The device being charged controls the charging current. Typically for smaller gadgets, a single resistor connected to the charge controller IC sets the current. Bigger things like laptops may be more sophisticated.

▲

0_____0 3 days ago | parent [-]

Re: more sophisticated

USB-PD now has a standard for having sink devices request a particular current from the charger, meaning that you could actually remove a converter from the sink device side, because with a programmable source, the current limiting happens on the power supply side. Cool stuff especially for small electronics like wearables.

Generally though if you want to charge with USB-PD and accept all kinds of chargers, the sink device will have to have its own charge control PMIC.

All this negotiation happens over a side band via the "CC" pins on the USB-C connector.

	▲	lxgr 2 days ago \| parent \| next [-]
		That's an optional USB-PD feature, though (called PPS). Devices that support it can potentially charge more efficiently (and by extension more quickly, if they're limited by heat dissipation), but they can't rely on any USB-PD adapter supporting it. It's also different from AVS (which Apple's adapter seems to support), which allows controlling the voltage in finer steps, but not limiting the current in the same way that PPS does.
	▲	3 days ago \| parent \| prev [-]
		[deleted]