How much current may a USB device draw before and after enumeration, and from a charger?

For USB 2.0 the rules are tied to the device's state. After attach and until the host has configured it, a device may draw at most 100 mA (one unit load). The configuration descriptor contains a bMaxPower field in units of 2 mA, so a value of 250 declares 500 mA, the maximum for USB 2.0. Once the host selects that configuration with SET_CONFIGURATION, the device may draw up to the declared amount. While the bus is suspended it must drop to a very low current: 2.5 mA under the current rules, 500 µA in the original specification.

USB 3.x ports raise the numbers to 150 mA before configuration and 900 mA after, and the field is then counted in 8 mA units.

For a 450 mA design that means two operating modes. Before configuration, keep the total under 100 mA: hold the charger IC at its low-current setting and leave the display backlight off. When the USB stack reports the configured state, raise the charger's input current limit. Many charger ICs have a pin or register for exactly this, with 100 mA and 500 mA settings intended for USB.

Declare the truth in bMaxPower. A bus-powered hub can give each downstream port only 100 mA, and a host is entitled to refuse a configuration that asks for more than the port can supply. Most PC ports do not actively limit at 100 mA, which is why the board appears to work regardless, but their overcurrent protection will cut the port at some point above 500 mA.

Inrush is part of the rules too. At the moment of plugging in, the device may present no more than 10 µF directly across VBUS (the specification's range is 1 to 10 µF). A larger reservoir, such as the 100 µF people like to put on a supply input, makes the host's rail dip and can reset other devices on the same hub. If you need bulk capacitance, put it behind a load switch or a current-limited soft-start circuit.

Plan for voltage drop as well. The port may be below 5 V to begin with, and the cable and connectors drop more under load: 450 mA through 0.5 Ω of total cable and contact resistance loses 0.23 V. Make sure the design still works with VBUS around 4.4 V.

Wall chargers do not enumerate, so a different mechanism tells a device that more current is available. Under the Battery Charging 1.2 specification, a dedicated charging port shorts D+ and D- together (through no more than 200 Ω). The device detects this by driving about 0.6 V onto D+ and checking whether that voltage appears on D-. If it does, the port is a charging port and up to 1.5 A may be drawn; if not, it is a standard data port and the enumeration rules apply. A charging downstream port on a PC or hub, which offers both data and 1.5 A, is told apart from a dedicated charger by a second detection step.

You do not need to build this from discrete parts: many charger ICs and some microcontroller USB peripherals include BC1.2 detection.

With a USB-C connector there is a third, simpler source of information. The source advertises default, 1.5 A or 3 A at 5 V through the pull-up on the CC line, and the device reads the CC voltage across its 5.1 kΩ pull-down. On a Type-C source advertising 1.5 A or more, a 450 mA load may be drawn without waiting for enumeration. If the advertisement is only the default level, the classic rules above still apply.

A robust design combines the methods: start at 100 mA, step up when the device is configured, when a charging port is detected or when CC indicates more, and as a last safeguard reduce the charging current if VBUS sags. Many charger ICs do that automatically with an input-voltage regulation loop.