Is using the power down mode like this a good idea, or wont this work?

Hi,
I am working on a project where I am building a device working on a battery. It’s coming along nicely, and I am looking into ways to reduce my part count.

As an on/off switch, I am currently using the following circuit to make a tactile button latch the circuit on and off:

This work quite well, but it uses quite some parts. To make the part count lower, I thought I would use the power down mode on the Arduino chip. The chip I am using is an ATmega4809.
As I understand it, the power down mode simply turns most parts of the chip off, leaving only what is needed to turn the chip back on, while consuming very little power. Using this functionality, I want to make a new circuit like this (ATmega4809 not shown)
image
In which a “permanent” (from the battery) power source supplies power to a switch controller (which is also currently used by the circuit shown earlier), and to the ATmega4809. Then a button is connected to one of the digital pins (interrupt capable), which notices when the on/off button is pressed. This triggers the chip to disable the switch controller, and put the chip itself in power down mode.
Now if the button is pressed again, the chip should wake up again, and enable the switch controller, thereby powering up the device again.

Can the power down mode be used like this? Or am I miss-understanding something about its functionality? Is there anything else I should look out for if I implement this?

The ATMega must stay powered for this to work and needs to turn all pins connected to the load to the INPUT state to prevent phantom powering.

You can configure an ATmega 4809 in such a way that button toggles it between ON and a sleep mode with a power consumption in the low microamp range. On waking, it can enable the load switch to power any additional modules etc… However, as pointed out, it has to be permanently connected to a power source.
This may be useful, but not specifically for the 4809: https://www.microchip.com/content/dam/mchp/documents/OTH/ApplicationNotes/ApplicationNotes/AN2515-AVR-Low-Power-Techniques-00002515C.pdf

Good point about the phantom powering, thank you.

Thank you for linking that document, I will definitely give that a read!

To clarify: the phantom powering here is a potential problem for the other devices in the system (e.g. sensors and what not), not for the chip itself right?
i.e. I can connect the positive lead of the battery to the atmega4809 without worrying about phantom powering the chip? (As the chip is already powered anyway)
But I do need to look out for the chip not powering the devices that should be off so that current leaks away via that path?

Correct.

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