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Original question:

TS3A27518E: Unused pin connections

TS3A27518E: Unused pin connection when SD signal doesn't transmit

Shinichi Inoue
Guru 20161 points
Part Number: TS3A27518E

Tool/software:

Dear Specialists,

The customer is considering to use TS3A27518E for ESD card switch and has a question.

I would be grateful if you could advise.

---

We are considering using the TS3A27518E as an SD card switch.

When connected NC pins (pull-down resistors connected), they are not expected that an SD card signals will be transmitted.

According to the datasheet p.20 9.2.2 Detailed Design Procedure, 50ohm is recommended.

But in this case, is it possible to use 1kΩ?

I think that there is no need to use 50Ω, is this understanding correct?

---

I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    Guru 267920 points

    The "N.C." pin does not need to be connected, or can be grounded directly.

    Termination of unused I/O pins (COMx, NCx, NOx) with 50 Ω would be recommended only if there are high-frequency signals. But are these pins really unused? Please show a schematic.

  • 0 in reply to Clemens Ladisch
    Guru 20161 points

    Hi Clemens,

    Thank you for your reply.

    I attached the customer's circuit.

    The power is OFF, these are connected to NC1-NC6 and GND via1kohm resistor.

    The power is ON, it is connected to NO1-NO6 for SD card interface.

    In this case, are these possible to use 1k ohm resistor?

    Question about TS3A27518E.pdf

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 267920 points

    50 Ω is recommended only for unused pins. But these pins are not unused; they are connected to COMx and the processor. You can use 1 kΩ or any other value required.

    But what exactly do you mean with "power off"? When the TS3A27518E does not have VDD power, then it will not work at all. I hope you mean that VDD stays on, and that the power to the SD card is disconnected.

  • 0 in reply to Clemens Ladisch
    Guru 20161 points

    Hi Clemens,

    Thank you for your reply.

    I understand.

    I shared your suggestion with the customer.

    The pull-up power supply of the SD card's signal line power on before the SD card's VDD power up, so the customer is considering using TS3A27518E as a workaround.

    I have an additional question,

    Clemens Ladisch said:
    Termination of unused I/O pins (COMx, NCx, NOx) with 50 Ω would be recommended only if there are high-frequency signals.

    Could you please let me know why 50Ω resistor needed. 

    What would happen if there was no 50 Ω resistor?

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi

  • 0 in reply to Shinichi Inoue
    Guru 267920 points

    An unused pin might act as an antenna for certain frequencies. 50 Ω matches the typical trace impedance and terminates the line without reflections.