An ESD event rapidly forces current (see Figure 1-1), I_ESD , into a system, usually through a user interface such as a cable connection, or a human input device like a key on a keyboard. Protecting a system against ESD using a TVS relies upon the TVS being able to shunt I_ESD to ground. Optimizing a PCB Layout for ESD suppression is largely dependant on designing the path to ground for I_ESD with as little impedance as possible. During an ESD event, the voltage presented to the protected integrated circuit (Protected IC), V_ESD, is a function of I_ESD and the impedance presented to it. Since the designer has no control over I_ESD, lowering the impedance to ground is the primary means available for minimizing V_ESD.

Lowering the impedance presents several challenges. Mainly, it cannot be of zero impedance, or the signal line being protected would be shorted to ground. In order for the circuit to have a realistic application, the protected line needs to be able to maintain some voltage, usually under a high impedance to ground. This is where the TVS becomes applicableLowering the impedance presents several challenges. Mainly, it cannot be of zero impedance, or the signal line being protected would be shorted to ground. In order for the circuit to have a realistic application, the protected line needs to be able to maintain some voltage, usually under a high impedance to ground. This is where the TVS becomes applicable

A TVS is an array of diodes (see Figure 1-2 for a typical example) arranged to present a very high impedance to the voltages normally present in the circuit, but if voltages exceed the design, the TVS diodes will breakdown and shunt I_ESD to ground before it can damage the system being protected. The system designer is then challenged to lower the impedance for I_ESD from the ESD Source through the TVS to ground.

The impedance presented to I_ESD is a function of any impedance inherent with the TVS (in the diode array and the package of the TVS) and the PCB Layout between the ESD Source and the TVS ground. A TVS is generally designed to offer as low of an impedance to ground for I_ESD as its overall design constraints will allow. With the proper TVS selected, a critical phase of the design is to lower the impedance in the PCB Layout between the ESD Source and the TVS ground.

Another concern created by the rapidly changing I_ESD is its associated rapidly changing electromagnetic field (EM) causing interference (EMI) to couple onto other circuits of the PCB. This is especially true in the area between the ESD Source and the TVS. Once the TVS shunts I_ESD to ground, the trace between the TVS and the Protected IC should be relatively free of EMI. Therefore, unprotected circuits should not be adjacent to an ESD protected circuit's traces between the ESD Source and the TVS. In order to keep EMI emissions at a minimum, circuit traces between the ESD Source and the TVS should have corners which do not exceed 45° or, ideally, which are curved with large radii.

In today's PCB Layout, board space is at a premium. ICs, including TVSs, are designed to be very compact. Also, the density of their placement on the PCB is continually increasing. Multiple layer PCB boards and routing lean heavily upon VIAs for maximizing the density to increase the system's feature set while decreasing the system's size. This PCB architecture, particularly related to layer switching and VIAs, plays an important role in shunting I_ESD to ground through the TVS. Large differences in V_ESD at the Protected IC can be induced by the manner in which the circuit is routed to the TVS using VIAs. Generally, placing a VIA between the ESD Source and the TVS is detrimental, but in some circumstances the designer is forced to do so. Even in these circumstance, if properly done, V_ESD can still be minimized at the Protected IC.

Grounding schemes are critical in protecting against ESD. Having a chassis ground for the TVS that is separated from the digital and/or analog ground by inductance provides very good protection against ESD related failures. Yet it presents great challenges when routing high speed circuits across multiple ground planes. For this reason, many designs use one common ground for the protected circuits. Ground planes are necessary for the TVS to have success in dissipating I_ESD without increasing V_ESD. Electrical connections to an earth grounded chassis, like a PCB grounded through-hole for a chassis screw, immediately adjacent to the TVS ground and the ESD Source's ground (for example, a connector shield) provide a sound methodology in keeping ground shifts at Protected ICs to a minimum. If a system cannot utilize a chassis earth ground, tightly coupled multiple layer ground planes can help keep ground shifts at Protected ICs to a minimum.

To summarize these parameters, successfully protecting a system against ESD includes:

• Controlling impedances around the TVS for dissipating ESD current, I_ESD
• Limiting the effects of EMI on unprotected circuits
• Properly using VIAs to maximize ESD dissipation by the TVS
• Designing a grounding scheme which has very low impedance for the TVS

Outside of controlled RLC values, PCBs have inherent parasitics which contribute to overall board performance. Usually these parasitics are detrimental to the functionality of the design. An important parasitic to consider when designing a circuit to dissipate ESD is inductance. Because (see Note 1, below) V_ESD = V_{br_TVS} + R_DYN(TVS)I_ESD + L(dI_ESD/dt), and the term dI_ESD/dt is very large, the forced current in an ESD event will cause large voltages to drop across any inductance. For example, in an 8 kV ESD event as specified by IEC 61000-4-2, the dI_ESD/dt = (30 A)/(0.8×10^-9 s) = 4 × 10¹⁰ A/s. So even with 0.25 nH of inductance an additional 10 V is presented to the system.

In Figure 2-1 four parasitic inductors are shown: L₁ and L₂ is the inductance in the circuit between the ESD Source (typically a connector) and the TVS, L₃ is the inductance between the TVS and ground, and L₄ is the inductance between the TVS and the Protected IC. Not considering VIAs, the inductors L₁ and L₄ are generally dependant upon design constraints such as impedance controlled signal lines. However, I_ESD can still be "steered" towards the TVS by making L₄ much larger than L₁. This is accomplished by placing the TVS as near to the ESD Source as the PCB design rules allow while placing the Protected IC far away from the TVS, for example near the middle of the PCB. This effectively creates L₄ >> L₁, helping shunt the I_ESD to the TVS. Placing the TVS adjacent to the connector also mitigates EMI from radiating into the system. The inductor shown at L₂ should not be present in a well designed system. This represents a stub between the TVS and the line being protected. This design practice should be avoided. The Protected Line should run directly from the ESD Source to the protection Pin of the TVS, ideally with no VIAs in the path. The inductor at L₃ represents the inductance between the TVS and ground. This value should be reduced as much as possible, and perhaps represents the most predominant parasitic influencing V_ESD. The voltage presented to the node "Protected Line" will be V_ESD = V_{br_TVS} + I_ESDR_DYN(TVS) + (L₂ + L₃)(dI_ESD/dt). Thus the PCB designer needs to minimize L₃ and eliminate L₂. Minimizing L₃ is covered in Section 2.4. Minimizing L₁ is covered in Section 2.2 and Section 2.3.

Summary

• Minimize any inductance between the ESD Source and the path to ground through the TVS
• Place the TVS as near to the connector as design rules allow
• Place the Protected IC much further from the TVS than the TVS is to the connector
• Do not use stubs between the TVS and the Protected Line, route directly from the ESD Source to the TVS
• Minimizing inductance between the TVS and ground is critical