The layout example in this section shows
the recommended placement for de-coupling capacitors and ESD protection diodes. A
continuous ground plane is recommended below the D+/D- signal traces. Small
footprint capacitors (0402/0201) are recommended so that these can be placed very
close to the supply pins and corresponding ground pins and connected using the top
layer. There must not be any vias in the routing path between the decoupling
capacitors and the corresponding supply and ground pins. The capacitors on V1P8Vx supplies are
higher in priority when considering placement close to the IC. The ESD
protection diodes must be placed close to the connector with a strong connection to
the ground plane. Pins 4 and 11 for
V1P8V1 and pins 18 and 25 for V1P8V2 are connected
together, but this connection is after the de-coupling capacitors. If more than
2 layers are available in the PCB, this connection must be made in an inner or
bottom layer (ex. Layer 3 or 4) so as to not interrupt the ground plane under
the D+/D- traces. The example shown is for an isolated host or hub, but
similar considerations apply for isolated peripherals also. The 120-μF capacitor on
VBUS only applies to host or hub and must not be used for peripherals. A ferrite
bead, with DC resistance less than 100 mΩ, can be optionally placed on the VBUS
route, after the 100-nF (and 120-μF) capacitors to prevent transients such as ESD
from affecting the rest of the circuits.
For best performance, minimizing the
length of D+/D- board traces from the MCU to ISOUSB211, and from ISOUSB211 to the connector is recommended. Vias and stubs on D+/D- lines
must be avoided. Avoiding Vias and stubs on D+/D-
lines is especially important for High Speed Operation.
Connect a small plane (for example, 2 mm x 2 mm) to the GND pins on the top
layer to improve thermal performance. Connect this to the ground player in the
second layer with multiple vias.