Power consumption is a concern in many applications. 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 on the input impedance of the receiver. The TIA/EIA-485A standard defines a unit load as allowing up to 1 mA. With up to 32 unit loads allowed on the bus, the total current supplied to all receivers can be as high as 32 mA. The HVD308xE is rated as a 1/8 unit load device. As shown in Figure 6-1, the bus input current is less than 0.125 mA, allowing up to 256 nodes on a single bus.
The current in the termination resistors depends on 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-Ω resistor at each end, this sums to 25 mA differential output current whenever the bus is active. Typically the HVD308xE can drive more than 25-mA to a 60-Ω load, resulting in a differential output voltage higher than the minimum required by the standard (see Figure 6-3).
Overall, the total load current can be 60 mA to a loaded RS-485 bus. This is in addition to the current required by the transceiver itself; the HVD308xE circuitry requires only about 0.4 mA with both driver and receiver enabled, and only 0.3 mA with either the driver enabled or with the receiver enabled. In low-power shutdown mode, neither the driver nor receiver is active, and the supply current is low.
Supply current increases with signaling rate primarily due to the totem pole outputs of the driver (see Figure 6-2). 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.