nRF52 Solar Reference Design

[K. Townsend]

We're quite interested in producing an nRF52 design based on the solar beacon (great design, and clean software). I appreciate the relative simplicity and manageable BOM cost of the reference design.

I read through the documentation, but there aren't any notes on adjusting the solar power front end that provides power to the nRF52 is controlled intervals via the four FETs and the resistor divider networks.

We'd like to be able to adjust the solar supply based on the specific solar cell used, or different timing or current requirements ... I don't suppose you happened to have a spreadsheet detailing values used in the reference design to be able to tweak them for different use cases? Or would it be possible to add a quick description of the design choices made on the front end to enable slight modifications?

I don't mind digging into this myself, but figured it was worth asking before I started digging into it. I suspect adding a quick spreadsheet to the hardware download would help a great many people though.

[awneil]

You mean this: infocenter.nordicsemi.com/index.jsp ?

[K. Townsend]

Yes, that's the board I'm talking about. I should have added the link for the sake of other viewers sorry.

[K. Townsend]

To further clarify, there is some minimal info on the hardware design here: infocenter.nordicsemi.com/index.jsp

Unfortunately, there are no notes on adjusting the resistor divider values, or any design notes on the specific values chosen, which is unfortunate since this is a particularly inexpensive solution to an interesting problem. A simple spreadsheet in the downloadable hardware folder would help a lot of people out taking a nice design and making some minor changes to it ... or even a quick one or two paragraph explanation of the decisions behind the design with the values and output clearly stated.

The dual FET ICs and resistor dividers I'm talking about can be seen here:

[TORS]

The U5 switch is the actual switch that turn ON/OFF the radio. This switch is controlled by the Vgs treshold voltage in U4. The Vgs voltage level is again controlled by voltage devider R11, R12 and R13. R11 could in theory be merged with R12, but are implemented to create the hysteresis characteristics of the switch. The U4 turns on when the voltage level at pin 2 in U4 is higher than 1.4 V, as documented in its datasheet. To reduce the power consumption of the hysteresis switch, resistor values is made as high as possible. Because of small leakages, non-linear and parasitic effects in U4 and U5, the thoery doesen't correlate perfectly with practice. The theory should anyhow give you some ballpark values, while the last tuning has to be done manually. Unfortunately, I don't have any spreadsheets with optimal resistor values that represents the different voltage levels.

[K. Townsend]

Thanks for the design notes. It might be useful to add to the infocenter guide for people who don't stumble across this post, but thanks for posting some more info here at least.