Datasheet

www.ti.com
APPLICATION INFORMATION
OPTO-ISOLATED DATA BUSES
SUPPLY SOURCE IMPEDANCE
+
–
+
–
V
S
R
S
R
S
I
L
R
L
V
L
= V
S
– 2R
S
I
L
DC-to-DC
Converter
Opto
Isolators
DC-to-DC
Converter
Opto
Isolators
Local Power
Source
Rest of
Board
Local Power
Source
Rest of
Board
SN75HVD08 , SN65HVD08
SLLS550C – NOVEMBER 2002 – REVISED JULY 2006
As electrical loads are physically distanced from their not be ignored and decoupling capacitance at the
power source, the effects of supply and return line load is required. The amount depends upon the
impedance and the resultant voltage drop must be magnitude and frequency of the load current change
accounted. If the supply regulation at the load cannot but, if only powering the SN65HVD08, a 0.1 µF
be maintained to the circuit requirements, it forces ceramic capacitor is usually sufficient.
the use of remote sensing, additional regulation at
the load, bigger or shorter cables, or a combination
of these. The SN65HVD08 eases this problem by
Long RS-485 circuits can create large ground loops
relaxing the supply requirements to allow for more
and pick up common-mode noise voltages in excess
variation in the supply voltage over typical RS-485
of the range tolerated by standard RS-485 circuits. A
transceivers.
common remedy is to provide galvanic isolation of
the data circuit from earth or local grounds.
Transformers, capacitors, or phototransistors most
In the steady state, the voltage drop from the source
often provide isolation of the bus and the local node.
to the load is simply the wire resistance times the
Transformers and capacitors require changing
load current as modeled in Figure 15 .
signals to transfer the information over the isolation
barrier and phototransistors (opto-isolators) can pass
steady-state signals. Each of these methods incurs
additional costs and complexity, the former in clock
encoding and decoding of the data stream and the
latter in requiring an isolated power supply.
Quite often, the cost of isolated power is repeated at
each node connected to the bus as shown in
Figure 16 . The possibly lower-cost solution is to
generate this supply once within the system and then
Figure 15. Steady-State Circuit Model
distribute it along with the data line(s) as shown in
Figure 17 .
For example, if you were to provide 5-V ±5% supply
power to a remote circuit with a maximum load
requirement of 0.1 A (one SN65HVD08), the voltage
at the load would fall below the 4.5-V minimum of
most 5-V circuits with as little as 5.8 m of 28-GA
conductors. Table 1 summarizes wire resistance and
the length for 4.5 V and 3 V at the load with 0.1 A of
load current. The maximum lengths would scale
linearly for higher or lower load currents.
Table 1. Maximum Cable Lengths for Minimum
Load Voltages at 0.1 A Load
WIRE RESISTANCE 4.5 V LENGTH 3-v LENGTH
SIZE AT 0.1 A AT 0.1 A
28 Gage 0.213 Ω /m 5.8 m 41.1 m
24 Gage 0.079 Ω /m 15.8 m 110.7 m
22 Gage 0.054 Ω /m 23.1 m 162.0 m
20 Gage 0.034 Ω /m 36.8 m 257.3 m
18 Gage 0.021 Ω /m 59.5 m 416.7 m
Figure 16. Isolated Power at Each Node
Under dynamic load requirements, the distributed
inductance and capacitance of the power lines may
11
Submit Documentation Feedback