High current drawn by nRF51422 chip during brown out reset

[Rana]

I am using a solar powered energy harvested circuit to power the nrf51 module. However, when the generated voltage drops below 1.8v, the nrf51422 enters into brown out reset and is drawing close to 500uA of average current. Is this expected? The same amount of current is drawn by the device when the reset pin is pulled low (active low reset). Is there a way to reduce the drawn current when the reset is active?

[Stefan Birnir Sverrisson]

Hi Rana

The current consumption in Brownout reset or when reset button is pressed is not specified, but experience shows that it is quite high, few hundred microamps. The strategy is to avoid to enter Brownout reset by shutting down the chip gracefully before the voltage drops below 1.8V, with either using the Power-Fail Comparator or the internal ADC, see the nRF51 Reference Manual. The brownout reset should not trigger however until you go below 1.7V, which typically occurs during a radio event when the current consumption is the highest.

[Rana]

Unfortunately, in my current design there is no way to avoid brownout since the solar harvester drops to 0v at night. When sunlight becomes available and the system is coming up from a cold-start, the nrf51 draws very little power for harvested voltage between 0v to 0.8v. The issue is when the voltage raises above 0.8v, nrf51 draws 500uA. This delays the time it take for the harvester to charge my super capacitor. FInally, when the generated voltage is above 1.8v, nrf51 is active and then my software puts it to low power thereby drawing very little current and everything is happy. I am trying to remove the additional cold-start time due to the unusually large current being drawn during brownout (0.8v to 1.8v). Would you recommend using a low voltage load switch (FDZ8040L or TPS22921) in the energy harvester design to cut-off the nrf51 when the harvested output voltage is below 1.8v?

[Stefan Birnir Sverrisson]

Yes, that is exactly what I would recommend, to use a switch that feeds the nRF51 with power only if the supply voltage is sufficient.