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Using Arduino => Project Guidance => Topic started by: CrossRoads on Aug 20, 2014, 08:20 am

Title: self-controlled power switch
Post by: CrossRoads on Aug 20, 2014, 08:20 am
I'm looking to add a power control switch to a Teensy 3.1 card.
Here's my thinking - Vbat and VUSB feed thru low Vf diodes to the FET source; either LiPo battery, or USB if connected.
http://www.diodes.com/datasheets/ds30492.pdf

That provide at least 3.4V to the P-channel FET
http://www.fairchildsemi.com/ds/FD/FDC604P.pdf
The gate is pulled high to keep the P-FET off.

When the gate is pulled low by the switch, the processor wakes up  and it then holds the gate low to stay awake via the N-channel FET. The N-channel keeps the pullup voltage off the processor pin when there's no power on Vreg-in. It's gate is pulled low to keep from floating and turning on unintentionally.

This makes sense, yes?

I should probably have cap C2 below the P-channel instead of above it, next to pin 8.



Title: Re: self-controlled power switch
Post by: MichaelMeissner on Aug 20, 2014, 06:35 pm
You might want to ask this over at http://forum.pjrc.com/forum.php (http://forum.pjrc.com/forum.php), which is Paul Stoffregen's (designer of the Teensy) forum, where he is more likely to notice it.
Title: Re: self-controlled power switch
Post by: CrossRoads on Aug 20, 2014, 10:27 pm
Why? Is not specifically a Teensy problem, just where I happen to be adding it.
The concept could be used with Arduino too. Just imagine the processor above as any 8 MHz arduino, which will run happily at 8 MHz from ~2.7V and above.

Need a circuit that can turn off power after some inactivity time, or perhaps a low battery, vs a non-mechanical (slide, toggle) switch that the uC cannot possibly move.
Pololu has a little board, I cannot determine the chip used, I think this covers the practical part of it tho.
Title: Re: self-controlled power switch
Post by: Kinnishian on Aug 21, 2014, 05:02 am
I haven't checked your schematic, but this definitively makes sense. It's been done before, for sure. I intend on using it in a future project actually, though I'll be trying to do high-side.

I know of a guy on endless-sphere.com who has powered some MCU projects where a momentary on button gives power to the unit before it then powers a mosfet to sustain energy.

I don't think you need to mess with the switch turning the mosfet on. Just have the switch give power to your microcontroller, and deal with the fet during the microcontroller setup.
Title: Re: self-controlled power switch
Post by: Jiggy-Ninja on Aug 21, 2014, 06:11 am

I haven't checked your schematic, but this definitively makes sense. It's been done before, for sure. I intend on using it in a future project actually, though I'll be trying to do high-side.

I know of a guy on endless-sphere.com who has powered some MCU projects where a momentary on button gives power to the unit before it then powers a mosfet to sustain energy.

I don't think you need to mess with the switch turning the mosfet on. Just have the switch give power to your microcontroller, and deal with the fet during the microcontroller setup.

So something like the old seal-in circuits used by relays and PLCs?

I think a momentary switch on the high side in parallel with a P-channel FET with the gate pulled to VCC with a resistor, connected to a uC pin is what he's talking about. Press the momentary, and it provides power to the uC, which pulls the P-FET's gate low. When you want to kill the power, HIGH-Z the pin or push it HIGH.

I'd test it, but my breadboard is already occupied with my current project.
Title: Re: self-controlled power switch
Post by: rogerClark on Aug 21, 2014, 06:56 am
I was going to recommend modelling it in LTSpice, especially as it uses special diodes.
Title: Re: self-controlled power switch
Post by: CrossRoads on Aug 21, 2014, 08:31 am
Special? They're just low voltage schottky diodes.

"with the gate pulled to VCC with a resistor, "
"it provides power to the uC, which pulls the P-FET's gate low."

That violates the requirement to keep IO pins <= VCC +0.5V tho.
Need the transistor between IO and P-FET gate so that a gate pulled high does not leak current into the pin and "phantom power" the chip.
Making the pin High-Z should let the N-FET gate pull down keep the power off.

I'm working 2 designs at once (with 3 others pending!), think I'll dig up some FETs and try this before I get too far on this board's layout & routing.
Design-wise, I think I'm on the right track tho.
Title: Re: self-controlled power switch
Post by: rogerClark on Aug 21, 2014, 09:27 am
Quote
They're just low voltage schottky diodes


If you have some to test with that's great, I just meant they were not regular diodes.
i.e. I have a rack of components and low voltage Schottky diodes are not in my stock

But they probably should be ;-)
Title: Re: self-controlled power switch
Post by: TomGeorge on Aug 21, 2014, 10:57 am
Hi, is this what you are looking at?
http://www.pololu.com/product/751

No circuit diagram unfortunately, though reverse engineered.........

Tom... :)
Title: Re: self-controlled power switch
Post by: CrossRoads on Aug 21, 2014, 04:33 pm
I believe so TomG, I can't open pololu from here to confirm (network issues it seems).
Title: Re: self-controlled power switch
Post by: TomGeorge on Aug 22, 2014, 12:33 am
Hi, I'd build your circuit if I was you Crossroads.
See  how it goes, I'd say that switch on will be  fine.
Its making sure that switch off is glitch free to ensure a clean off, that needs to be tested in prototype.

Tom..... :)