I am looking at using an off-the-shelf current sense amplifier IC, but I have a couple of questions about things.
I had initially looked, and tried, a discrete op-amp solution; but as I have changed strategy with my project, there became the need to do bi-directional current sensing, which I understand is pretty tricky to do using op-amps. It requires many components, so it's way easier, convenient and cheaper to use a ready-made IC for this.
But, I'm not confident I've made a good choice of IC, and there are some things I don't fully understand, so I would appreciate some advice.
Firstly, the whole high- or low-side thing. Is this really a factor with bi-directional sensing? I mean, if current is flowing in either direction, technically aren't both topologies in effect? And yet, I see datasheets describing bi-dir ICs as 'high-side'. Why?
Second, most bi-dir. current amplifiers seem to have a focus on battery charge/discharge monitoring - that is, with continuous current flow but at different voltages (i.e. as battery discharges). But I want to use one to current sense for small DC motors, with very off/on operation. I'm slightly concerned I'd be using it outside of it's intended application.
Lastly, I'm wondering if I need to add some additional components to my design if I add a bi-dir current amplifier into the motor circuit. My motor switching is to be done with relays, so I have nothing else in that side of the circuit, but I have a feeling that inductive spikes may be damaging to a current-sense amp (even though it's measuring the relatively small voltage drop across the current-sense resistor) so I may need to add flyback diodes. Do I?
The IC I'm looking at using is the
Silicon Labs TS1101, probably the x25 gain version. But I'm not confident it would be an appropriate choice. However, it is cheap, and in a SOT23-6 package that I have a hope of hand soldering, rather than a more difficult SC70 with 0.65mm lead spacing as most others (particularly from TI) seem to be.
If it helps, here is the scenario in which I'd be using it:
- Steady-state motor current: ~90-100mA
- Peak motor current: ~400-500mA
- Motor operating voltage: 12V nominal (realistically 10-14V)
- 5V supply
- Sense resistor <= 0.5 Ohm
- Output to be fed to ADC of an AVR (Atmega328P)
One other option I've been looking at is the Allegro ACS712. However, I have my reservations about this one: at the steady-state current draw its output would need further amplification, as it only gives 185mV/A. It's also pretty expensive. But, I believe because it's hall-effect, there is no concern with flyback inductive spikes as I mentioned above (right?).