XR21B1420 Datasheet, PDF (9/60 Pages) Exar Corporation – Enhanced 1-Ch Full-Speed USB UART

XR21B1420
USB Power Modes
The XR21B1420 device may be configured in any of the following power modes: bus-powered, self-powered 5V, or self-
powered 3.3V. In all three modes, the VBUS power signal from the USB host must be connected to the VBUS_SENSE pin
of the device.
The default power mode for the XR21B1420 is bus powered. In this mode, the USB device’s maximum power requirement
from the host must be specified. In this context, the USB device includes all components on the PCB that will draw power
from the USB host VBUS power. The default maximum power for the XR21B1420 is 100mA. This may be changed using
the Attributes field in the OTP.
Bus-Powered
In bus-powered mode, VBUS from the USB cable supplies 5V to the XR21B1420 device. The VCC pin will supply a 3.3V
output.
VBUS
Dp
Dm
GND
USB Connector
XR21B1420
VCC_REG
VBUS_SENSE
Dp
Dm
GND
VCC
Figure 1: Bus-Powered Mode
Self-Powered 5V
In self-powered 5V mode, a local source provides 5V to the XR21B1420 device. The USB attributes should be changed in
the OTP to correctly report self-powered mode. The VCC pin will supply a 3.3V output.
5V
Supply
VBUS
Dp
Dm
GND
USB Connector
XR21B1420
VCC_REG
VBUS_SENSE
Dp
Dm
GND
VCC
Figure 2: Self-Powered 5V Mode
© 2014 Exar Corporation
9 / 60
exar.com/XR21B1420
Rev 1A