Datasheet

SN65HVD3082E, SN75HVD3082E
SN65HVD3085E, SH65HVD3088E
SLLS562G –AUGUST 2009–REVISED MAY 2009 www.ti.com
POWER USAGE IN AN RS-485 TRANSCEIVER
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 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 , the bus input current is less than 1/8 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 16.)
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 totum pole outputs of the driver (see Figure 15).
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.
LOW-POWER SHUTDOWN MODE
When both the driver and receiver are disabled (DE low and RE high) the device is in shutdown mode. If the
enable inputs are in this state for less than 60 ns, the device does not enter shutdown mode. This guards against
inadvertently entering shutdown mode during driver/receiver enabling. Only when the enable inputs are held in
this state for 300 ns or more, the device is assured to be in shutdown mode. In this low-power shutdown mode,
most internal circuitry is powered down, and the supply current is typically 1 nA. When either the driver or the
receiver is re-enabled, the internal circuitry becomes active.
If only the driver is re-enabled (DE transitions to high) the driver outputs are driven according to the D input after
the enable times given by t
PZH(SHDN)
and t
PZL(SHDN)
in the driver switching characteristics. If the D input is open
when the driver is enabled, the driver outputs defaults to A high and B low, in accordance with the driver failsafe
feature.
If only the receiver is re-enabled (RE transitions to low) the receiver output is driven according to the state of the
bus inputs (A and B) after the enable times given by t
PZH(SHDN)
and t
PZL(SHDN)
in the receiver switching
characteristics. If there is no valid state on the bus the receiver responds as described in the failsafe operation
section.
If both the receiver and driver are re-enabled simultaneously, the receiver output is driven according to the state
of the bus inputs (A and B) and the driver output is driven according to the D input. Note that the state of the
active driver affects the inputs to the receiver. Therefore, the receiver outputs are valid as soon as the driver
outputs are valid.
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