Starting a new design..
I’d been meaning to build a USB isolator for a long time, especially since people tend to rely on and my laptop/pc for programming and testing of the new boards they’ve built. Regular hub’s don’t offer much protection and the USB chips will shut down if they detect something, so an opto-isolated USB adapter is a good thing.
I found a USB isolator from circuits@home, had BatchPCB make a few of the PCB’s ( Not recommended as they are too expensive and their customer support is non existent, try Silver Circuits instead )
After krs built and I tested the USB Isolator it seemed to work pretty well, so the next step is to adapt it for an NSL project. In this case I went back to the chip itself and decided to start from scratch. We’re using the Analog Devices ADuM4160 USB Isolator.
Step 1 – Find Chip, datasheet and Design
Find the datasheet, and another other points the OEM has, Analog are generally really good at this.
Easy enough , a neat chip and its used in the Circuits@home, we looked at the http://www.analog.com/en/interface/digital-isolators/adum3160/products/product.html but its 4-5V and only does 2.5KV of protection.
Even better a whole article discussing the chip, problems and sample designs, it also has a design for the ADuM5000 Isolated power supply.
So the above shows an isolated hub design. That would make a good project and its different enough from the Circuits@home to make it worthwhile.
Step 2 – Find availability of the chips.
So this is probably one of the most frustrating aspects of design, sourcing the chips. We’ve decided to look for the ADuM4160, the ADuM5000 and the Atmel AT43301 chip… Uh oh its my nemesis, Atmel… Almost every design I’ve ever done that’s relied on Atmel has either met out of stock for months, or discontinued.. Ok so off to findchips.com to do a quick survey of the land.
Ok good, a spread of stock over different suppliers with a decent quantity for a project of this size. Prices as usual vary widely. Newark often come in the lowest, $9.78 vs $12.23 at digikey and oddly $19.41 at element14. Even odder given Farnell/Newark/element14 are basically the same place.
Also I notice there is a eval board, so we’ll note this part number and look up the datasheet for that too, it’ll more than likely give us the schematic to work from, as well as layout tips. The chip datasheet covers local layout aspects for the PCB for the chip itself, but the evaluation board gives you it all usually.
Ok the ADum5000 ( which we may not use in the hub since a hub potentially requires more power )
It is in stock, but a low count/spread if I were doing a commercial production run I’d look into it further. Again Newark has beaten Digikey pricing (Newark also ship everything in separate white envelopes inside the bag with markings on the outside vs digikeys plastic baggies )
On to the Atmel….
What a shocker! no stock… no spread. Ok so time to head over to Atmel, and see what’s up with the chip. Ok –AC version is non ROHS, so it is gone. Its a few years old so its not surprising its been EOL’d
Over to http://www.atmel.com/dyn/quality/quality_prod_obsl_result.asp and lets see if it is on there.
AT43301-AC AT43301 5/20/2008 11/20/2008 No replacement, same for the -au… Well that tales the biscuit, obsoleted, no replacement. checking eBay turns up no results, and its too difficult to deal with the part finders on Google to source a chip. So we’ll have to start looking for a different chip. So far my Atmel experience is staying true.
Step 3 – Find a replacement chip or design.
I’ve had mostly good luck with TI , I like the Stellaris series they bought, though the chip i used wasn’t in mass production last time i checked. Poking around we get the TUSB2046B its available in LQFP32 too
Data Sheet –: http://focus.ti.com/lit/ds/symlink/tusb2046b.pdf
3.3V, which isn’t uncommon. Hugely available pretty much everywhere, ( 56,779 in stock for one sku ) slightly more complex to implement.