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.

[hmolesworth]

VBUS from USB complicates things as it is +5V and therefore must connect to VDDH. VDD when VBUS is present will output the programmed voltage on REG0 and so REG0 must be set to the maximum expected CR2032 voltage to ensure no power is taken via the MAX40203 by the CR2032. This implies i/o pins and external circuitry will be at (say) 3.4 volts with VBUS present; internal MCU and RADIO will be 1.3V. With VBUS absent the CR2032 will provide power via MAX40203 to VDD; this implies i/o pins and external circuitry will be between (say) 3.4 volts and 2.6 volts with VBUS not present; internal MCU and RADIO will always be 1.3V. Pulse currents will cause even lower voltages on the CR2032-driven VDD, and so a lot of bulk capacitance is required (ceramic, 3 x 3.4V or say 10V rating). In this scenario, typically the MAX40203 is only required on the CR2032 battery assuming that when VBUS is absent nothing connects to the USB. Adding a MAX40203 to the VBUS input provides better protection against potential back-drive of stuff on the USB port.

The alternative is to use an external 3.4V DC-DC on VBUS and only attach that to VDD with a MAX40203 together with the CR2032 battery; probably a less power-efficient (and more expensive) solution

[Serafino Convertini]

Hi, I choose to use LTC4411 to select exclusively VBAT or VBUS and connect the output to VDDH of the nRF

I doubled the capacitors on VDD pin to avoid dropout voltage during radio transmission

[Serafino Convertini]

I know that the nRF52840 can use linear regulator or dcdc regulator. Using one of them is set automatically or should I set any flag registers?

[hmolesworth]

The LTC4411 is a good choice if the battery is not (say) a CR2032 used for long periods; the Iq current (quiescent current to Gnd) is much higher for the LTC4411 (10s uA) compared with the MAX40203 (<1uA) which might not be important if only sparse use on the battery. Using daisy-chaned (ie both) DC-DC is also slightly less efficient, but only a few percent. This becomes an isse if (say) a 1 year shelf-life is required before use.

Both DC-DC need to be enabled by registers, and the VDD output voltage from REG0, via registers DCDEN0, REGOUT0  and DCDEN. The nRFConnect and SDK IDEs are different however, but there are examples.

[Serafino Convertini]

So your proposal is:

1. using 2x MAX40203 in place of 1x LTC4411 for the less quiescent current;
2. Connect the VBUS (via MAX40203) to VDDH;
3. Connect VBAT (via MAX40203) to VDD;
4. Enabling only the DCDCEN0;

Are you agree?

[hmolesworth]

Actually provided the coin cell is not used for long periods (the Iq issue) your latest circuit should work well as you can now set VDD to a lower voltage, typically 1.8V, which reduces current drain. However that requires all external peripheral devices (if any) on VDD or a nRF52 port pin to be specified at 1.8V supply, which is generally true nowadays. 1.8V halves the current from a 3.4V coin, and is still pretty good at 2.7V coin; I generally use 1.8V. Note if you have LEDs those require a buck-boost from the battery or a simple voltage doubler from the battery; when on battery power both of those will be required in any case.

What value is the doubled capacitor? I usually recommend low-leakage 150uF 10V on the coin cell, much higher than most reference designs.

[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