nRF51 power management for energy harvesting applications

[mnhs]

Hi,

as a university project, I am currently developing a ultra-low-power temperature and barometer sensor powered just by RF and laser-beam energy. Please see this link to read more about the whole system and how it works: hackaday.io/.../6243-rf-and-laser-beam-harvesting-for-future-sensors

I use TI BQ25570 as a harvesting chip. The amount of energy which I can harvest to the storage capacitor is tiny, the average current flowing to the capacitor is about 50 microamps - 470uF cap gets charged to 4.5 volts in about half a minute. The BQ25570 includes a buck converter which generates stable voltage for uC from the storage capacitor. Unfortunately, I can't connect the nRF51 directly to this supply, because it consumes a few mA's during start-up and init and discharges the cap very quickly.

What I need is to read one temperature and barometer sample using I2C and broadcast it using BLE non-advertising packets.

My idea is to design a simple nanopower supply management as following:

• the power managament will be powered directly from storage capcaitor with a maximum standby power consumption of microamp or two.
• at around 1.5 volts, the low-voltage comparator with reference and load switch will start working and start monitoring the storage capacitor voltage
• after the storage capacitor voltage is over let's say 4 volts (will be properly calculated afterwards), turn on the load switch and use the energy which is in the capacitor to init the nRF51 and broadcast a few packets
• if the storage capacitor voltage is under around 2.2 volts, turn off the load switch.

My questions are following:

• how to minimize the start-up duration of the chip and S110 stack? I use rev3 chips and internal RC 32kHz oscillator (I read in one post that the external 32kHz crystal may take up to one second to start) ? I won't be powering the nRF51822 at the lowest possible voltage - 1.8V, but with a bit higher, about 2.2 volts to avoid the delays in initialization.

• when I am thinking about it, maybe I could save some time by not using the stack at all. It is possible to send non advertising BLE packets using nRF24L01 chip:

dmitry.gr/index.php

This implementation would take a lot less time than the initialization of the stack, initialization of BLE broadcast etc, right? I am not targeting for certification, it is only an explorative project to prove the concept.

• I would welcome any other suggestions if I missed something.

Thank you in advance, Marek.

[Marcio Montenegro]

Why not use supercap? en.wikipedia.org/.../Supercapacitor

[mnhs]

The supercap has too much capacity for me. I need just enough energy to charge a cap quickly and use it to read two values from the sensor and send let's say three or four advertising packets. As far as I know, the supercaps also have high leakage currents when compared to conventional capacitors. Even 5 microamps will reduce the performance of my solution.