MAX13413EESA Datasheet, PDF (21/24 Pages) Maxim Integrated Products – RS-485 Transceiver with Integrated Low-Dropout Regulator and AutoDirection Control

RS-485 Transceiver with Integrated Low-Dropout
Regulator and AutoDirection Control
R_t
ISO_V_CC
V_SYS
UNREGULATED ISOLATED
POWER SUPPLY
MCU AND
RELATED
CIRCUITRY
V_SYS
+
ISO_V_CC
RO
1
R
ISO_V_CC
RE ₂
ISO_V_CC
V_{REG 3}
C_S
1μF
DETECT
CIRCUIT
DI
4
D
LDO 8 ^VCC
0.1μF
B
7
A
6
GND
5
MAX13412E
MAX13413E
ISO_V_CC
R_t
Figure 10. Typical Application Circuit for the MAX13412E/MAX13413E
ground loops. The typical application circuits show an
isolated RS-485 interface using the MAX13410E–
MAX13415E. The transceiver is powered separately from
the controlling circuitry. The AutoDirection feature of the
MAX13412E/MAX13413E (see the AutoDirection Circuitry
section) requires only two optocouplers to electrically
isolate the transceiver.
An isolated RS-485 interface electrically isolates differ-
ent nodes on the bus to protect the bus from problems
due to high common-mode voltages that exceed the
RS-485 common-mode voltage range. An isolated RS-
485 interface has two additional design challenges not
normally associated with RS-485 design. These are 1)
isolating the control signals and 2) getting isolated
power to the transceiver. Optical isolators are the most
common way of getting the control signals across the
isolation barrier.
Isolated power is typically done using a transformer in
either a push-pull or flyback configuration. The MAX845
is an example of an inexpensive, unregulated push-pull
converter (see Figure 12). While in theory, the output of
an unregulated push-pull converter is predictable, the
output voltage can vary significantly due to the non-ideal
characteristics of the transformer, load variations, and
temperature drift of the diodes, etc. Variances of ±20%
or more would not be uncommon. This would require the
addition of a linear regulator to get standard RS-485
transceivers to work. Since the MAX13410E–
MAX13415E have the linear regulator built in, this exter-
nal regulator and its associated cost and size penalties
are not necessary. A nominal +7.5V output with a ±20%
tolerance would provide a +6V to +9V supply voltage.
This is well within the operating range of the
MAX13410E–MAX13415E. If the output tolerance is even
greater than ±20%, adjust the design of the power sup-
ply for a higher output voltage to ensure the minimum
input voltage requirements are met.
Flyback converters are typically regulated. A TL431 type
error amplifier and an optical isolator usually close the
loop. The MAX5021 is an example of a small, inexpen-
sive, flyback controller (see Figure 13). While the prima-
ry output of the flyback converter is tightly regulated,
secondary outputs will not be. As with the unregulated
push-pull converter, the MAX13410E–MAX13415E are
ideally suited for use with these secondary outputs.
______________________________________________________________________________________ 21