Current limiting for battery protection
Having figured out how to add under voltage shutdown, and reverse voltage protection for lithium battery powered modules, I obviously also need to look at over-current protection: I don't want a motor stall to set a battery on fire.
Current limiting with discrete components isn't that hard, but it involves lots of components and lots of testing. The result is probably just too big. I wanted to see if there was a reasonably priced integrated solution.
Anyway, it's worth observing that accuracy of a simple transistor pair is ±30%, just to set expectations on accuracy.
These turn out to be called smart switches, or current limit switches. It's not easy to do parametric search on these: they have different features, as well as basics like current, voltage handling and Rds(on).
Some searches: Farnell
Some of these have current feedback. I.e. they have a pin which is a current source that's some fraction of the output current. They all have thermal shutdown to protect the device itself from destruction.
The devices with feedback that seem relevant are AUIPS71451G, BTS5030-1EJA, BTS5045-1EJA, BTS5090-1EJA.
One group if devices has a programmable current limit 0.4-2.0A: FPF270[012], which doesn't simply shut down immediately, but goes into current source mode. There are three versions: one of which latches off if the over current state persists for long enough. Another which shuts down, and then resets repeatedly, and the third where what to do is left to an external monitor.
This device seems the most interesting, since all I have to do to ensure the battery is protected is calculate the resister value correctly. The problem with it is that the control signal needs to be limited to 5.5V, and I think that's tricky to use in my previous latching switch designs, which just use a pull up to Vin.
I'm also wondering if an LDO regulator might do the job - you can use it as a current source, so presumably it can work as a current limiter too?
I could test this idea with a regulator I have to hand. It's not exactly LDO: at 1A it has a 1.5V drop. Can I find something better? Yes: the AZ1084C apparently has a 1.1V drop at 2A. It's cheap, too. That's still going to waste 15% or so of a pair of lithium batteries, which would be disappointing.
I had been thinking about current limiting as sensing + feedback, but of course just feedback is better: because there's only one active device (an op amp) it's more likely to be stable. Also, it seems like it would be way more accurate: Op amps with ±3.5mV offset voltage are common and give 3.5% accuracy with a 100mV drop. RR IO makes the design simple, and a MCU/DAC could be used to set the limit and also do soft start.
This last idea has made me think about using a micro-controller in the switch: There are 8 pin micro controllers. If it had the right peripherals, maybe that would simplify things.
I still have the thermal shutdown requirement though. Perhaps I'll come back to this category of devices.
Current limiting with discrete components isn't that hard, but it involves lots of components and lots of testing. The result is probably just too big. I wanted to see if there was a reasonably priced integrated solution.
Anyway, it's worth observing that accuracy of a simple transistor pair is ±30%, just to set expectations on accuracy.
These turn out to be called smart switches, or current limit switches. It's not easy to do parametric search on these: they have different features, as well as basics like current, voltage handling and Rds(on).
Some searches: Farnell
Some of these have current feedback. I.e. they have a pin which is a current source that's some fraction of the output current. They all have thermal shutdown to protect the device itself from destruction.
The devices with feedback that seem relevant are AUIPS71451G, BTS5030-1EJA, BTS5045-1EJA, BTS5090-1EJA.
One group if devices has a programmable current limit 0.4-2.0A: FPF270[012], which doesn't simply shut down immediately, but goes into current source mode. There are three versions: one of which latches off if the over current state persists for long enough. Another which shuts down, and then resets repeatedly, and the third where what to do is left to an external monitor.
This device seems the most interesting, since all I have to do to ensure the battery is protected is calculate the resister value correctly. The problem with it is that the control signal needs to be limited to 5.5V, and I think that's tricky to use in my previous latching switch designs, which just use a pull up to Vin.
I'm also wondering if an LDO regulator might do the job - you can use it as a current source, so presumably it can work as a current limiter too?
I could test this idea with a regulator I have to hand. It's not exactly LDO: at 1A it has a 1.5V drop. Can I find something better? Yes: the AZ1084C apparently has a 1.1V drop at 2A. It's cheap, too. That's still going to waste 15% or so of a pair of lithium batteries, which would be disappointing.
I had been thinking about current limiting as sensing + feedback, but of course just feedback is better: because there's only one active device (an op amp) it's more likely to be stable. Also, it seems like it would be way more accurate: Op amps with ±3.5mV offset voltage are common and give 3.5% accuracy with a 100mV drop. RR IO makes the design simple, and a MCU/DAC could be used to set the limit and also do soft start.
This last idea has made me think about using a micro-controller in the switch: There are 8 pin micro controllers. If it had the right peripherals, maybe that would simplify things.
I still have the thermal shutdown requirement though. Perhaps I'll come back to this category of devices.
