How to configure cell battery and USB power

[Serafino Convertini]

Hi to all,

this is my first public ticket ;)

I would power a nRF52840 on my custom board both with a cell battery (specifically CR2032) and the VBUS provided by the USB connection.
I was thinking to use two schottky diode in the way showed in the attached image with using the internal DCDC of the nRF52840 as in the development board.

But I'm not sure to use schottky diode since the voltage drop on the diode. In the proposed configuration for the nRF52840, I cannot find any solution that could fit with this question.

Has anyone done this task before?

Thanks in advance

[hmolesworth]

My grandfather used schottky diodes, nowadays there are better low-drop solutions. Have a look at these:

max40203 Ideal Diode

[Serafino Convertini]

Let's assume that using MAX40203 or TCK106 (as used in nRF52840-DK). Does make sense to connect the VBUS (when present) to VDDH and VBATT (qhen present) on VDD, or should I connect the output voltage (VBUS or VBATT) to VDDH?

In both case I think I configure the nRF power stage using internal DCDC.

[Serafino Convertini]

Whereas the device must work for long period, if I should use 1.8V as VDD and I have a coin cell with 3V. How could I provide a 1.8V voltage on the nRF. Should I use a DCDC stage to generate 1.8V starting from 3V (coin cell)?

[hmolesworth]

I would suggest just use your latest circuit as-is, but change the LTC411 to the MAX40203 and set VDD to 1.8V via REGOUT0. Best test before commiting to PCB layout ..

Keep in mind that the coin cell will not work well in low voltage situations (ie below 1.8V + a bit) as REG0 might misbehave, hence the need for large capacitance on the coin cell (and some on VDD).

Edit: VDDH High voltage mode operating voltage 2.5 minimum, so coin cell (as seen at VDDH) must not dip below 2.5V

[hmolesworth]

In case you are not familiar with why I always use 3xvoltage for the capacitor voltage rating as a rule-of-thumb, have a look at this White Paper. 3x is perhaps over cautious, but 2x is mandatory (my view). Ceramics are the only suitable capacitor type, others (Tant, Electrolytic, Super, etc) are too leaky or have short life. Capacitor De-rating Issues nwp_030.pdf

This paper specifically refers to nRF52840 design

[Serafino Convertini]

Hi, if I use 150uF 10V I must use a lot of capacitor and I lost the requirement of smal size :O. At the moment I have 2x 4.7uF 6.3V

[hmolesworth]

Ah, good point. 3 x 47uF are not that big but I see the problem. 2 x 4.7uF will shorten the useable coin cell battery life, perhaps by 10%, as more of the pulse current has to be supplied from the coin rather than the coin trickle-charging the capacitors and the risk of current-pulse-induced nRF52840 resets will occur sooner. Mitigate by using PPI and keeping the CPU in sleep as much as possible and set VDD to 2.8V. Rapid advertising Tx burst raise the risk of resets, reducing the attempts will also help; instead of (say) 20 Tx advertising per second 1 per second is much better even if then it takes longer to connect. Ditto in Beacon Mode. Worst case change the design to use an  external DC-DC battery-to-VDD at 1.8V instead of 2.8V, but that takes as much physical space as 3 x 47uF.