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  • Case ID: 313833
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Clarification on using DCDC with 1.8V external supply

robca

I'm designing a new nRF52840 board. In the past, I used a 1.8V LDO and never enabled the DCDC option, so I was using Circuit configuration #3 from the reference circuit.

In playing with the power profiler, it looks as if I can save a few mA when transmitting by using the DCDC converter instead of the internal LDO, so I was thinking of switching to circuit # 4 here https://infocenter.nordicsemi.com/index.jsp?topic=%2Fps_nrf52840%2Fref_circuitry.html&cp=5_0_0_6_2_3&anchor=unique_609532902

I will connect my 1.8V LDO to VDD_HV and the rest of the board peripheral ICs, leaving the VEXT net unconnected and the same layout as in the reference circuit for VDD_nRF. Is this the correct understanding?

Considering that the device will transmit quite a lot of data at 50Hz, is it reasonable to expect some power savings? Also, the device will need to spend a lot of time in "system off" mode, will having the DCDC enabled have a negative impact on the power consumption? Should I disable DCDCEN before entering system off?

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  • +1

    Hi, Roberto!

    If you intend to use the High Voltage mode, which is what the VDDH port is used for, you will need a supply with a voltage of at least 2.5 V. This is due to the following note,

    which can be found here.

    If that isn't the case and you only want to use the DCDC regulator to save power, as you're describing, it might be a better option for you to use configuration 5.

    Regarding your question about disabling the DCDC, this isn't necessary because when DCDC is enabled the chip automatically switches between the DCDC and LDO modes as the current draw changes.

    To help you find the best solution for your power supply it would be nice to know a bit about what the rest of your system looks like. If the rest of the components that you're using need 1.8 V, I guess it may be convenient with the external LDO. If not, it might be a solution to discard the external LDO and connect your battery to the HV regulator (REG0) instead.

    Best regards,
    Mathias

  • 0 in reply to mathiaso

    I'm glad I asked, since I had missed the "minimum 2.5V" requirement. Thanks so much for the prompt reply, and for catching my mistake.

    Configuration 5 seems to be the one for us, then. I didn't realize that it was an option until you pointed it out.

    We need an LDO, because we have an analog front end with 3 quad opamps, plus an IMU and QSPI flash, all working at 1.8V and drawing more than what the nRF52840 couple provide as Vext. Also, we use a standard lipo pouch battery, and the voltage would be above the max VDDH for the nRF52840, requiring some sort of LDO/PMIC anyway.

    As a feedback for your documentation people, I was confused because in the reference circuits, there are mentions of DCDCEN0 and DCDCEN1, but the nRF52840 Power section refers to registers DCDCEN and DCDCEN0. I was searching for "DCDCEN1 and nRF52840", and could not find much. The reference circuits should refer to DCDCEN instead of DCDCEN1 (at least, I think that's what it means)

  • 0 in reply to robca

    I'm happy to hear that you seem to have found something that's working for you.

    Thank you for your feedback regarding the confusion in the documentation. I have now reported the issue.

    If you don't have any further questions, I'll close this case.

    Best regards,
    Mathias

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