SN65HVD3082E_17 Datasheet, PDF (21/36 Pages) Texas Instruments – Low-Power RS-485 Transceivers, Available in a Small MSOP-8 Package

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SN65HVD3082E, SN75HVD3082E, SN65HVD3085E, SN65HVD3088E
SLLS562I – AUGUST 2009 – REVISED SEPTEMBER 2016
10 Power Supply Recommendations
To ensure reliable operation at all data rates and supply voltages, each supply must be decoupled with a 100-nF
ceramic capacitor located as close to the supply pins as possible. This helps to reduce supply voltage ripple
present on the outputs of switched-mode power supplies and also helps to compensate for the resistance and
inductance of the PCB power planes.
11 Layout
11.1 Layout Guidelines
Robust and reliable bus node design often requires the use of external transient protection devices in order to
protect against EFT and surge transients that may occur in industrial environments. Because these transients
have a wide frequency bandwidth (from approximately 3 MHz to 3 GHz), high-frequency layout techniques must
be applied during PCB design.
• Place the protection circuitry close to the bus connector to prevent noise transients from entering the board.
• Use V_CC and ground planes to provide low-inductance.
NOTE
High-frequency currents follow the path of least inductance and not the path of least
impedance.
• 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.
• Apply 100-nF to 220-nF bypass capacitors as close as possible to the V_CC pins of transceiver, UART, and
controller ICs on the board.
• Use at least two vias for V_CC and ground connections of bypass capacitors and protection devices to
minimize effective via-inductance.
• Use 1-kΩ to 10-kΩ pullup or pulldown resistors for enable lines to limit noise currents in these lines during
transient events.
• Insert series 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 pins. These resistors limit the residual clamping current into
the transceiver and prevent it from latching up.
• 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 200 mA.
11.2 Layout Example
5
Via to ground
R
C4
Via to VCC
6R
MCU
6R
SN65HVD3082E
R
7
R
R
5
1
5 _TVS
Figure 26. SNx5HVD308xE Layout Example
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