Filtering USB input - Page 1
Forum section: electronics / projects

author: DenLi
author_rank: Newbie
timestamp: April 27, 2017, 10:55:44 am
content: Greetings to all EEVBlog Community Members,I have a question regarding using USB Power Adapter as a Power supply. I am using following module as a part of my HW design:https://www.fs-net.de/de/produkte/efus/efusa7ul-mit-nxp-imx6-ultralite/.It really lacks information in datasheets. The only thing it has on power supply requirements, is that it requires 5V+/-5% (https://www.fs-net.de/assets/download/docu/efus/efusA7UL_Hardware_eng.pdf,page 25). I guess this module has some kind of power regulator on it, but I have zero info on it.I've contacted tech. support and they told me that the module doesn't have any particular start-up requirements (such as "supply voltage rise time". I am really conscious of UVLO, had pleasure debugging it once), apart from the start should be "more or less" continuous. Well it's all nice, but I would like to be sure that everything works fine, so my question is:I know that my customer is going to use a USB power supply adapter, but I have no idea which one. It can be even some cheap Chinese one, the only info I have is that it will be capable of 2A. How would you suggest to filter USB input to be on the safe side? I've attached a pdf with a snippet from my power supply schematics(don't mind my notes). Would this filter be enough? How do you usually filter USB input? It should be more like a rule of thumb, since I have basically no info on the efus module and on the USB power supply, which is kind of annoying. So please don't start with "you have to give us more info", I don't have it myselfAny suggestions or links to info on the subject are very welcome.Kind Regards,Den

author: cowana
author_rank: Frequent Contributor
timestamp: April 27, 2017, 12:20:29 pm
content: Have a think about the things you want to protect against. These might include:Undervoltage. You probably want an UVLO to prevent it trying to power up with too-low supply.Overvoltage. Are you worried about a faulty adapter shoving too many volts in? Could be a crowbar short or disconnect.HF noise. Ferrite beads are very handy for soaking up very high frequency noise.Mid-frequency noise. A large inductor (as you have) will do a good job of smoothing out the supply.Low frequency noise. You'll probably want a large bulk capacitor to provide pulse current. However, watch out for the maximum allowed capacitance on V_USB if you want to be USB compliant.Overcurrent. Handy in case anything goes wrong - maybe a PTC?

author: DenLi
author_rank: Newbie
timestamp: April 27, 2017, 01:51:49 pm
content: Hi cowana,thank you for your post.- I won't have any UVLO since, as stated by Efus, it doesn't have any critical power up timing requirements. it requires "more or less" continues startup.- Overvoltage and Overcurrnet protection, that is actually a very good idea in case those adapters go haywire. Thanks for the tip!- Noise: What I have there is a FB: BLM18EG221SN1D. So I put it there against HF. I thought of adding one more low L-filter with an inductor and cap for mid range. However I am not sure about values, since I don't really know which noise frequencies I will have there. Anything you use as a rule of thumb? Suggestions for specific parts (2 A) are welcome too.- USB compliance, as far as I remember the overall capacitance shouldn't be more then 10uF, am I right?Thanks again,Den

author: SeanB
author_rank: Super Contributor
timestamp: April 30, 2017, 07:37:56 pm
content: I would add the UVLO though, as many USB power banks simply stop converting and apply the battery voltage ( less a single diode drop) to the output when they do a low battery shut off. Leave your device connected and drawing power will kill the battery pack fast, and might even cause it to do a Note7 when connected to charge again.