How to choose TVS tube for MSP430FR2633 touch chip

alan000345

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 tests will be performed during the product's EMC test. 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 higher 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 to more than 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: 102)]

Figure 3 Proximity sensing principle diagram of parallel TVS tubes

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

Through the internal block diagram of MSP430FR2633 touch, we found that if there is a variable capacitor outside the capacitance monitoring proximity sensing, the proximity sensing will be triggered in the process of reaching steady state. If the capacitance of the external variable capacitor no longer changes after reaching the problem, it will be offset by the offset module inside MSP430FR2633, and will not cause false triggering.

Figure 5 MSP430FR2633 touch block diagram

Next, let's share this TVS tube (ESD9D5C). In fact, TVS has another parameter called Cpf. This capacitance will change with the change of frequency. The junction capacitance of TVS changes from 0 to its nominal value from power-on to steady state, and finally tends to be stable. This also leads to the false triggering of Proximity proximity sensing 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 phase, 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.

Picture 8 Electrical parameters of TVS with Cpf=0.3pF

The influence of parallel connection of 11pF TVS and 0.3pF TVS on the internal monitoring data of proximity sensing is tested under the same conditions, and the following chart is summarized.

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

It is obvious from Figures 9 and 10 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 a TVS tube of about 0.3pF, is required to solve the impact of the capacitance on the TVS tube on false triggering of proximity sensing.

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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, resulting in the false triggering of the proximity sensor.

Figure 7 Equivalent model after TVS and capacitor are connected 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.

Picture 8 Electrical parameters of TVS with Cpf=0.3pF

The influence of parallel connection of 11pF TVS and 0.3pF TVS on the internal monitoring data of proximity sensing is tested under the same conditions, and the following chart is summarized.

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

It is obvious from Figures 9 and 10 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 a TVS tube of about 0.3pF, is required to solve the impact of the capacitance on the TVS tube on false triggering of proximity sensing.

png[/img][/url]

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, resulting in the false triggering of the proximity sensor.

Figure 7 Equivalent model after TVS and capacitor are connected 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 Electrical parameters of TVS with Cpf=0.3pF

The influence of parallel connection of 11pF TVS and 0.3pF TVS on the internal monitoring data of proximity sensing is tested under the same conditions, and the following chart is summarized.

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

It is obvious from Figures 9 and 10 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 a TVS tube of about 0.3pF, is required to solve the impact of the capacitance on the TVS tube on false triggering of proximity sensing.

Figure 8 Electrical parameters of TVS with Cpf=0.3pF

Test of 11pF TVS in parallel under the same conditions The impact of 0.3pF TVS on the internal monitoring data of proximity sensing is summarized in the following chart: 102)]

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

It is obvious from Figures 9 and 10 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 a TVS tube of about 0.3pF, is required to solve the impact of the capacitance on the TVS tube on false triggering of proximity sensing.

Figure 8 Electrical parameters of TVS with Cpf=0.3pF

Test of 11pF TVS in parallel under the same conditions The impact of 0.3pF TVS on the internal monitoring data of proximity sensing is summarized in the following chart: 102)]

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

It is obvious from Figures 9 and 10 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 a TVS tube of about 0.3pF, is required to solve the impact of the capacitance on the TVS tube on false triggering of proximity sensing.

蓝雨夜

Very good!

alan000345

Blue Rain Night posted on 2018-12-3 09:43 Very good!

Thank you. I also think it is very good so I share it with you.

蓝雨夜

alan000345 posted on 2018-12-3 10:27 Thank you. I also think it is very good, so I share it with you.

It is not your original work?

qwqwqw2088

The junction capacitance of TVS is an important parameter for selecting TVS in data interface circuits. The capacitance of TVS Cpf will change with the frequency. I don't know how to test it when using it.

alan000345

qwqwqw2088 posted on 2018-12-3 12:02 The junction capacitance of TVS is an important parameter for selecting TVS in data interface circuits. The capacitance of TVS Cpf will change with the frequency. I don't know...

It should be measured with an oscilloscope. This is the test officially provided by TI. I feel that there is no other instrument that can measure it except the oscilloscope.

tiankai001

Very good, learned a lot of knowledge

跳动的心

Why does it take a minute? Shouldn't it be fast to power on? Could you explain this to me?

alan000345

Blue Rainy Night posted on 2018-12-3 10:30 It's not your original work?

Master ti's masterpiece is very valuable, so I share it with you