SN65HVD72
SN65HVD75
SN65HVD78
SLLSE11B –MARCH 2012–REVISED JUNE 2012
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Table 6. Bill of Materials
Device
Function
Order Number
SN65HVD72D
Manufacturer
XCVR
3.3V, 250kbps RS-485 Transceiver
10Ω, Pulse-Proof Thick-Film Resistor
Bidirectional 400W Transient Suppressor
Bidirectional.
TI
R1, R2
TVS
CRCW0603010RJNEAHP
CDSOT23-SM712
Vishay
Bourns
Bourns
TBU1, TBU2
TBU-CA-065-200-WH
200mA Transient Blocking Unit 200V, Metal-
Oxide Varistor
MOV1, MOV2
MOV-10D201K
Bourns
Vcc
10k
Vcc
Vcc
Vcc
10k
0.1μF
0.1μF
TBU1
R1
R1
1
2
3
4
8
1
2
3
4
8
7
6
5
RxD
R
Vcc
RxD
MCU
R
Vcc
A
MOV1
TVS
TVS
7
RE
DE
D
A
B
RE
DE
D
MCU
DIR
XCVR
XCVR
6
5
DIR
TxD
B
MOV2
TxD
GND
GND
R2
R2
TBU2
10k
10k
Figure 22. Transient Protections Against ESD, EFT, and Surge Transients
The left circuit provides surge protection of ≥ 500 V transients, while the right protection circuits can withstand
surge transients of 5 kV.
Design and Layout Considerations For Transient Protection
Because ESD and EFT transients have a wide frequency bandwidth from approximately 3 MHz to 3 GHz, high-
frequency layout techniques must be applied during PCB design.
In order for your PCB design to be successful start with the design of the protection circuit in mind.
1. Place the protection circuitry close to the bus connector to prevent noise transients from penetrating your
board.
2. Use Vcc and ground planes to provide low-inductance. Note that high-frequency currents follow the path of
least inductance and not the path of least impedance.
3. Design the protection components into the direction of the signal path. Do not force the transients currents to
divert from the signal path to reach the protection device.
4. Apply 100 nF to 220 nF bypass capacitors as close as possible to the Vcc-pins of transceiver, UART,
controller ICs on the board.
5. Use at least two vias for Vcc and ground connections of bypass capacitors and protection devices to
minimize effective via-inductance.
6. Use 1k to 10k pull-up/down resistors for enable lines to limit noise currents in theses lines during transient
events.
7. Insert pulse-proof resistors into the A and B bus lines if the TVS clamping voltage is higher than the specified
maximum voltage of the transceiver bus terminals. These resistors limit the residual clamping current into the
transceiver and prevent it from latching up.
8. While pure TVS protection is sufficient for surge transients up to 1 kV, higher transients require metal-oxide
varistors (MOVs) which reduce the transients to a few hundred volts of clamping voltage, and transient
blocking units (TBUs) that limit transient current to some 200 mA.
16
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