SN65HVD485/ Current consumption

Hello Kuramochi

Power supply current is delivered to the bus load as well as to the transceiver circuitry. For a typical RS-485 bus configuration, the load that an active driver must drive consists of all of the receiving nodes, plus the termination resistors at each end of the bus. The load presented by the receiving nodes depends of the input impedance of the receiver. The TIA/EIA-485-A standard defines a unit load as allowing up to 1 mA. The current in the termination resistors depends of the differential bus voltage. The standard requires active drivers to produce at least 1.5 V of differential signal. For a bus terminated with one standard 120 Ohms resistor at each end (60 Ohms in total), this will be 25 mA differential output current whenever the bus is active. Typically the HVD485E can drive more than 25 mA to a 60 Ohms load, resulting in differential output voltage higher than the minimum required by the standard (see figure 15 on datasheet datasheet.octopart.com/SN65HVD485EDGKR-Texas-Instruments-datasheet-116604.pdf)
If you consider that you have 120 Ohms with 1.5 V of differential signal, you will have 12.5 mA (with 100 Ohms 15 mA).
Supply current increases with signaling rate primarily due to the full H-bridge outputs of the driver. When these outputs change state, there is a moment when both the high-side and low-side output transistors are conducting and this creates a short spike in the supply current. As the frequency of state changes increases, more power is used.

Please, let me know if with this is more clear to you and if you have any other question.

Regards
Francisco