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  • TI Thinks Resolved

TS3A4751: off-isolation issue with 1Mhz signal

Expert 5130 points

Replies: 3

Views: 370

Part Number: TS3A4751

Hello team,

My customer has met some issue using TS3A4751. The simplified circuit is as following:

All the NOx are connected together and pulled up to 3.3V through a 100Ohm resistor.

Below are the waveforms that captured with all INx pulled to low. CH1 is NO1, CH2 is COM1.

Normally, all COMx should be low as all INx are low no mater what signal is on NOx.

Using a larger pull up resistor, like 1kOhm will have lower COM output but customer can only accept 100Ohm pull up due to the heavy capacitive load.

Could you help to advise what may cause this issue?

Thanks and best regards,

Victor

  • Victor,

    Placing the control logic pins at ground logic low will create a high impedance path between COM and NO. This doesn't automatically mean that COM will go to ground unless you have pull down resistors on the COM side of the circuit. Can you give more clarity on what is attached to the COM side of the circuit?

    In the first wave form it is difficult to understand what is happening on the COM pin without knowing what is attached to that side of the circuit. Are you able to show what is on the other side of the COM pins? I can't tell what is causing the constant 1V then slow rise to 3.3 V and then drop voltage down.

    Are the IN pins in a transient state in the scope captures or are they grounded through the whole duration of the scope capture?

    Thank you,
    Adam
  • In reply to Adam Torma:

    Hi Adam,

    According to customer, the test is down with COM floating(nothing is connected to COM). In real application, COM will connect to a BUS.
    For the IN pins, they are controlled by MCU I/O and kept low during the whole test.

    Best regards,
    Victor

  • In reply to Victor Chen_HK:

    Victor, 

    If the COM node is floating it is hard to predict the behavior.  It looks like there is some paracitic leakage current passing through the switch and slowly charging up the floating node and then a slow discharge when the signal stops.   

    What is the result if you have a weak pull up or pull down on the COM pin instead of it being floating?  I suspect the bus will remain high or low and not be influenced by the MCU signal when the TS3A4751 is Hi-Z.   

    Thank you, 

    Adam

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