Since I couldn't find the information of how the supply current would change with different supply voltage for nrf51 chip, this is the result of a little test that I made to get this info.
Setup: nrf51 DK with variable voltage source connected to P21 and reverse voltage protection diode shorted.
Code:
int main(void){
NRF_CLOCK->EVENTS_HFCLKSTARTED = 0;
NRF_CLOCK->TASKS_HFCLKSTART = 1;
while (NRF_CLOCK->EVENTS_HFCLKSTARTED == 0){
}
while(true) {
}
}
Result:
- 1.770V - 4.65mA
- 2.109V - 4.67mA
- 2.501V - 4.68mA
- 2.957V - 4.71mA
- 3.421V - 4.72mA
Analysis: The current consumed is almost the same since the CPU is not driven directly from the supply voltage, but through a LDO. The power consumed (V x I) by the nrf51 is almost half at 1.77V than at 3.42V, with this difference in power dropped over the LDO. So it would make sense to use an efficient external DC-DC switching regulator which have >90% efficiency. When sleeping it'll be good to have a bypass mode that just switches off this external DC-DC and connects its Vin to Vout (Vin<3.3V). Also the start-up time of the external DC-DC regulator used is an important consideration based on the application.
