TS3A5223: About the behavior when switching

Hello,

Thank you for your question and welcome to E2E!

It seems that the image you are referring to is not showing in the post. Could you try re-posting the image, maybe as an attachment?

Let me see if I understand your question-- is the following statement your main concern? "If no measures were taken, it would have exceeded about 0.5V at room temperature with n = 1."

Are you observing that the voltages at CPU1 and CPU2 are 0.5V above the expected voltages (3V, 1.8V)?

Could you help me understand your 2nd question: are you asking about the time taken to switch from NO to NC? Or, are you asking if there is any delay from input to output of a channel, for example COM to NC?

Best regards,
Kate

Hello, Kate.

Thank you for your quick response.

sorry. I'll send you the file again.

My concern is the cause of this phenomenon and how to deal with it.

The following is just an explanation of this phenomenon.

"If no measures were taken, it would have exceeded about 0.5V at room temperature with n = 1."

The second question I wanted to know was about the time taken to switch from NO to NC.

Best regards.

Hello,

Thank you very much for the image block diagram, this is very helpful for me to understand your system. 

I want to double check that I understand before I move forward with helping you troubleshoot. VCC is 3.3V and SEL voltage levels are 0V/3.3V. You are observing nominal voltage at NO, 3.3V, when switch is connecting COM to NO. Where you have a concern is when COM is switched to NC and the transient voltage at NC is higher than 1.8V (by 0.1 to 0.5V, depending on the presence of R). Is this correct?

Have you measured voltage at the COM node (at thermistor) during all cases? And, thermistor is set to 100kohm during all cases?

Regarding switching time from NO to NC, I would like to point out a few things from the datasheet. This device has the break-before-make feature, which ensures that NO and NC will never be shorted to COM at the same time. This means that there will always be a minimum switching time from NO/NC-- characterized by tBBM in the datasheet. Typical tBBM for this device is 8 ns. Because there is no enable pin in this device (only SEL, so the switch cannot be set to "high-Z"), the timing specifications tON and tOFF characterize the maximum switching time from NC/NO (70 ns) or NO/NC (75 ns). 

Best regards,
Kate

Hello, Kate.

Thank you for your response.

>I want to double check that I understand before I move forward with helping you troubleshoot. VCC is 3.3V and SEL voltage levels are 0V/3.3V. You are observing nominal voltage at NO, 3.3V, when switch is
>connecting COM to NO. Where you have a concern is when COM is switched to NC and the transient voltage at NC is higher than 1.8V (by 0.1 to 0.5V, depending on the presence of R). Is this correct?
Yes. Your perception is correct.

>Have you measured voltage at the COM node (at thermistor) during all cases? And, thermistor is set to 100kohm during all cases?
Wrong. The resistance value of the thermistor changes depending on the ambient temperature.100kΩ is an example.
10 kΩ at 25 ° C. The resistance value drops at high temperatures. The resistance value increases at low temperatures.

>Regarding switching time from NO to NC, I would like to point out a few things from the datasheet. This device has the break-before-make feature, which ensures that NO and NC will never be shorted to COM at the
>same time. This means that there will always be a minimum switching time from NO/NC-- characterized by tBBM in the datasheet. Typical tBBM for this device is 8 ns. Because there is no enable pin in this
>device(only SEL, so the switch cannot be set to "high-Z"), the timing specifications tON and tOFF characterize the maximum switching time from NC/NO (70 ns) or NO/NC (75 ns).
I understand that it doesn't turn on at the same time. Then, is the cause of this time the input capacity of COM? (115pF)
In the above case, what is the maximum input capacity of COM according to the design value?
Is this method correct? (Add resistor)

Best regards.

Hello,

Thanks for the additional information. I have one more question-- what is the purpose of the two 1000pF capacitors in your system block diagram? Are these intended to be decoupling capacitors for the voltage sources or serving a different purpose?

Here are my thoughts on the addition of the resistor R: By adding R, you are creating a parallel resistor network (R in parallel with the thermistor + Ron of the switch). The resistor R is decreasing the equivalent resistance of the switch network and potentially undermining the effect of the thermistor in your system and therefore lowering the node to your expected voltage. This leads me to believe that there is an issue elsewhere in the system, likely with the thermistor/system temperature, that is causing the unexpected behavior, this is something you may want to investigate. The Ron flatness of the switch is very small, it is unlikely that the switch is a factor in this behavior, unless absolute maximum ratings are not being observed, such as excessive input/output current. Have you measured the current through your switch/resistor network?

The on-capacitance specification provided for input/output COM is 115pF, which is a factor in turn-on/off time. I recommend that you watch this TI Precision Labs video to learn more about switch on-capacitance and its effect on switch timing.

Best regards,
Kate

Hello, Kate.

Thank you for your response.

1000pF has two purposes. One is noise suppression. The other is to reduce the impact of the input capacity of the A/D port.

By the way, is there a capacity limit that can be attached to the outside?

I think that R is not the cause because there is a problem even if I do not add R.

I am sorry. Therefore, the current is not measured.

In addition, the input resistance of A / D is large enough, R does not affect the measurement results.

I will send you the cause that I am assuming here by attachment now, so please confirm the contents.

I have an additional question.

When I look at the data sheet, I can read it as follows, but what about it?

Best Regards.

Hello, Kate.

Thank you for your response.

Immediately I see TMUX1237 .

By the way, "The Break Before Make delay occurs at every switching event, meaning it is triggered by a change in control signal. "

means that it will be turned on at the same time during the green square in the figure below?

(It doesn't occur with tbbm >> toff.)

Best regards.

Hello, Kate.

Thank you for  the detailed explanation.

At last, I could understand the internal operation of this IC.

But the ”a faster discharge” you recommend is not possible for me.

The reason is that the resistance value of the thermistor changes greatly depending on the operating temperature.

In addition, in our usage this time, the port may be switched even when the thermistor is not connected.

As I asked before, is it difficult to provide a design value for the COM input capacity?

Best regards.

Hello, Kate.

Thank you for your response.

I know it's 115pF(typ).

I would like to know the maximum input capacity, please tell me.

If it's difficult to answer, just tell me the order?  2x, 3x, 5x?

In addition, is the input capacity of each port for each individual almost equal? Or do they change apart?

Do both of the following two examples exist?

example 1 : COM=100pF, NC=105pF, NO=95pF

example 2 : COM=150pF, NC=85pF, NO=110pF

Best regards.

Hello, Kate.

Thank you for your response. And I'm sorry for the late thank you.

I understand the contents of the answer.

Doesn't it mean that you don't recommend using anything other than the datasheet conditions?

Best regards.