing a 20 ohm load (causing the output voltage to drop to about 4.5=
V)
Input capacitor 1uF 1206 ceramic X7R

With an output capacitor of 100nF the overshoot was about half a volt. =20
With 1uf it was less- about 300mV with a duration of about 25us.

If I remove the capacitor entirely I can see a huge overshoot.

Rise & fall times of the order of 75ns.

Of course a fast fall-time load with layout inductance can lead to
a voltage spike on the board if there are not bypass capacitors
properly situated to absorb the energy stored in the inductance.
The fault current can be in the hundreds of mA, so a relatively
large capacitance (hundreds of nF or 1uF) would be good.

Best regards,
Spehro Pefhany




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