I'm developing a small micro-ohm meter that I will be powering from USB, due to the ubiquity of USB power bricks and computer power.
I've run into a few questions:
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What do I do with the data lines of the USB? I'm not using them, I'm just interested in USB power, should I tie them to ground, to each other, to +5V?
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Do I need to switch from "low power mode" to "high power mode"? This seems to indicate that you need to ask for high power using something called "enumeration", but the same article also talks about a 2.25W max… which I know is no longer true.
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Max capacitance for the USB 2.0 specification according to the above is 10µF. I've got significantly more than that. This leads me to believe that I should be using something like the NCP380, MIC2545, or TPS2112. These are high side switches that can limit inrush current, allowing me to have more capacitance after the switch. Is this still necessary? Or has the capacitance limit increased since the above linked document was written?
Best Answer
For your particular purpose, you don't need to do anything, just connect GND (black wire) and VBUS (red wire) to your design.
(1) leave D+/D- unconnected;
(2) don't worry about any "low" or "high" power modes. These modes are for host to supervise and police overall power that host believes is available to supply. It never worked correctly. If you don't pull any data wires up, the host will not even know if your cable is even plugged in.
(3) Do not worry much about 10uF limit. In worst case you might get "port overcurrent" message on a host, but it is rare. Or you might disrupt other USB devices that might be working in adjacent ports. But there are plenty of DC-DC converters that use input caps of 10uF or less, so it is advisable to use a better DC-DC converter in your design. If you are not using any DC-DC de-coupling from VBUS supply line, it is a very bad idea for a sensitive analog design.
Regarding the BC1.2, forget this completely. All you need to care is not to exceed 500mA DC limit.
Clarification for (2): The OP is not building a USB device. The 100mA is a requirement for USB devices, not hosts. The OP wants to use the port capability of a USB host. Every host port must have an ability to supply 500mA, regardless if there are "low" or "high" powered devices. So 500mA is always fine. If the port will be behind a bus-powered hub, there could be a different story.