Abstract:
Thanks to the progress of advanced manufacturing process, more sophisticated functions and lower power consumption are realized in electronic products with smaller footprint and volume. However, this trend makes main chips more vulnerable to electrostatic discharges. As the repair rate has everything to do with failures due to electrostatic discharges, major manufacturers have defined the test specifications for interface ports vulnerable to transient event, and products are tested during mass production according to the system-level electrostatic resistance test of IEC 61000-4-2 before sold in the market.
However, as test conditions become increasingly vigorous and chips to be protected become more and more vulnerable, there are rising protection devices that provide lower clamping voltage with the risk of latch-up. In this study, two cases study are performed on TVS and latch-up and solutions are also proposed for how to select the right TVS to prevent latch-up.
Case 1: HDMI 1.4 interface ESD protection element and latch-up effect
Re-timer and re-driver are often used in high-speed communication interface to reshape the integrity of signals. The most commonly used are USB and HDMI re-timers, which provide the conversion between protocols and flexibility in PCB circuitry design. On the other hand, such a circuit application results in direct connection of pull-up resistors to power supply, and the IC’s output driver has greater power driving ability, which is one of the sources of latch-up in TVS.
For a latch-up case, a screen is connected via an HDMI 1.4 port and an oscilloscope is used to capture the level of changes in the TMDS channel. As we check that the system is working as expected, an ESD gun is used to inject electrostatic charges to the HDMI port, and the ESD protection decive protects the IC in the front, as shown below.
Fig. 1 HDMI Experiment with Wrong TVS Selection
It is observed in the experiment that the screen turns blank after the ESD discharge. An oscilloscope is used to display that the voltage of latched-up TVS (not an Amazing Microelectronic product) drops from the normal 2.8V to 1.8V, enough to cause communications failure. A curve tracer, Tektronix 370A, is used to measure the IV characteristics of TVS under large power. It is found that the lowest turn-on voltage, Vhold, is 1.8V as the TVS is in a low-resistance conduction state, as shown below. (This TVS is provided with an SCR structure, and the snap-back characteristic provides lower clamping voltage.)
Fig. 2 IV Curve of TVS with Latch-up Risk
Root cause breakdown: the HDMI 1.4 re-timer voltage output is 2.8V (minimum at 2.6V) and it is 1.8V for the TVS Vhold. It is found in the re-timer datasheet that the TMDS short-circuit output current can be as high as 50 mA, since a path of extremely low-resistance is created after TVS conduction and considered the IC output short circuit to ground. A power driving ability like this pushes the TVS into latch-up and makes it unable to turn-off after trigger. The screen display and voltage level cannot be restored if the IC power is not cut off by disconnecting the cable. With current running for a long period of time, there can be irreversible potential threats to the IC and TVS, such as thermal failure or significantly increased electric leakage.
For HDMI TMDS channels, therefore, Amazing Microelectronic recommends TVS with VRWM≧3.3V as the protection element; for example, AZ1143-04F for HMDI 1.4/2.0 and AZ1123-04F for HDMI 2.1. With the highest quality and reliability in mind, Amazing Microelectronic is working hard to ensure Vhold>VRWM and zero latch-up risk in all series of Amazing Microelectronic TVSs as we are creating ESD protection with low clamping voltage!
Case 2: Power Management Circuit Protection in Portable Device
Battery and its power management IC – LDO (low-voltage dropout regulator) is the core to power up a portable device. For the stable voltage output and best reliability in a device, ESD protection is recommended by using TVS at the voltage supply pins. However, it is a big problem to introduce a TVS with lath-up risk at the power output pins, since the current output is very strong and can cause battery depletion in a very short time and large current in TVS. In some worst cases, the components are fried and hardware damaged. In a real case, a client found a problem in 3.3V LDO output after ESD test. We measured the voltage level during the experiment and found that the voltage dropped from 3.3V to 2.85V, which is inconsistent with LDO output. It was necessary to disconnect the battery and cut the power off in order to restore the normal working voltage. This phenomenon is a clear indication of latch-up in TVS. We checked the TVS specifications with the client and found that the Vhold was as low as 2.8V for the TVS with a working voltage, VRWM, of 3.3V. It was confirmed that the inappropriate selection of TVS led to system failure caused by latch-up. This case was solved successfully with the replacement of Amazing Microelectronic AZ5123-01H with VRWM at 3.3V.
Fig.3 LDO ESD Test and the Waveform Diagram
To sum up, Amazing Microelectronic is still working hard for the development of TVSs with even lower clamping voltage, while helping clients improve their products’ specifications and reliability. We are trying to put ourselves in clients’ shoes without leaving clients at risk. It is our promise to provide clients with TVS of low clamping voltage and zero latch-up risk.
Amazing offers latch-up free TVS: Vhold > VRWM≧Voperating!