FSUSB30 circuit questions - Page 1
Forum section: electronics / beginners

author: gmajor
author_rank: Newbie
timestamp: June 27, 2016, 10:30:09 pm
content: Good day everyone!I'm in the midst of yet another project. A part of this project requires me to:1. Switch a USB 2.0 input between two devices2. Light an LED depending on which device has been selectedI found the FSUSB30https://www.fairchildsemi.com/datasheets/FS/FSUSB30.pdfwhich looks perfect for this application. I read a few tutorials on using EAGLE and came up with this:Thisseemslike it will work. I saw a few examples of wiring the FSUSB30, but they were supplying 5V to Vcc and Vsw which, according to the data sheet, pushes close to the maximum rating so I included the LD1117 to drop closer to recommended.What I'm really not certain about is my LED lighting. One switch position is straight-forward, but the other doesn't seem right. I feel like I'm missing something obvious, but I'm having trouble putting my finger on it. Any suggestions are greatly appreciated!Cheers!Greg

author: gmajor
author_rank: Newbie
timestamp: June 29, 2016, 09:31:34 pm
content: Hello again! So I should probably ask a more specific question, eh?Based on my circuit, I think that with the switch thrown as shown in the diagram the FSUSB30 Vsw input should see 3.3V and, additionally, the LED should light. My question is whether the 10K pulldown that's on Vsw does anything useful. I added it before I added the LED, but I'm questioning whether or not it's necessary.Cheers!Greg

author: Audioguru
author_rank: Super Contributor
timestamp: June 29, 2016, 10:38:23 pm
content: There are six versions of the LD1117 and each version has its own output voltage. You did not say which one.LEDs have a forward voltage drop depending on the part number and the color. We do not know which one.

author: ovnr
author_rank: Frequent Contributor
timestamp: June 30, 2016, 12:48:31 am
content: I don't see any major issues, but here are some things to keep in mind:You will need to route the D+/D- signals as controlled impedance differential pairs. For short distances it's fine (ish...) on a 2-layer board, but you will want to use a calculator to figure out the specifics. Generally speaking, a thinner board is easier to deal with in this case.Your devices may not notice being switched if they keep getting VUSB (+5V) all the time. I'd strongly suggest also switching their power supply, even if they're self-powered devices. This is easily accomplished by some p-ch mosfets and resistors.The FSUSB30 chip expects 3.3V, so be sure to supply it; you cannot connect it - or any pins on it - straight to 5V.Yes, your LEDs will light as expected. Remember that you're using 3.3V; blue or white LEDs may have difficulties.

author: gmajor
author_rank: Newbie
timestamp: June 30, 2016, 03:14:02 pm
content: Quote from: Audioguru on June 29, 2016, 10:38:23 pmOops! Great catch. The LD1117 is, in fact, an LD1117V33. I haven't selected a specific LED as of yet, but I anticipate the one I select with have a forward voltage drop of around 2V.Cheers!Greg

author: gmajor
author_rank: Newbie
timestamp: June 30, 2016, 03:51:55 pm
content: Quote from: ovnr on June 30, 2016, 12:48:31 amWhen I first read this I said to myself, "I understand the individual words, but whaaaa?!?"I did some reading and Ithinkthat I get the gist of the concern. I'm not sure this project will find it's way to a custom PCB, but if it did then it seems that keeping the traces equal length, close together, and an equal distance apart is generally a good idea due to EMI. Beyond that, I'm afraid solving for this is beyond my current skill set.Quote from: ovnr on June 30, 2016, 12:48:31 amI hadn't considered that (obviously), but what you're recommending makes good sense. I'll have to scratch my head a bit on how to incorporate the mosfets as I've not used them before. Thanks for the pointer!Quote from: ovnr on June 30, 2016, 12:48:31 amWhen I first started searching online, I was surprised to see examples using the FSUSB30 all putting 5V straight on Vcc and Vsw which is in stark contrast to the datasheet. That's why I used the LD1117V33 to bring things to the datasheet's recommendation. I've learned that nothing substitutes the datasheet and some thought when it comes to designs I come across online!Quote from: ovnr on June 30, 2016, 12:48:31 amYeah, I'll have to keep a keen eye on the Vf of whatever LED I use.Thank you for the feedback! I sincerely appreciate the time and experience!Cheers!Greg

author: ovnr
author_rank: Frequent Contributor
timestamp: July 01, 2016, 12:26:34 am
content: Quote from: gmajor on June 30, 2016, 03:51:55 pmIf you do want to actually get USB2 performance, I'm afraid you will need to have a board fabbed for it. You can't really run a 480Mbit differential signal on a breadboard or veroboard. Maybe if you solder the chip to an adapter board, then solder the data pairs straight onto the pins.As for the actual design of the board, it's pretty simple. Use a calculator - I usedthis one(you want the differential microstrip one), and enter your parameters. You're shooting for a 90 ohm differential impedance..And yes, you need to keep the pairs close together as you said, and the same length. This isn't for EMI reasons, but for signal integrity - if one trace is longer than the other, the signals will go out of sync. Yes, this is a thing, and it happens sooner than you'd expect.