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APPLICATION INFORMATION
R
T
R
T
POWER USAGE IN AN RS-485 TRANSCEIVER
SN65HVD485E
SLLS612C – JUNE 2004 – REVISED MARCH 2007
NOTE: The line should be terminated at both ends with its characteristic impedance (R
T
= Z
O
). Stub lengths off the main line
should be kept as short as possible.
Figure 16. Typical Application Circuit
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-485-A
standard defines a unit load as allowing up to 1mA. With up to 32 unit loads allowed on the bus, the total current
supplied to all receivers can be as high as 32mA. The HVD485E is rated as a 1/2 unit load device, so up to 64
can be connected on a bus.
The current in the termination resistors depends on the differential bus voltage. The standard requires active
drivers to produce at least 1.5V 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 HVD485E can
drive more than 25mA to a 60 Ω load, resulting in a differential output voltage higher than the minimum required
by the standard. (See Figure 15 .)
Supply current increases with signaling rate primarily due to the totum pole 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.
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