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TS5A3166-Q1: RS485 termination using Analog Switch

uri itzack68
Prodigy 50 points
Part Number: TS5A3166-Q1
Other Parts Discussed in Thread: THVD1419, SN74CB3Q3306A, SN3257-Q1

Hi All,

 

I'm planning to design a system with RS485 transceiver (THVD1419), and want feature of turning ON/OFF the termination through micro-controller.

As of now I don’t see any part in that would meet this requirement.

 

The nodes of RS485 share common ground, so i'm planning to use TS5A3166 as shown below:

 

 

The differential output A & B of RS485 would swing rail to rail (0 to 5V) when not terminated and swing 2v around the common node of 2.5v when terminated.

My switch control signal is 3.3V rated and i'm going to supply  TS5A3166 with 3.3V as well.

Do you see any concern of using the analog switch for this application? Please do suggest

Thanks alot,

Uri

  • 0
    TI__Mastermind 43295 points

    Hi Uri,

    Do you know what the voltage range will be for your system from terminal B to GND and terminal A to GND. The switch itself doesn't care too much about the voltage across it, unless it is large enough for an over-current event, which in this application isn't my biggest concern. The rated voltages come down the voltage at the input pin with respect to GND. 

    Please let me know so I can see what the best option for this application is.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 50 points

    Hi Parker,

    voltage range in the system from terminal B to GND and terminal A to GND will be 5V.

    will also use only 120ohm termination

    Best,

    Uri Itzack

  • 0 in reply to uri itzack68
    TI__Mastermind 43295 points

    Hi Uri,

    The only concern I have is that this part will most likely sustain damage when the termination is unconnected. We will have a max of 4.5V on one of the input pins. If you could increase the power supply to at least 4.5V but up to 5.5V this part can be used in this application. 

    If that is not possible we may have some other solutions but they will be a bigger package size - please let me know which option you would like to pursue. Parts such as  the SN74CB3Q3306A would have an extra channel but can handle five volt signals using a 3.3V supply.

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    Prodigy 50 points

    Hi Parker,

    In my application I don't have power supply rather than 3.3V.

    will apricate any suggestion you have and will prefer using package small as possible

    Thanks,

    Uri 

  • 0 in reply to uri itzack68
    TI__Mastermind 43295 points

    Hi Uri,

    I have two possible options for you. None of them are the same type of switch as the one you are currently using. Unfortunately the architecture that supports voltage beyond supply typically leads to bigger packages, and unfortunately we don't have any other 1:1 1 channel switches that have this feature. However these two switches can still work for your application.  

    The first is the SN74CB3Q3306A which is a 2 channel 1:1 switch in the VSSOP (DCU) package we have that can meet the applications needs  it is a 6mm^2 package. You can leave the unused channel floating, but high speed operation it is best practice to terminate with a 50 Ohm resistor to ground

    The other option is the SN3257-Q1 is a 4 - channel 2:1 switch and in the SOT-23-THN (DYY) package is only 8mm^2 can can handle voltage above supply rails. You can leave unused pins floating, and leave one of the active switch channels floating to simulate the Hi-Z mode of a 1:1 switch. 

    After that you start getting into some larger package sizes - these are the two smallest that will work. The first one mentioned is the smallest and has the least unnecessary amount of connections. However both of them have slightly higher resistances and may need to account for their increased resistances when selecting your termination resistor. 

    If you have any other questions please just let me know.

    Best,

    Parker Dodson