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AR# 50053: 7 Series maximum allowed current in terminating resistors
7 Series maximum allowed current in terminating resistors
Hot-plug or hot-insertion could cause high current peaks in terminating resistors. What is the maximum current peak allowed in GT terminating resistors?
The maximum current peak allowed in terminating resistor is 14 mA. When current exceeds 14 mA, a cumulative time is given as reference for two cases:
If the current through the terminating resistor is 15.5 mA, the cumulative time is 6.9 years.
If the current through the terminating resistor is 31.0 mA, the cumulative time is 50 days.
An equivalent model for the insertion should be analyzed for each particular case. Here below a simple case is investigated as example:
Tx is already powered and transmitting. Rx daughter board is plugged in. The worst case is considered: the terminating resistor has been calibrated to the lowest value (40 Ohm) and the highest power supply 1.23V is provided to the transmitter side. There are two series decoupling capacitors and the daughter board GND pins are the first to connect.
Immediately after the insertion, Rx is 0V and current flux is from Tx side to Rx:
With this circuit in mind, the max current peak is 15.4 mA. The decoupling series capacitor drives the pulse timing. Four decoupling caps sizes are reported below to represent high rates (10 nF), normal rates (100 nF), STM1 (1000 nF), Video applications (47000 nF). The pulse duration for each capacitor size is reported.