Author Topic: Using the MCP16251 with CR2032 battery. A good choice?  (Read 779 times)

doublec4

  • Newbie
  • *
  • Posts: 47
Using the MCP16251 with CR2032 battery. A good choice?
« on: May 11, 2018, 11:10:40 PM »
Hi all,

So I recently made a two button remote with a regular pcb trace Moteino. I used the MCP16251 boost regulator with a CR2032 battery to power it (directly to the 3.3V pin on the Moteino). In the programming I used sleep mode for the MCU and radio sleep for the radio. The unit wakes up on a button press and only stays awake for a few seconds to receive the ack and a possible error code from the receiver.

The first time I tested the remote, it seems to work fine. I got a little carried away and was pressing buttons over and over for a little while. Then I left the unit to sleep, came back a few hours later and the remote was dead. When I removed the battery the voltage eventually came back up to 3V on the coin cell but when inserted back into the remote I believe it can no longer support enough current for the radio to work. Pressing a button will not send a packet and the unit seems unresponsive. When I calculated the battery life before designing and building the remote I estimated that the battery would last ~1 year with ~2 button presses per day. However, by aggressively using the remote for a short period of time, does this somehow drain the battery faster than using it with longer intervals of rest in between?

Also, the MCP16251 chip has an enable/shutdown pin... one version of the chip has input to output disconnect when this pin is grounded, and the version that I have has input to output bypass when the enable pin is grounded. I'm assuming this means that voltage is no longer regulated and the datasheet says current consumption drops to ~0.6uA in this state. Right now I am not using this feature, but would it be as simple as connecting the enable pin to a digital I/O on the Moteino and pulling the pin low before sleeping the Moteino? Then the interrupt could wake the Moteino, I could pull the enable pin high again and the regulator would kick back on to boost for a reliable 3.3V output? Would this be better than leaving the boost regulator on the entire time? Or is it more complicated than this? Thanks!

Schematic:



Pictures:

upload a picture

upload picture



Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 5303
  • Country: us
    • LowPowerLab
Re: Using the MCP16251 with CR2032 battery. A good choice?
« Reply #1 on: August 03, 2018, 01:38:01 PM »
Nice little project!
IMO probably not a great match between COIN-cell and step-up booster because the up-boost will be inneficient especially at high currents required by the transmitter.
You could probably do much better with a tiny LiPo which will eliminate the need for boosting while being able to support the TX even at max power.

BTW at no load the coin cell will tend to show a good-looking voltage, you really need to put a small load on it to get the real voltage.
You could perhaps do quite a bit better by eliminating the booster and using a Moteino-8Mhz which runs down below 3V directly from a coin cell. Don't forget to always dial TX power down as much as possible, perhaps use RFM69_ATC with a low enough RSSI parameter.

doublec4

  • Newbie
  • *
  • Posts: 47
Re: Using the MCP16251 with CR2032 battery. A good choice?
« Reply #2 on: September 10, 2018, 12:40:43 AM »
Thanks for the reply Felix,

I put this project away for a while but I'd like to revisit it. Do you have the eagle design files available for the Moteino with PCB trace antenna, or the 8Mhz moteino? I checked your Github page but don't see those variants.

Rather than combine two PCBs like I have shown in the pictures, I would like to integrate everything into one PCB for a slimmer design. (8Mhz, PCB antenna, coin cell mount and buttons)

Is there any danger of damage or unintended/unpredictable behavior with running the battery down too low without any regulator?

Thanks!


Felix

  • Administrator
  • Hero Member
  • *****
  • Posts: 5303
  • Country: us
    • LowPowerLab
Re: Using the MCP16251 with CR2032 battery. A good choice?
« Reply #3 on: September 10, 2018, 10:07:01 AM »
Only what is published is available.
The antenna was designed for the form factor of the Moteino, changing it in any way might render the antenna ineffective or useless.

Damage? No. Weird behavior? Yes, brownouts when VCC reaches the limits stated in the datasheet of the 328p.