LDO vs DCDC (nRF51822)

I am not quite certain what the advantages/disadvantages are of using LDV vs DCDC if you are using an external regulator. In my case i use a LiPo battery outputting ~4.2V so i need to drop the voltage down to something 1.8-3.6V for the nRF51822.

Would it be more power efficient to use the LDO @ 1.8V or DCDC at some higher voltage like 2.0V? Either is an option im just not sure since i hear DCDC is more power efficient than LDO.

  • The DCDC is safe to use with the third revision nRF51 hardware and softdevices S110 8.0.0, S120 2.0.0, S130 1.0.0, S210 5.0.0 or S310 3.0.0. See the nRF51 compatibility matrix to identify third revision hardware. For information for other softdevices, look at the release notes for the softdevice in question. For the second revision of the nRF51 hardware, we do not recommend customers to use the DC/DC together with the BLE softdevices, see “limitations” section in the S110 softdevice release notes. Also, the DCDC on the third revision hardware should not be enabled with softdevice S110 v7.1 or older.

    If you do not have a softdevice enabled, use the DCDCEN register to enable/disable the DCDC. If you have a softdevice enabled, use the sd_power_dcdc_mode_set function.

    You must manually disable the DCDC when supply voltage goes below 2.1V, and enable it when the supply voltage goes up again.

    In the nRF51 Reference Manual 3.0 it says: Enabling the DC/DC converter will not turn it on, but set it in a state where it automatically gets turned on when the radio is enabled and goes off again when the radio gets disabled. This is done to avoid wasting power running the DC/DC in between the radio events where current consumption is too low. The automatic operation is done by hardware so it will work with or without softdevice enabled.

    How a DC/DC converter actually saves power compared to LDO is described on this thread

    DC/DC vs LDO schematics

    In nRF51822/nRF51422 Product Specification (PS), chapter about Reference Circuitry, there are different schematics for different nRF51 setups. The difference in functionality between LDO and DCDC schematics is as follows:

    DCDC schematics: You can fully control in software if the nRF51 is in DCDC or LDO mode, i.e. by writing to DCDCEN register or by calling the sd_power_dcdc_mode_set function. If you disable the DCDC in software, the nRF51 will be in LDO mode. If you enable DCDC in software, the nRF51 will be in DCDC mode.

    LDO schematics: You can not enable DCDC in software. The nRF51 will stay in LDO mode no matter what is set in software.

    DC/DC recommendation for the second revision nRF51 hardware

    For a non-BLE application you can use the DCDC, but you need to carefully consider the startup time of the DCDC which greatly depends on the current consumption of the device during DCDC startup, which does not include current drawn by the external 16MHz crystal.

    For the DC/DC, we only recommend enabling it during radio activity since it is only more efficient to use the DCDC then the internal LDO when the device is consuming more than ca 5 mA, see figure 8 in the nRF51 Series Reference Manual v2.1, section If your supply voltage is 3.6V then it is sufficient to enable the DCDC when the chip current is above ca 3mA. Current above 3mA is normally just the case during a radio event so the strategy is to enable the DCDC before a radio event and disable it when the radio event is finished. The DCDC consumes 300uA when it is enabled so it should be disabled during low current consumption. If you have e.g. application processing that needs to be performed you could optimally do that just before a radio event and after enabling the DCDC, this way you will shorten the startup time of the DCDC because of higher current consumption of the device.

    According to the data sheet, DC/DC can be used down to 2.1V but the LDO can be used down to 1.8V. When the battery voltage drops to 2.1V you must manually disable the DC/DC. Do so by writing to the DCDCEN register, see nRF51 reference manual.

    Since a lithium battery voltage is up to 4.2V but the supply voltage for the nRF51822 is 1.8V-3.6V, then I guess an efficient option is to have an external LDO drop the voltage down to 3.6V for the nRF51822, and have the DCDC enabled. Lithium batteries normally have voltage range of 3.0V-4.2V, so you should bypass your external LDO as your battery voltage drops below 3.6V. With this setup and Lithium battery, the DCDC will be most efficient as the supply voltage for the chip will most of the time be 3.6V. But again, do not use the DCDC if you are using the BLE S110 softdevice.

    Some customers have also applied an external DCDC to drop voltage down to 1.8V. This option might be worth considering when battery voltage range is above 3.6V.

    It might also be power efficient to use external DCDC for lower voltages, but that depends on the specification of the external DCDC. The nRF51822 Power Efficiency guide is available here.

  • Hi, what do you mean by "You must manually disable the DCDC when supply voltage goes below 2.1V, and enable it when the supply voltage goes up again."? Does this means we have to detect the supplied voltage periodically?

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