Quote:
Originally Posted by ilovejedd
Highest consumer chargers you can find with USB-A is 5V/3A for Qualcomm QuickCharge and that one also downgrades to 500mA (iirc) if the requesting device doesn't provide the correct signaling.
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There is NO signalling by normal clients on USB-A. The host doesn't downgrade as such. The client decides how much current can be drawn based on the voltage/resistances on the D- and D+ pins, used for the data transfer. That's why to ensure you get 2A on an Apple thing, you need an Apple host port or charger, or one that's 100% compatible.
A gadget using the "Qualcomm QuickCharge" spec likely only takes 3A from a 100% compliant Qualcomm QuickCharge host. It doesn't seem to be in any actual proper USB standard, so I've no idea if the Qualcomm QuickCharge host will deliver 3A to a dumb load. Certainly ORDINARY USB -A hosts and chargers are supposed to simply supply UP TO their rated spec to a dumb load and shut down till disconnected if excess current is taken as that would be a fault condition. Actual gadgets are supposed to be cautious and only take 0.1A or 0.5A (often only 0.1) if the SW/HW in the gadget can't figure the D- and D+ line conditions. That sensing is done before negotiation of device class, if any.
There are loads of "dumb" things you can plug into USB ports. Most don't check anything and assume 500mA is OK. A 30W audio amp that claims to run on 5V @ 500mA is an amusing piece of marketing nonsense. Class AB would give maybe 75% of 2.5W, both channels, = 3.75W/2 = 1.875W per channel. Class D (basically a SMPSU modulated by audio) might manage 95%, or nearly 2.4W, 1.2W per channel.
Invent a fictitious PMPO and x2 because it's stereo? Or leave out the decimal point?