STF202-22 and STF202-30
PROTECTION PRODUCTS
Applications Information
Device Connection
Figure 1 - STF202 Circuit Diagram
The STF202 is designed to provide termination, EMI
filtering and ESD protection for two USB I/O lines. The
equivalent circuit diagram is shown in Figure 1. The
device is connected as follows:
.
1 . Full-Speed Devices: For full-speed devices the pull-
up resistor is connected to the D+ line. Pin 1 is
connected to the voltage supply line (VBUS). The
input of the D+ line is routed into pin 3 and out of
pin 4. The input of the D- line is connected at pin 2
and the output at pin 5. Pin 6 is connected to
ground.
Figure 2 - STF202 Connection Diagram
(Full Speed Devices)
2 . Low-Speed Devices: For low speed devices the
pull-up resistor is connected to the D- line. Pins 1
is connected to the voltage supply line (VBUS). The
input of the D- line is connected at pin 3 with the
output taken at pin 4. The input of the D+ line is
connected to pin 2 and the output is at pin 5. Pin
6 is connected to ground.
1
2
3
6
5
4
USB Port Design Considerations
The Universal Serial Bus (USB) specification requires
termination and filtering components for proper opera-
tion. In addition, an open USB socket is vulnerable to
hazardous ESD discharges in excess of 15kV. These
discharges can may occur on the data lines or the
voltage bus. The STF202 is an easily implemented
solution designed to meet the termination and EMI
filter requirements of the USB specification revision
1.1. It also provides ESD protection to IEC 61000-4-2,
level 4.
Semtech provides a complete solution to simplify USB
port design (Figure 4). The STF201 and STF202
integrate all of the components necessary for line
termination, bidirectional EMI filtering, and ESD protec-
tion on downstream (STF201) or upstream (STF202)
ports. The SC5826 is a dual port power switch that
provides individual or ganged port switching, fault
reporting, and inrush current limiting as required by the
USB specification. The SC5205 ULDO provides a
stable voltage to the USB controller.
Board Placement and Layout Guidelines.
A simplified USB port is shown in Figure 3. USB line
termination is achieved with series resistors on both
the D+ and D- lines. These resistors preserve signal
integrity by matching the cable impedance to that of
the differential driver. 15kΩ pull-down resistors are
used to identify a downstream port while an upstream
port is identified with a 1.5KΩ pull up resistor on either
the D+ (full speed devices) or the D- (low speed de-
vices) data line. Capacitors are used to bypass high
frequency energy to ground and for edge rate control
of the USB signals. TVS diodes are added for ESD
protection of both (D+ and D-) data lines and the
voltage bus (VBUS). A power distribution switch and
voltage regulator provide the power management
functions of the port.
Designing a USB hub to meet EMI and ESD immunity
requirements requires a combination of optimum
component placement, trace routing, and good circuit
design practices. Some general guidelines are given
below:
z
Avoid running D+ and D- signal line traces near high
speed clock lines or similar signal lines.
Avoid running critical signal lines near board edges.
Locate the USB controller chip near the USB
connectors.
z
z
z
z
Place the STF202 near the USB connector to
restrict transient coupling.
Minimize the path length between the USB connec-
tor and the STF202
www.semtech.com
2004 Semtech Corp.
4
SEMTECH [ SEMTECH CORPORATION ]
