AN-202 Datasheet, PDF (1/4 Pages) Exar Corporation – USB UART BOARD DESIGN CONSIDERATIONS FOR USB COMPLIANCE

APPLICATION NOTE
AN202
USB UART BOARD DESIGN CONSIDERATIONS FOR USB COMPLIANCE
1.0 INTRODUCTION
The XR21V1410, XR21V1412 and XR21V1414 USB UARTs have passed compliance for USB 2.0 Full-Speed
and are listed on the USB-IF Integrator’s List. This application note describes some scenarios to consider
when designing a commercial product using the XR21V141x to ensure that it has a higher possibility of
passing USB compliance.
2.0 DESIGN CONSIDERATIONS
The first design option to consider is whether the commercial product will be a self-powered or bus-powered
design.
In a self-powered design, an alternative power source is supplied via a power jack or battery to the USB UART
and other components on the board (See Figure 1). Design considerations for a self-powered design are
discussed in Section 2.1.
In a bus-powered design, the 5V from the VBUS of the USB connector (from the USB host) provides power
source for the USB UART and possibly other components on the board (See Figure 2). Design considerations
for a bus-powered design are discussed in Section 2.2.
2.1 Design Considerations for Self-Powered Applications
This section discusses the various design considerations for a self-powered application to ensure that it will
pass USB compliance. A typical block diagram for a self-powered application is shown in Figure 1 below
(UART channels are not shown).
FIGURE 1. TYPICAL BLOCK DIAGRAM FOR SELF-POWERED APPLICATIONS
External 5V
VBUS
USB
Connector
D+
D-
GND
Voltage
Regulator
EN
3.3V
VCC
USBD+
USBD-
XR21V141x
VCC
10K
I²C
EEPROM
10K
SCL
SDA
LOWPOWER
1