Two parallel VTT Drivers for QDR and GN4124 or only one
The VTT drivers (TPS51200DRCT, ICs 31 and 23) and help maintain DC voltage at the end of the 50Ohm addapted lines. At the beginning of the PFC design there was foreseen separate VTT drivers for the QDR II and the GN4124. As this complicated the VTT distribution, both VTT planes where joined. If the number of 1 and 0 transmitted over the QDR and GN4124 the current through the VTT Driver should be close to 0A. In case all the data lines are stuck to 0 the VTT will have to provide 0.9V/50Ohm * 100 lines~ 2A. The voltage drop over the TPS51200DRCT will be in that case 3.3V-0.9V=2.4V, which will result in a power of ~5W dissipated on the TPS51200DRCT, which is only able to dissipate 1.9W at 25C. In principle this is an unlikely state and in case of happening it should only happen during a transitory period.
The default configuration is:
IC23 ON, IC 31 OFF, R179 OFF, R186 OFF, C92 OFF
Back to Configuration-Options