NRF52840 best power supply option for specific case

[Lorenzo_B]

Hello everyone, I am developing a wearable project using nrf52840 board and ANT communication protocol (max 60 Hz transmission from nrf to PC).  I am interfacing the micro with two sensors (1.8 to 3.3 VDD) and an SD card (3.3 VDD). My goal is to power the entire system with a rechargeable 3.7 V LiPo battery.

By consulting the online power profiler i noticed that when the radio si used at short interval it is better to power the board with higher voltages, it is correct?

I am now strugglng to find the most power efficient way to power the system, i got three options:

1) LiPO --> 3.3 converter --> all peripheral at 3.3 V

2) LiPO --> VDDH micro --> VDD micro to power the sensors (by doing so i can select REGOUT0 to 1.8V so reduce sensors power consumption)

    LiPO --> 3.3 converter --> Sd card VDD

3) LiPO --> 3.3 converter --> sd card --> 1.8 converter --> micro and sensors

Another topic is related to depowering the sd card during inactive state using a mosfet to save power, what is the best configuration in this case high side configuration (using a p-mos) or a low side (n-mos)?

I hope I made the situation clear, thank you in advance.

Regards Lorenzo

[Jared]

Hi,

2) LiPO --> VDDH micro --> VDD micro to power the sensors (by doing so i can select REGOUT0 to 1.8V so reduce sensors power consumption)

    LiPO --> 3.3 converter --> Sd card VDD

3) LiPO --> 3.3 converter --> sd card --> 1.8 converter --> micro and sensors

What's the difference between these two? Are you using the internal regulator on the nRF52840 for the sensors in option 3 as well?

In general I would recommend the option that make it possible to drive the sensor at the lowest possible voltage, assuming that they pull less current at that domain. 

Another topic is related to depowering the sd card during inactive state using a mosfet to save power, what is the best configuration in this case high side configuration (using a p-mos) or a low side (n-mos)?

High-side, that way the SD card will always be connected to gnd. 

regards

Jared

[Lorenzo_B]

In option 3 I am using two external converters in series: 3.3 converter power the sd card while the 1.8 converter power the sensors and the micro: this option makes sense if only the power cosumption of the nrf chip is lower while been powered from the VDD pin but i think is not my case since I am using the radio at 60 Hz TX interval.

In option 2 i am using only an external 3.3 regulator for the sd card, while i am using the VDDH pin to power the micro directly from the LIPO and the internalr egulator to power the sensors. An external 3.3 converter is neede for the sd card since it drains a current of about 100 mA during write.

[Jared]

Hi,

I would go with the second option since it has the least necessary amount of components,

regards

Jared 

[Lorenzo_B]

Yes I think the second one si the best cost saving option for a future PCB too; doing so two problems come out:

1) if the sd card is powered at 3.3 V by the external converter and the nrf is at 1.8 VDD (REG0 out) is there the need of logical level shifters for spi communication between the two?

2) When the sd card is in idle state (high side mosfet off) the regulator is always active causing an avoidable power loss right?

One solution could be putting the mosfet before the converter input so it is activated only when the micro wants to write, i think of something like this:

Is it correct in your opinion? is the converter off when the mosfet is not conducing?

Thank you!

[Jared]

Lorenzo_B said:

1) if the sd card is powered at 3.3 V by the external converter and the nrf is at 1.8 VDD (REG0 out) is there the need of logical level shifters for spi communication between the two?

YES!

Lorenzo_B said:

2) When the sd card is in idle state (high side mosfet off) the regulator is always active causing an avoidable power loss right?

The converter doesn't have an enable pin to shut it off?

Also, note that we have a PMICs that has this feature with buck regulators that can be used to supply different voltage domains,

regards

Jared

[Lorenzo_B]

Jared said:

The converter doesn't have an enable pin to shut it off?

Good idea, if i swith the converter On only when i want to write i can get rid off the mosfet and simply activate it when i need. 

Thank you Jared! I have no more question for now ... We can close the topic.

[Jared]

Perfect,

I'll mark this ticket as resolved then,

Have a good day,

regards

Jared