THVD1500 Datasheet, PDF (20/29 Pages) Texas Instruments – 300 kbps RS-485 Transceivers With 8-kV IEC ESD Protection

THVD1500
SLLSEY4 – JULY 2017
11 Layout
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11.1 Layout Guidelines
Robust and reliable bus node design often requires the use of external transient protection devices in order to
protect against surge transients that may occur in industrial environments. Since these transients have a wide
frequency bandwidth (from approximately 3 MHz to 300 MHz), high-frequency layout techniques should be
applied during PCB design.
1. Place the protection circuitry close to the bus connector to prevent noise transients from propagating across
the board.
2. Use V_CC and ground planes to provide low inductance. Note that high-frequency currents tend to follow the
path of least impedance and not the path of least resistance.
3. Design the protection components into the direction of the signal path. Do not force the transient 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 V_CC pins of transceiver, UART and/or
controller ICs on the board.
5. Use at least two vias for V_CC and ground connections of bypass capacitors and protection devices to
minimize effective via inductance.
6. Use 1-kΩ to 10-kΩ pullup and pulldown 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 pins. 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 less than 1 mA.
11.2 Layout Example
5
Via to ground
R
C4
Via to V_CC
MCU
6_R
6R
THVD1500
1
R
5
TVS
5
Figure 24. Layout Example
20
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