NRF52810 using CR2032 in freezing temps

Question

Hello Nordic team, 
 We have developed a smart tag using the NRF52810. We are looking at using it in asset monitoring application. 
 It is powered by 2x cr2032 in parallel. 
 My question: 
 1. Have your customers been able to power the nrf52 via cr2032 coin cell in freezing temperatures (-10 deg C)? Is there a better coin cell for this application. 
 2. what would be a good capacitor size to ensure brown outs do not happen, given that BLE adverts take (~10ma for 380 us) x 3, while the CR2032 can only supply a 200ua current. 
 The reason I ask is that I've tested 8 tags and they all fail in the freezer in 2 days. When I inspect the cell voltage, its at 1.5v or 1.6v. When I run the tags outside, they last months. I am assuming there was a brownout event, which pulled the mcu into reset which then discharged the cells. 
 
 Kind regards, 
 Hari

haricane · Accepted Answer

Hi Hugh, 
 I have since resolved the issue with a 47uf cap. 
 thanks for the whitepaper, it will surely be useful for me in the future.

hmolesworth · Answer

Stored charge Q is the ticket; Q=CV so either use lot of (ultra low-leakage = ceramic) capacitance C or use a high voltage V or better still do both. The current surges on the coin cell are predictable, typically during BLE transmission but also (say) for flashing an LED. The CR2032 pair in parallel are giving a useable voltage of typically between 2.4 and 3.4 volts but due to the high internal impedance at low temperatures or near end-of-life cannot source anything like the pulse transmission current required. The nRF52810 can PWM a voltage doubler or tripler to raise the voltage V just prior to a high-current event. Ceramic capacitors allow this high voltage to be maintained for longish periods but the voltage rating on ceramics has to be at least 2 x the maximum voltage to get the full capacitance value. High leakage Tantalum capacitors don't have that voltage rating problem, but can only be used as charge storage devices for short periods due to the extremely high leakage current. What is a high voltage V? Without an external DC-DC it is limited to the max input voltage the nRF52810 DC-DC can handle, 3.6V, but with an external DC-DC regulator much higher voltages can be used, say 5 or 6 volts or more. 
 Naturally using an external DC-DC means a buck-boost regulator can be used instead to boost the voltage without the nuisance of having to PWM a voltage doubler, although now the maximum voltage is that the nRF52810 DC-DC can handle, 3.6 volts, or (safer) a tad less such 3.55V. 150uF at 3.5 volts is much better than the same capacitance at 2.4 volts. The coin cells should be treated as a trickle charger to the true power source, which is the capacitors. 150uF (6.3V rated ceramics passable or better 10V ceramics) is a starting point; better to use a lot more. The nRF52810 DC-DC is a lot more efficient at 3.55V than 2.4V and less likely to be susceptible to a brown-out. 
 Should there be other high-discharge components which need to be managed, typically those not subject to a brown-out, isolate the nRF52810 supply with an ideal schottky diode such that the nRF52810 storage capacitors are not drained by those other components. Ensure the buck-boost DC-DC does not drain backwards or supply other stuff; if it does add an ideal diode in that path as well. A good ideal diode for this purpose is the Maxim MAX40203 max40203.html

NRF52810 using CR2032 in freezing temps

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