EOS Protection for Main Chip Core Power in System Circuitry

Common EOS events occur mostly to external ports, such as USB and HDMI, at a system level. That’s why TVS devices are added at external ports for better EOS protection. In addition to these external ports, however, cases have been reported that system chip core power was fried by EOS energy. For example, a TV client of Amazing Microelectronic Corp. (AMC) examined several burned main chip core powers returned from the market and revised the tests for TV products during design stage. The client requested the EOS test on main chip core power in addition to those on external ports. At present, EOS tests are performed according to the IEC61000-4-5 (8/20us) specifications; i.e., EOS 36V tests on external ports and main chip core powers through a 2Ω output impedance. 

The cause of such a main chip core power damage could be the EOS spikes generated at the instance the system is turned on/off. These spikes have too much energy for the main chips and eventually the chips are damaged. However, effective protection can be achieved by installing TVS to protect main chip core power (Fig. 1). With the evolution of advanced miniaturized semiconductor processes, the voltage of these main chip core powers drops from previously common 3.3V to 1.8V or even lower. As a result, the main chips themselves become weaker against external surge energy and, in turn, we see increasing attention on main chip core power burned by EOS. 

Figure 1 Main chip core power + TVS protection

TVS production devices come with unidirectional and bidirectional designs. For the protection on power source, the unidirectional TVSs are recommended as they provided better protection in this regard. Fig. 2 (a) provides an example of comparison between 2 3.3V TVS devices on TLP, where AZ5123-01F is a bidirectional TVS (green curve) and AZ5013-01F a unidirectional TVS (red curve). The test results of positive TLP indicate that both devices provide pretty similar performance in terms of trigger voltage and clamping voltage. It is safe to say, therefore, that they provide similar protection against positive voltage surges. However, the test results of negative TLP tell a different story, as it is found that the trigger voltage is -6.2V for the bidirectional TVS AZ5123-01F and -0.7V for the unidirectional TVS AZ5013-01F, which is to say that the unidirectional TVS AZ5013-01F triggers into protection earlier when a negative voltage surge attacks. 

The system shown in Fig. 2 (b) needs a protection device for the 3.3V power source. If a bidirectional TVS such as AZ5123-01F is incorporated for protection and the system is subject to -3V surge energy, AZ5123-01F cannot trigger the protection until the negative voltage exceeds -6.2V, and the main chip circuitry may be fried if it triggers earlier than TVS or simply cannot withstand the surge energy. On the other hand, if the TVS is replaced with a unidirectional device like AZ5013-01F and a -3V surge comes, AZ5013-01F will bypass the vast majority of this energy to ground as the surge already exceeds the trigger voltage of AZ5013-01F at -0.7V, thus shielding the main chip from the surge energy impact and achieving better protection.

Figure 2 Unidirectional TVS vs. bidirectional TVS with negative trigger voltage

For the choice for power source, the unidirectional TVSs are recommended when system is subject to negative surge energy. As product designs are increasingly miniaturized, there may not be enough property on a PCB for TVSs, which makes small package TVSs a better choice for system designs. 

AMC has a series miniaturized TVSs developed specifically for power sources, ranging from previously common 5V and other voltages to common 1.8V sources in main chip core power. In the past, 0201-packaged TVSs (0.6mm L x 0.3mm W) were mostly bidirectional, but AMC has come up with a new series of 0201-packaged TVSs designed for main chip core power (Table 1). These miniaturized devices not only fit very snugly and easily around the already crowded main chip core power circuits, but also provide better EOS protection if they are placed even closer to core power. 

Table 1 Unidirectional TVS protection devices for power source of MCSP0603P2YS (0201 package)