usb pow

  
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The USB standard restricts the current consumption of all USB devices to either 100mA or 500mA depending on the specifics of the USB hub and/or host controller (i. e. computer) that is supplying the power. Ideally, no USB device should ever pull more than 500mA of current from a USB port. In reality this is not always the case. There are two main
usb pow - schematic

issues that can cause a USB device to attempt to exceed the 100mA or 500mA limit. The first is high "inrush" current during the startup of some USB devices. For example, a USB hard drive trying to spin up can temporarily attempt to pull much more than 500mA from its USB port. The second issue is the connection of high-power USB devices through long USB cables or active USB "repeaters" (really really long USB cables. ) In both cases, the large voltage drop that can occur over the length of such USB cables will cause some USB devices to be under-powered and fail to work reliably. or at all. Lastly, some USB host controllers are notorious for being "weak" and unable to provide the full power required by the USB standard. Weak host controllers are found in some laptops and peripherals with integrated USB hubs (such as computer monitors, printers, etc. ) USB POW is also ideal for allowing high-powered USB devices to work reliably with under-powered host controllers and hubs. Below are the schematics for USB POW. The latest schematic is revision 1. 3. Older schematics are also shown, but only for historical information. They are not guaranteed to work correctly. The printed circuit board for USB POW is a standard 2-layer board. Board revision 1. 3 allows USB POW to be used with DC power supplies whose polarity is tip = positive, ring = negative. Board revision relies on the less common polarity tip = negative, ring = positive for...



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