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Voltage regulator activation only above threshold voltage?

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tightmopedman9

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I'm powering an MCU through either 4 alkaline/Ni-MH or 2 Li-Ion cells. The maximum voltage the MCU can handle is 6V and the BOD level is set for 3V. When 2 Li-ion cells are used the input voltage will be ~7.4 volts and I'd like to regulate this voltage to prevent damage to the MCU. When the device is powered with alkaline cells no voltage regulation will be needed, but it will be needed with the Li-Ion cells.

I'd like to just use a LDO with a fixed output at 5.5V, however, all LDOs I've seen feature under voltage lockout. Therefore, they won't power my device unless the voltage is above 5.5V + the dropout voltage.

Is there any way I can have a voltage regulator that is free conducting below a certain voltage, and above it regulates the voltage?
 

How much current do you need? Maybe you can use a shunt regulator.
 

At max maybe 300ma. Any recommendations on a shunt regulator, my searches didn't turn up much.

- - - Updated - - -

Just checked out the TL431, seems just like what I need. I'm gonna draw up a circuit and post back in a bit.

- - - Updated - - -

Looking at the TL431 it seems it can only sink 150ma of current, not enough. I'm also starting to think that just dumping the excess voltage may not be the best idea. It would be nice to have a shunt regulator feed into a voltage regulator, but that seems overly complex.
 

A simple transistor regulator can do what you want. A zener diode provides the reference V.

Schematic:



The scope traces tell the story. To operate a 5V device you must maintain supply above 5.6 V or so.

It will take some effort to contrive a method of bypassing the regulator when supply goes below 5.5 V.
 

In LDO , at the gate of the pass transitor, use conditional pull up resistor.It should pull the gate high only when VCC goes below 5V or 5.5V.
I guess it should solve your problem.
 

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