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Logic vs Power ground

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Mosaic

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I am designing a cct which has large power transients @ around 38VDC. A capacitive discharge system similar to ignition pulses @ 120Hz.

I want to protect the MCU from ground bounce or ignition noise on it supply rails.

Is it advisable to place ferrite beads in the MCu Vcc and Gnd before or after the 5V regulator?

Star topology for the grounds & decoupling caps are included.

Also I am looking at using an optocoupler between the MCu pin and the MOSFET driver (mcp1407) input.

https://www.newark.com/avago-technologies/4n35-000e/optocoupler-phototransistor-3550vrms/dp/36J9837

But with a 3uS latency, would this cause any pulses under 3 uS to be lost? What would happen for say a 5uSec pulse? Would it just be time shifted by 3uSec an remain 5uSec long?
 
depending on the nature of the power transient you may need more than just a ferrite bead.
38 volts may kill a cheap 7805 if the actual spike is higher but you can't see it on the scope...

that optical isolator is rather slow, it may not turn on at all with a 3 uS pulse, also note that it may take as long as 10uS to turn on.
 
I want to protect the MCU from ground bounce or ignition noise on it supply rails.
This is very easy to do with any combination of:

Tranzorb clamps

L-C filter

common mode choke filter


As stated above, using an automotive reg designed to handle load dump will protect the regulator but it may pass transients through and kill downstream circuitry.
 
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LM2931XX
The LM2931xx are very low drop regulators. The
very low drop voltage and the low quiescent
current make them particular suitable for low
noise, low power applications and in batterypowered systems. In the 8-pin configuration
(SO-8), fully compatible with the older L78Lxx
family, a shutdown logic control function is
available. This means that when the device is
used as a local regulator it is possible to put a part
of the board in standby, decreasing total power
consumption. Ideal for automotive applications,
LM2931xx is protected from reverse battery
installations or 2 battery jumps. During the
transient, such as a 60 V load dump, when the
input voltage can exceed the specified maximum
 
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