I’m putting together a tiny temperature sensor with LoRa radio (based on STM32WLE5) and I intend to power it directly from a CR2032 3V lithium battery without Vreg. I’m wondering whether I should add something for reverse polarity protection, i.e., me managing to insert the battery the wrong way around.
I haven’t settled on a battery holder yet, and most provide some degree of protection. Maybe that’s enough. Or perhaps using a P-ch mosfet is worth it. e.g.:
I could also use an N-channel version of the same circuit, I haven’t looked at Rds(ON) options vs. cost, etc…
The max expected current drawn by the stm32 is 30mA during TX (I will only use the low-power RFO) and the low-voltage cut-off is around 1.8V. The DMP1045U’s datasheet promises 75mOhm max @ Vgs=1.8V which results in less than 2mV drop.
Am I missing something? Is there a better option? Do I need the gate resistor given the low voltages (the DMP1045U has Vgss of +/-8V and a zener protection diode).
I just want to add that for commercial products the battery must be protected against shorts. Normally done with adding resistor and/or diode. Kind of annoying since it degrade the battery power
The renata doc doesn’t have anything to protect against short circuit, or perhaps I didn’t see it? The only thing is that their short circuit tests didn’t produce anything harmful.
It’s stated at page 3 and in the table of maximum abnormal current. You need to take appropriate action, ie add circuitry to protect the battery. In mobile phones the is often a small PCB on the lithium battery to handle this before it enters the main PCB
Thanks, interesting! However, my original question was about a purely battery powered temperature sensor. There is no charging connector or mains power connector. There is no “reverse charging” situation.
From the Renata document, page 55, short circuit shall not cause bulging, leaking, or temperatures exceeding 30°C … isn’t that effectively saying you don’t have to deal with it? I wonder if the other cell manufacturers agree.
Yup, and I was wondering what to do about it, if anything… I could put a 100mA (or thereabouts) PTC fuse, trips in 1 second at 250mA and drops ~20mV normally. That all sounds reasonable, dunno whether UL would approve…
Also for CE testing they require some protection when the batteries are placed in the wrong way. A single mosfet is the best way as you do not have the losses of a diode.
+1 to this, no need for a gate resistor. Depending on circumstances I may leave this protection off if the cell holder ensures it will not be connected if cell is the wrong way around, many do this with the design of their contacts.
Do you mean at the FET, i.e. a failure of that part shorting G&D, or do you mean a short somewhere on VDD to GND?
Normally I would assume there is some form of regulator downstream which I would rely on for VDD to GND protection. But I’ve just re-read the OP and seen it will be powered directly from this rail. So actually I was a bit rash there, good point to consider!
From the Renata UL approval document you linked to earlier it also says if you can say that the cell will only be replaced by a “trained technician” then the requirements to protect against shorts are removed. So it also depends on the use case, maybe you could pass with less protection in some cases e.g. battery fitted at the factory and intended to last the life of the product, but maybe not so much in this case. As with a lot of things, one resistor can turn in to a days work and a rabbit hole of standards
I do not see a valid reason for a gate resistor. There should be no gate current. Also is the gate not floating so there is no change the gate voltage goes out of range. You must just take care of selecting a mosfet with a low enough gs voltage in order to turn it on properly (say <2V). From a sourcing point of view I would go for a N-channel fet as they are easier available. Also is the Rdson normally lower.
Yes, shorting of the FET. The FET is on the PCB and UL/IEC short components for single fault. If the battery can handle that, then fine. If not, then the gate resistor could be added (0.0001 USD price .-))
I actually tested Panasonic CR2032 batteries at 100C under no load, pulsing shorts, and complete shorts and couldn’t get a single battery to do anything unusual.
