Microcontroller with separate linear regulator supply?

[treez]

Hello,

I have a microcontroller which controls an emergency light. This micro must charge the battery, which powers the light.
The micro also receives inputs from various opamps and comparators.
The micro and most of the opamps and comparators are all supplied by the same 3V3 linear voltage regulator. (this is the “main” 3V3 regulator)

The 3V3 regulator is supplied from the battery.
However, every now and again, the micro must simply switch the battery out, so that the battery does not get drained by the ~8mA current drawn by the opamps etc.

The problem is, that when the micro switches out the battery, it also switches off its own power supply, and this isn’t wanted.

Therefore, the micro must be fed by its own, separate, dedicated, “micro” linear 3V3 regulator. Then when the micro switches the battery out, this only means that the “main” 3V3 regulator looses power. The “micro” 3V3 regulator still draws power from the battery (but not much).

Anyway, do you see any problems with having two 3V3 regulators like this.? (ie one supplying the micro, and the other supplying the opamps and comparators etc).

One problem with this “two regulator solution” could be that some of the micro’s digital outputs connect up with circuitry supplied by the “Main” 3V3 regulator, and when the “main” 3V3 regulator powers down, the micro’s digital outputs could end up getting shorted to ground via the ESD diodes in the opamps etc.

Are there any known ways of preventing this kind of thing from happening?…or is it just a case of connecting series resistors to the micro’s digital outputs so that high short circuit currents don’t get drawn from the pins of the micro? What sort of size series resistors would be advisable?….(or more relevantly,what is the maximum current that could safely be allowed to flow through the ESD diodes of the opamps and comparators?)

 

[kam1787]

How long does the battery swap take? If quite short, then a large capacitor might be sufficient.

Can you change the power supply to 4V?
Connect 2 diodes - both anodes to the power supply, the cathode to the micro, other cathode to the rest of the circuit

after the diode to the micro, place a large capacitor in parallel to the power lines

 

[tpetar]

There is many possibilities to solve this, one is as Kam1787 says in previous post to use larger capacitor, or super capacitor, today you can buy for cheap or get sample. Second option is to use smaller second battery, and if this is very important safety device use one more third battery. These batteries can be on standby and always fully charged, if big battery is absent then use second if second is out use third. Small batteries can be Li-Po or one Li-Po and second Ni-Mh or some other chemistry.


Best regards,
Peter

 

[kam1787]

https://en.wikipedia.org/wiki/Button_cell

 

[betwixt]

Providing a short term alternative power source just for the micro is easy and described well in previous posts. The issue of sequencing power is important but with care it should be possible to do it safely. The micro will always be powered so the chances of a powered-down peripheral device trying to dump current into it is slim but there is a danger that the micro could supply to the peripheral device and cause latch-up when the power is re-applied. With suitable current limiting resistors in the micro's output pins and a scattering of protection diodes it should be safe to do.

Brian.

 

[kripacharya]

Why not simply set it up like this -- ?

 

[treez]

@Betwixt:

scattering of protection diodes

...do you mean adding a protection diode in parallel with say the opamps internal protection diode so that any current goes through the external diode rather than the opamps intrinsic protection diode?
I say this because the current capability of say opamp's internal protection diodes is a total unknown. Ive known places having say 2mA constantly running through such internal protection diodes but don't know how long their products last in the field.

@ Kripacharya...............that's basically what I am thinking of doing, but I still have connections from micro to unpowered opamps etc, so I need to think about series resistors etc.

 

[betwixt]

That's exactly what I mean. Assuming your opamps or whatever else the micro drives doesn't need much input current, you can add a series resistor to limit the current but not with high enough value to upset the normal operation then use small Schottky diodes to divert undesirable current to the supply/ground lines. Essentially, they are in parallel with internal protection diodes but will have significantly higher current ratings. Use Schottky types because they have a lower Vf than most internal clamp diodes. Types 1N6263 or even better BAT85 come to mind.

Brian.