Use 1.3V System Power for external circuit.

[Al Bundy]

I would like to use 1.3 V System power to supply some external circuit that may need max 10 mA. This voltage is available via pin DEC4.

I expect that you are going to write that it is not recommended, can decrease radio performance etc.

However I need to power my external circuit periodically and only for few seconds. During that time I can turn off radio and I don't perform any flash write or erase operations so theoretically I should be able to add some external load for LDO or DC/DC.

My questions are:

1. Are there any other contraindications?

2. What is the max current that I can draw from LDO or DC/DC? Or more generally can I find any characteristics for both of them?

[ketiljo]

There's no additional current from the DEC4 regulator available so you can't use it to power a external circuit. The same applies for all the regulators and DCDC converters. The only exeption is REG0 on the nRF52840.

[Al Bundy]

ketiljo I think you just do not understand the question. When RADIO is off I am sure I can draw some current from LDO or DC/DC for external circuit. The question is how much I can draw.

[ketiljo]

Nothing basically. The regulators will turn off and on depending if they are in use or not. None of them are meant to power anything else. 

[Al Bundy]

Ok but imagine that CPU is on then system regulator is on and all peripherals are off, then wouldn't I be able to draw the current that potentially could be drawn by peripherals?

[ketiljo]

The regulator switches mode depending on which and how many peripherals that are enabled at a given time. The power management unit have control over that. This is automated and can't be overridden.

If you don't use the radio, there's no extra current to be drawn from the regulator.  It's not a simple regulator that is turned on and off. For instance, when the radio is off, the regulator goes into refresh mode where it charges a capacitor in short bursts at relatively long intervals. The ripple is quite high in this mode as well.