A few days ago I wrote about my wanting to put together a gang USB charger that’s kind of smart.
Well, I’ve started going down the path further and am in the product-selection merry-go-round. Basically I have needs for several different ICs to do different tasks.
So here’s the idea as it stands right now:
- Microprocessor is something like an Arduino. Not super critical, as long as it has around four ADCs and supports I2C signaling we should be good.
- On the current monitoring we need to have a current sense resistor. It’ll be a very low-value part. Something like .02Ω. Full-scale at 2.1A we’ll get a 42mV voltage drop. We’ll need a current-sense amplifier to get the voltage up something the ADC can handle.
- Since we’ll have 32 of these sense signals, we’ll need a way to mux them into the micro controller. I’m figuring we can have four 8:1 analog switches to get them into the ADCs. These need to be I2C controlled.
- To switch between iOS and Android charging schemes we need a low resistance I2C switch to short out the D+ and D- lines of the USB. We’ll need 32 addressable switches to accomplish this.
- Finally we need a demux to drive the power switch MOSFETs. Again, I2C addressable.
The thing here is that I need to find things that I can get that are stocked so I can buy a few samples — and to build a board later. I2C addressing also comes into play as well because I can’t have overlapping addresses.
So I found some stuff from Analog Devices that do what I want… but the samples have a lead time of months. One of the limitations from this is that they only have two I2C address selection pins… so while I could substitute an 8:8 switch for the 8:1 mux, but I would run out of addresses.
Then I start looking at other suppliers… and see what’s out there.
While this is frustrating, it’s also kind of fun. It’s a puzzle of sorts. Find the right combination of parts that works well together.