AR# 11794


DCI/XCITE - What power rating (wattage value) is needed for the VRN and VRP resistors?


Is a recommended resistor wattage value available for the DCI resistor?


A 1/10 watt or larger value can be used safely for any impedance from 25 to 100 ohms.

The power is determined by the current going through the resistor and the voltage across it. VRN must be connected to VCCO, and VRP to GND. Hence, each reference resistor (one connecting VRN to VCCO, or one connecting VRP to GND) will have half of VCCO across it.

The calculation is as follows for SSTL3_1: 

RVRN, RVRP = 50 Ohm
VCCO = 3.6V max.
VR = 3.6/2 = 1.8V
LVRN, LVRP = 1.8/50 = 36 mA

The peak power (PVRN, PVRP) = IV = 0.036A * 1.8V = 64.8 mW

The actual power rating may be lower due to Vcco limits for the family in question or VRP/VRN reference resistor values used but the same methodology can be applied.  For example:

In 7 series, DCI is supported only in HP I/Os which only support up to VCCO of 1.89V (1.8 V + 5%). The example calculation in the AR uses a VCCO of 3.6V:

So, the calculation should be:

RVRN,RVRP = 50 ohms
VCCO = 1.89V
VR = 0.945V
LVRN, LVRP = 0.945/50 = 18.9 mA
Peak Power (PVRN,PVRP) = 18.9 mA * 0.945V  = 17.86 mW

For DC power consumption of DCI I/O, please see (Xilinx Answer 11661).

AR# 11794
日期 04/22/2013
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