10 Layout

10.1 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 VCC 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 decoupling capacitors as close as possible to the VCC pins of transceiver, UART

and/or controller ICs on the board.

5. Use at least two vias for VCC and ground connections of decoupling capacitors and protection devices to

minimize effective via inductance.

6. Use 1-kΩ to 10-kΩ pull-up and pull-down resistors for enable lines to limit noise currents in these 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.

10.2 Layout Example

MCU

R

R

Via to ground

R

R

R

C

5

6

6

1

4

R

5

Via to VCC

TVS

7

5

Figure 10-1. Layout Example for SOIC package

THVD1400, THVD1420

SLLSF78B – DECEMBER 2020 – REVISED OCTOBER 2021

www.ti.com

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