MAX1480 Maxim, MAX1480 Datasheet - Page 17

• Manufacturer Part Number: MAX1480
• Description: Complete / Isolated RS-485/RS-422 Data Interface
• Manufacturer: Maxim

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 entire common-mode range (see Typical
Operating Characteristics ). In addition, a thermal shut-
down circuit forces the driver outputs into a high-
impedance state if the die temperature rises
excessively.
Typical propagation delays are shown in the Typical
Operating Characteristics using the test circuit of
Figure 5. Propagation delay skew is simply the differ-
ence between the low-to-high and high-to-low propaga-
tion delay. Small driver/receiver skew times help
reduce EMI and reflections by maintaining balanced
differential signals.
The receiver skew time,
100ns for the MAX1480A/MAX1490A and under 1µs for
the MAX1480B/MAX1480C/MAX1490B.
The driver skew time is typically 25ns for the
MAX1480A/MAX1490A and 100ns for the MAX1480B/
MAX1480C/MAX1490B.
Figure 14. Typical Full-Duplex RS-485/RS-422 Network
RO
DI
OR EQUIVALENT
V
74HC86
5V
IN
DRIVER INPUT
C1
22 F
RECEIVER OUTPUT
DI
______________________________________________________________________________________
´
0.1 F
C2
R1
R2
Driver Output Protection
Propagation Delay Skew
I
t
PLH
LOGIC GROUND
GND1
GND2
V
V
V
V
CC1
CC2
CC3
CC4
RO
D1
D2
SD
FS
DI
- t
10
11
12
1
2
3
4
5
6
7
8
9
PHL
Complete, Isolated RS-485/RS-422
ISOLATION BARRIER
MAX1490A/B
I
, is typically under
17
24
23
22
21
20
19
18
16
15
14
13
ISO V
A
B
Z
ISO COM1
ISO DI DRV
ISO V
ISO RO LED
AC1 (MAKE NO CONNECTION)
AC2 (MAKE NO CONNECTION)
ISO RO DRV
Y
CC1
CC2
R3
ISOLATED COMMON
R4
___________Applications Information
DI and DE are intended to be driven through a
series current-limiting resistor. Directly grounding
these pins destroys the device.
The MAX1480A/MAX1480B/MAX1480C are designed
for bidirectional data communications on multipoint
bus-transmission lines. The MAX1490A/MAX1490B are
designed for full-duplex bidirectional communications
that are primarily point-to-point. Figures 13 and 14
show half-duplex and full-duplex typical network appli-
cation circuits, respectively. To minimize reflections,
terminate the line at both ends with its characteristic
impedance, and keep stub lengths off the main line as
short as possible. The slew-rate-limited MAX1480B/
MAX1480C/MAX1490B are more tolerant of imperfect
termination and stubs off the main line.
The MAX1480A/MAX1480B/MAX1480C/MAX1490A/
MAX1490B pinouts enable optimal PC board layout by
minimizing interconnect lengths and crossovers.
For maximum isolation, the “isolation barrier” should
not be breached except by the MAX1480A/
MAX1480B/MAX1480C/MAX1490A/MAX1490B.
R5, 100
R6, 100
SH2
SH1
A
Y
B
Z
NOTE: RESISTORS R5 AND R6 PROTECT
THE MAX1490A/B FROM TRANSIENT
CURRENTS BETWEEN SHIELD AND
TRANSMISSION LINES.
120
120
Data Interface
SHIELD (OPTIONAL)
SHIELD (OPTIONAL)
Layout Considerations
120
120
Y
Z
B
A
D
R
DI
RO
17