CN-0313 Datasheet, PDF (3/5 Pages) Analog Devices – EMC Compliant RS-485 Transceiver Protection Circuits

Circuit Note
The ADM3485E is a 3.3 V, low power data transceiver suitable
for half-duplex communication on multipoint transmission lines.
It has a data rate up to 12 Mbps with a common-mode range on
the bus pins (A and B) of −7 V to +12 V. Data transmits on the
DI pin, and it is received on the RO pin. Both the driver and
receiver outputs can be enabled or disabled, that is, put into a
high impedance state, by changing the logic levels on the DE
and RE pins, respectively.
Power and ground are connected via a screw-wire connector (VCC
and GND). This connector supplies all three ADM3485E devices.
Logic inputs DE and RE are set using LK1 to LK6. For each
ADM3485E, LK2, LK4, and LK6 relate to DE and LK1, LK3, and
LK5 relate to RE. For each link, Position A connects the logic pin
to VCC, Position B connects the logic pin to GND, and Position C
connects the logic pin to the four terminal, side screw-wire
connector. The input DI and output RO pins are connected
directly to the four terminal screw connector.
The EVAL-CN0313-SDPZ is also compatible with the Analog
Devices, Inc., ezLINX™ board (EZLINX-IIIDE-EBZ) and the
system development platform (EVAL-SDP-CB1Z). Connector J8
links the UART and GPIO interfaces on the SDP or the ezLINX
board to the logic I/O of the ADM3485E devices. The I/O
connections and jumper configurations are shown in Table 2.
Table 2. ezLINX and SDP I/O Connections and Jumper
Configurations
ADM3485E I/O Pin SDP/ezLINX Connector Selection
TVS
RO
UART_RX
LK7 (A)
RE
GPIO_0
LK1 (C)
DE
GPIO_3
LK2 (C)
DI
UART_TX
LK8 (A)
TVS/TBU/TISP RO
UART_RX
LK7 (B)
RE
GPIO_1
LK3 (C)
DE
GPIO_4
LK4 (C)
DI
UART_TX
LK8 (B)
TVS/TBU/GDT RO
UART_RX
LK7 (C)
RE
GPIO_2
LK5 (C)
DE
GPIO_5
LK6 (C)
DI
UART_TX
LK8 (C)
The ADM3485E transmitter and receiver share the same differential
bus pins (A and B). The protection circuitry is used to protect
these bus pins.
CN-0313
In the first protection circuit, shown in Figure 1 as TVS, it uses one
component, the Bourns CDSOT23-SM712. This is the transient
voltage suppressor (TVS) array shown on the EVAL-CN0313-
SDPZ. It consists of two bidirectional TVS diodes optimized to
protect RS-485 systems with minimal overstress while allowing
the full range of the RS-485 signal and common-mode excursions.
The TVS has high impedance to ground in normal operating
conditions. When an overvoltage occurs, the TVS enters into
avalanche breakdown mode and clamps the pin voltage to a safe
predetermined level. It then diverts the transient current away
from the ADM3485E to ground.
This protection scheme offers up to 8 kV contact and15 kV air
discharge ESD, 2 kV EFT, and 1 kV surge protection.
As described in the CDSOT23-SM712 data sheet, the part is
designed specifically for RS-485 devices. The next two protection
schemes add to the CDSOT23-SM712 to provide higher levels
of circuit protection against surge.
In the second scheme, shown in Figure 1 as TVS/TBU/TISP,
the CDSOT23-SM712 TVS provides secondary protection, and
Bourns TISP4240M3BJR-S provides the primary protection. The
TISP4240M3BJR-S is a totally integrated surge protector (TISP).
The TISP is a solid-state thyristor. When its predetermined voltage
is exceeded, the TISP provides a low impedance path to ground,
diverting a majority of the transient energy away from the
ADM3485E.
Bourns TBU-CA065-200-WH transient blocking unit (TBU) is
a nonlinear overcurrent protection device between the primary
and secondary protection devices that ensures coordination
occurs. The TBU is an overcurrent blocking device that becomes
an open circuit at a predefined current. In blocking mode, TBU
has very high impedance to block transient energy. This protection
scheme offers up to 8 kV contact and15 kV air discharge ESD,
2 kV EFT, and 4 kV surge protection.
The third protection scheme, shown in Figure 1 as TVS/TBU/
GDT, operates in a similar fashion to Protection Scheme 2. In
this scheme, a gas discharge tube (GDT) is used instead of the
TISP. The GDT protects to higher overvoltages and overcurrents
than the TISP described in the previous protection scheme.
A GDT is a gas discharge plasma device that provides a low
impedance path to ground to protect against over voltage
transients. The selected GDT is the Bourns 2038-15-SM-RPLF.
This third protection scheme offers up to 8 kV contact and
15 kV air discharge ESD, 2 kV EFT, and 6 kV surge protection.
The ADM3485E has a 120 Ω termination attached to the bus pins.
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