Datasheet

MAX3480A/MAX3480B
Complete, Isolated, 3.3V
RS-485/RS-422 Data Interface
12 ______________________________________________________________________________________
Figure 8. Receiver Propagation Delays
V
OH
V
OL
V
ID
-V
ID
1.5V
0V
1.5V
OUTPUT
INPUT
0V
RO
V
B
- V
A
t
PLH
t
PHL
__Switching Waveforms (continued)
____________________Function Tables
Table 2. Transmitting
INPUTS
DE´ DI´
B A
1 1 0 1
1 0 1 0
0 X High-Z High-Z
INPUTS OUTPUT
DE´
A-B
–
R
—
O
–
0 ≥ +0.2V 0
0 ≤ -0.2V 1
0 Inputs open 0
OUTPUTS
Table 3. Receiving
X = Don't care
High-Z = High impedance
goes to approximately double the supply voltage.
Since the circuit is symmetrical, the two outputs can be
combined with diodes, filtered, and used to power the
oscillator itself.
The diodes on the primary side may be any fast-switch-
ing, small-signal diodes, such as the 1N914, 1N4148,
or CMPD2838. The nominal value of the primary filter
capacitor C3 is 0.01µF.
Driver Output Protection
There are two mechanisms to prevent excessive output
current and power dissipation caused by faults or by
bus contention. A foldback current limit on the output
stage provides immediate protection against short cir-
cuits over the whole common-mode voltage range (see
Typical Operating Characteristics ). In addition, a ther-
mal shutdown circuit forces the driver outputs into a
high-impedance state if the die temperature rises
excessively.
Resistor R8 provides additional protection by current
limiting between the shield and the two signal wires. In
the event that shielded cable is used and an external
voltage or transient is inadvertently applied between
the shield and the signal wires, the MAX3480 can be
damaged. Although unlikely, this condition can occur
during installation.
The MAX3480 provides electrical isolation between
logic ground and signal paths; it does not provide
isolation from external shields and the signal paths.
When in doubt, do not connect the shield. The
MAX3480 can be damaged if resistor R8 is shorted
out.