Designers Maximize TVS Diode Performance

Mar 1, 2006 12:00 PM
By Jim Lepkowski, Senior Applications Engineer, ON Semiconductor, Phoenix

Printed circuit board layout is key to achieving the highest level of surge immunity on power and data lines.

A transient voltage suppressor (TVS) diode is an effective, low-cost option to protect sensitive electronic components from electrostatic discharge (ESD) and electromagnetic interference (EMI) surge pulses. However, to maximize the surge immunity provided by a TVS diode, designers should follow certain printed circuit board (PCB) layout guidelines. Layout examples help illustrate the tradeoffs between the different circuit configurations available with TVS diodes.

PCB Layout Guidelines

Location

TVS devices should be used on all data and power lines that enter or exit the PCB at the I/O connector. Locating the TVS devices as close as possible to the noise source ensures that a surge voltage will be clamped before the pulse can be coupled into adjacent PCB traces (Fig. 1). In addition, the PCB should use short TVS traces.

A short trace length equates to low impedance, which ensures that the surge energy will be dissipated by the TVS device instead of the IC's internal ESD protection circuit. Locating sensitive traces in the center rather than near the edge of the PCB is a simple method to protect against ESD, which can occur during handling. Fig. 2 provides an example of the PCB layout recommendations.

Ground selection

If possible, the protection circuits should shunt the surge voltage to either the reference or chassis ground, as shown in Fig. 2. Shunting the surge voltage directly to the IC's signal ground can cause ground bounce. The clamping performance of TVS diodes on a single ground PCB can be improved by minimizing the impedance with relatively short and wide ground traces.

Parasitic inductances

The PCB layout and IC package parasitic inductances can cause significant overshoot to the TVS's clamping voltage. The inductance of the PCB can be reduced by using short trace lengths and multiple layers with separate ground and power planes. The inductance contributed by the package can be minimized by selecting small surface-mount packages.

Loop area

Radiated emissions and radio frequency susceptibility are reduced by minimizing the loop area formed by high-speed data and ground lines. One effective method to minimize loop problems is to incorporate a ground plane in the PCB design, especially when the traces are relatively long. Maximizing the separation distance from the TVS device and IC provides isolation; however, it may increase the loop area, as shown in Fig. 3.

Device Selection Guidelines

Avalanche TVS versus diode array

The high surge rating but relatively large capacitance of an avalanche TVS diode makes this device the preferred option for applications such as load switch and dc power bus protection. In contrast, the modest surge rating but low capacitance of a diode array is a match for protecting high-speed data lines. Avalanche TVS and diode arrays often can be used interchangeably; however, some circuits require careful analysis to select an appropriate device.

Fig. 4 illustrates the back drive problem that occurs when a path exists for current to flow through the diode array via a data line. The data line can unintentionally provide power to Module 1 if V_DD2 is greater than V_DD1. This condition can cause powerup problems with logic ICs and anomalies such as the illumination of indicator lights in Module 1 when the unit is unpowered.

Uni-directional versus bi-directional avalanche TVS diode

The different breakdown voltages (V_BR) of uni- and bi-directional avalanche TVS diodes can provide distinct advantages in specific applications. Uni-directional devices have a reverse-bias breakdown voltage of V_BR and a forward-bias breakdown equal to the forward voltage (V_F) of a diode.

In contrast, the breakdown voltage of a bi-directional device is equal to ±V_BR. The low breakdown voltage of a uni-directional device for a negative surge voltage often is an advantage for dc power line and single-supply-powered ICs. In contrast, the symmetrical breakdown voltage of a bi-directional device typically offers better noise performance in a differential input or in output amplifiers.

External versus internal IC protection circuits

An ideal external TVS device will absorb the entire energy of a surge pulse; however, a portion of the surge current energy may pass through the IC's internal protection circuit. One option to limit the current to an internal protection circuit is to use a series resistor, as shown in Fig. 5.

Internal protection circuits function well at preventing the ESD failures that occur in assembly; however, the relatively small size of the protection devices limits their ability to withstand the surges that occur in normal product usage. The surge ability of a diode is proportional to its size and it is typically not practical for an IC to incorporate large protection devices.

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