I’m thinking of powering the Boron LTE via USB, and using the JST as a “backup” power source, powered by non-rechargeable batteries, say 4.5V or so (keeps LiPo batteries out of the system). Assuming I have the charger switched off, is there any issue with this setup? What’s the max/min voltage on the JST?
Your main problem would be when the BQ24195L starts to try and charge the battery if USB is plugged in. You could put an ideal diode or even a Schottky in series with VBAT which would prevent that. But this would require an external board/modification.
As with any LTE deployment, make sure your battery source is able to handle the higher currents. Looks like it’s 200mA worst case but this is sustained.
If you look at the current plot of an LTE transaction you may be surprised that there are spikes in there that wreak all types of havoc:
Keep in mind then recommended max voltage for VBAT is 4.4V according to the data sheet. The absolute is 6.
Also @michaelm, there looks like a way to disable the charging. I have not tested this though.
See pin 9. You can write to the PMU via I2C and disable it that way.
Very helpful. I was thinking along similar lines, and I’ve gone in and poked around the schematics and datasheets for the Boron and TI BQ24195.
So, short of cutting the trace to the CE pin, looks like it will always try to charge the battery (if USB connected) until a command shuts down charging. I might be able to get away with a Schottky as a protection device.
Your scope shot is interesting – what are your measurement points? Am I looking at a cellular transmission cycle? I’ll fire up my digital scope and compare. Thanks for the insight!
That particular measurement point was the rail current I believe. (As the battery current changes depending on voltage) This is a full CATM connection cycle (off, transmit, off).