How to choose TVS tube for MSP430FR2633 touch chip

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How to choose TVS tube for MSP430FR2633 touch chip [Copy link]

TI's touch chip MSP430FR2633 touch solution can support proximity sensing. Generally, the proximity sensing PCB Sensor will be placed close to the machine casing. Antistatic testing will be performed during the product's EMC testing. TI's touch chip itself can support higher antistatic properties. If you want to improve this antistatic line, you need to connect a TVS in parallel to the proximity sensing pin to increase the product's antistatic ability.

Let's first look at a basic proximity sensing schematic and read its three internal values, namely LTA (average value of Count), Count (count value), and Threshold (threshold). We can see that when our body approaches the proximity sensing PCB Sensor, the Count value decreases and exceeds the Threshold value, and a proximity sensing is triggered. Similarly, several proximity sensings are triggered later.

Figure 1. Normal proximity sensing schematic

Figure 2 Proximity sensing monitoring data

If a TVS tube (ESD9D5C) is added to the proximity sensor, what effect will it have on the proximity sensor? We will find that the chip, i.e. the proximity sensor, will be triggered within 1 minute after it is powered on. Why does this happen? The analysis and solution are as follows:

Figure 3. Schematic diagram of proximity sensing of parallel TVS tubes

Figure 4 Proximity sensing monitoring data after connecting TVS tubes in parallel

Through the internal block diagram of MSP430FR2633 touch, we found that if there is a variable capacitor outside the capacitive proximity sensor, the proximity sensor will be triggered in the process of reaching steady state. If the capacitance of the external variable capacitor does not change after reaching the problem, it will be offset by the offset module inside the MSP430FR2633 , and will not cause false triggering.

Figure 5 MSP430FR2633 touch block diagram

Next, let's share the TVS tube (ESD9D5C). In fact, TVS has another parameter called Cpf. This capacitance will change with the frequency. The junction capacitance of TVS changes from power-on to steady state, and its capacitance increases from 0 to its nominal value, and finally tends to be stable. This also causes the proximity sensor to be falsely triggered after a period of power-on. It is this capacitance that causes the above problem.

Figure 6 ESD9D5C electrical parameters

The final equivalent model is a variable capacitor connected in parallel to a TVS tube. At the beginning of the system power-on, the capacitor changes from transient to steady state, causing the proximity sensor to be falsely triggered.

Figure 7 Equivalent model after connecting TVS and capacitor in parallel

At the same time, we also conducted relevant tests to verify the impact of TVS tubes with different parameters of 11pf and 0.3pf on proximity sensing and how to choose TVS tubes to solve the problem of false triggering of proximity sensing.

Figure 8 TVS electrical parameters with Cpf=0.3pF

The impact of parallel connection of 11pF TVS and 0.3pF TVS on the internal monitoring data of proximity sensing was tested under the same conditions and summarized into the following chart:

Figure 9 Cj=11pf TVS Figure 10 Cj=0.3pf TVS

From Figures 9 and 10, it is obvious that reducing the junction capacitance of the TVS tube can greatly improve the impact on proximity sensing. Finally, if a TVS tube is to be connected in parallel in a proximity sensing circuit, a TVS tube with a smaller capacitance, such as about 0.3pF, needs to be selected to solve the effect of the capacitance on the TVS tube on false triggering of proximity sensing.