Advantech SOM-Express Design Guide
Chapter 5 Carrier Board Design Guidelines
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usually allows determining the correct trace width and spacing to achieve this
impedance, after the PCB stack-up configuration is known.
As per usual differential pair routing practices, the two traces of each USB pair
should be matched in length and kept at uniform spacing. Sharp corners should be
avoided. At the SOM-Express module and connector ends of the routes, loop areas
should be minimized. USB data pairs should be routed as far from other signals as
possible.
SOM-Express
USB[0]
USB Connector
USB[0]-
USB[3]
USB[3]-
Recommended
Not Recommended
Route to Minimum
Figure 5-8 USB Layout Guidelines
5.2.3.2 Overcurrent Protection
Overcurrent protection on external USB power lines is required to prevent faults in
external USB devices or cables from causing hardware damage and/or crashing the
system. The USBOC# signal is used to signal overcurrent conditions to the system
hardware and software. Note that overcurrent protection typically allows relatively
high currents to flow for brief periods before the current is limited or interrupted. The
system power supply must be able to provide these high currents while maintaining
output regulation, or else the SOM-Express module or other system components
may malfunction.
In case the simple resetable fuse (like shown on the reference schematic) does not
switch off fast enough, overcurrent caused by an external USB device may impact
the carrier boards internal power supply. In this case we recommend using active
protection circuits available from various vendors. These devices may be used for
per port protection of the USB power lines and allow direct connection to the
USB_X_X_OC# signal.
NOTE:
This circuit does not get implemented on SOM-Express. Please implement it
on the carrier board.
USBVFBO
F1
L38
1A
USBOC#
C72
0.01u
R49
470K
11P300S
USBVCC
R50
560K
C73
100
C74
0.1u
VCC
Figure 5-9 Overcurrent Circuit
