Often, car electronics including the VBUS regulator reside in a box in the dash while some of the USB sockets may be placed in the back seat. The cabling between the VBUS regulator and the socket can be several meters in length and twice that counting the return trip. Long cables mean high resistance, so the 5V VBUS supply drops out at high USB device current.
Power USB VBUS in cars with the Linear Technology LT8697
USB sockets appear in an increasing number of car models as portable devices become more ubiquitous. The USB interface delivers power to portable devices via the 5V VBUS and ground lines. Depending on the USB version and the portable device, the VBUS sources up to 2.1A. In a car, this high VBUS current presents a challenge.