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SN74CB3Q3306A: Use of this switch to connect/not connect RS485 termination

Part Number: SN74CB3Q3306A
Other Parts Discussed in Thread: TS5A3166, , TS5A3167

I am designing a system with an RS485 bus. The bus is <1 m but long enough to require termination in both ends. At one end of the bus there is a main module that always has a fixed termination resistor. Up to 10 expansion modules are then connected to the main module and in the module furthest away from the main module I want a termination resistor to be switched in.

The data rate is 7.5 Mbps and the rise/fall times of the selected transceiver is min 3 ns and max 20 ns. The transceivers are supplied by 3.3 V. The main module and expansion modules share a common ground.

I saw in the thread below that someone was planning to use TS5A3166 for this but I think it is not suitable for my application. This since losing power to one expansion module leads to loss of power to the TS5A3166 which seems to lead to that the TS5A3166 is not high impedance

https://e2e.ti.com/support/switches-multiplexers/f/388/t/509978

Is the SN74CB3Q3306A suitable for the application described above or do you propose some other circuit?

  • Yes, the SN74CB3Q3306A or any other SPST switch with powered-off-protection is suitable.

    If you do not need to disconnect both ends of the resistor, you can use a single-channel switch.

  • Hi John,

    As Clemens mentioned, the SN74CB3Q3306A suitable for your application. Please let us know if you have any further questions.

    Regards,

    Saminah

  • Thank you Clemens and Saminah,

    I do not plan to disconnect the termination resistor in both ends. I was planning to connect the two switches in SN74CB3Q3306A in parallel in order to lower the resistance. 

    I have looked at switches with "only" power-off protection, e.g. TS5A3167. It would of course be interesting to use TS5A3167 instead of SN74CB3Q3306A since TS5A3167 is smaller, cheaper and has lower ON-resistance than the SN74CB3Q3306A.

    In the datasheet for TS5A3167 I see the following:

    - Chapter 6.1 "Absolute Maximum Ratings": VNC, VCOM "Analog voltage range" -0.5 to VCC+0.5
    - Chapter 6.3 "Recommended Operating Condictions": NC, VCOM "Analog voltage range" 0 to VCC
    - Chapter 6.7 "Electrical Characteristics for 3.3-V Supply": INC(PWROFF) = ICOM(PWROFF)  is specified to max +/-25 uA

    The above seems slightly "inconsistent", i.e. in chapter 6.7 a voltage of VCC+3.6 V is applied and this is not allowed according to chapter 6.1 or chapter 6.2 

    Is a voltage of VCC+3.6 on NC or COM pin allowed only when VCC = 0 V?
    What happens in this case if VCC = 1 V or in a transition to VCC = 0 V?

    Perhaps the requirements in chapters 6.1/6.2 is only applicable when the switch is ON?
    What happens in that case if the switch is ON and then power is lost to the unit this switch is located in and power is still available on NC and/or COM in the transition to VCC = 0 V?

  • Thank you Clemens for your answer. I added a longer comment to the post from Saminah.

    Best Regards

    John

  • Hi John,

    The Absolute Maximum Ratings and the Recommended Operating Conditions analog voltage range is specifying what input signal is supported by the switch when a VCC is applied to the switch and highlighting what is the absolute max input voltage the switch can take and what is recommended when the switch is ON.

    The I(PWROFF) spec is specifically for the switches that support powered-off protection. Meaning when VCC=0V, there will be high impedance on the I/O pins and logic pins to protect the system from damage. 

    The Table 6.7 with INC(PWROFF) and  ICOM(PWROFF)  specifies +/-25 uA leakage current with Vcc = 0V showing that the I/O signal path is Hi-Z when a voltage from 0V-3.6V is applied to NC or COM at VCC=0.

    Here is a link to an FAQ that explains powered-off protection with Ioff spec.

    If the switch is ON ans power is lost, as soon as VCC hits 0V, a high impedance (Hi-Z) path is created between the input and output (I/O) signal paths and system power rails.  This prevents back-powering conditions when the voltage on the I/O signal paths are greater than the supply rail (VCC).

    Hope this explains your questions.

    Thanks

    Saminah