S2R72A21 Application Note
Seiko Epson Corporation
27
(Rev.1.00)
7. PCB design guide
7.1
Power supply and reset
The following is the explanation in regard to power supply related topic towards the S2R72A21.
•
Power on / Power down sequence
The power supplied to S2R72A21 is only HVDD (3.3V). There is no Power on / Power down sequence since
there is a built in regulator. 1.8V can be supplied internally.
•
Noise
Noise on the power supply may affect the USB waveform quality and cause USB communication problems. Care
must be taken when designing the power supply to avoid external noise or ripple noise due to irregular series
regulator oscillation or if the switching regulator circuit constants are inappropriate.
•
Reset
Please release the reset by setting the XRESET pin from Low to High after power supply to S2R72A21 is
finished. The OSC circuit would start the oscillation after the reset is released. So there is no necessity of any
sequence such as reset release after the oscillation is stabled.
7.2
DP/DM signal line
7.2.1
Circuit board wiring
The following points should be taken into consideration for the DP/DM signal wiring to ensure impedance
matching and prevent reflection.
•
The differential impedance in the DP/DM signal line must be 90
Ω
.
•
Sufficient attention must be paid to impedance matching if other connectors and cables are inserted between
the S2R72A21 and the USB receptacle.
•
The inner layer right under the signal line must be a non-separated GND plane.
•
Signal lines that could be noise source (e.g. clock, high speed bus line) must be kept away from the DP/DM
line.
•
A pair of DP/DM signal lines should be parallel and of equal length, and should be kept as short as possible.
Branches should be minimized, and curved lines should be subject to curved wire treatment.
7.2.2
Additional component
•
Common mode choke coil
The common mode choke coil prevents the occurrence of common mode noise by inhibiting current flowing in
the same direction in the differential signal line. Use on the DP/DM signal line can be effective in improving
skew and in reducing unnecessary radiated noise. It is not directly involved in improving Eye-pattern opening.
Example components used for USB High-speed are listed below. Note that components should preferably be
installed linearly with respect to the signal line to ensure signal quality.
•
Panasonic
EXC24CE900U
•
Murata
DLW21SN900SQ2
