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TS3L500AE: DC bias after AC coupling magnetics

Kazuki Itoh
Genius 9865 points
Part Number: TS3L500AE

Hello team,

My customer is considering to use TS3L500AE between AC coupling magnetics and RJ45 connector. The purpose is mechanical relay replacement. 

Please find internal link here for the block diagram. (Please connect to VPN for download).

In previous e2e thread like this, it was mentioned that it needs DC bias on the bus because the switch doesn't support negative voltages.

However, I don't exactly get how to design the DC bias voltage. (IC selection, input voltage source, etc.)

Could you please elaborate on that? If you have a reference schematic or app note, it would be very helpful.

Regards,

Itoh 

 

  • 0
    TI__Intellectual 2694 points

    Hi Itoh-san,

    Just to clarify previous comments, the TS3L500E device will support voltages from 0V to VCC on the switch channels (Ax/Bx/LEDx pins). The allowed supply voltage for this device is 3V to 3.6V which means the device will support signals from 0V to 3V/3.6V. Any voltage outside this will result into damage to the device.

    Most commonly used schematic is --> PHY - SWITCH - MAGNETICS/TRANSFORMER - RJ45 connector. 

    Based on the block diagram you provided, I see that that there are two paths

    1) RJ45/Remote to RJ45/?C --> Blue path through IC

    2) RJ45/Remote to TRNS/PHY --> Through IC

    It is not clear what the GREEN arrow means as based on the block diagram this will never connect. Could you help clarify.

    Also, what is TRNS?

    Thank you.

  • 0 in reply to Sandesh Rawool
    TI__Genius 9865 points

    Hello Sandesh-san,

    My apologies, you are correct.

    Please find corrected schematic from this link. The common-side of the switch is connected to RJ45/Remote.

    TRNS means pulse transformer. It should be same as AC coupling magnetics.

    I think some people discussed similar use case in previous threads, and TI experts mentioned DC bias should be cared.
    But I don't get what they are trying to say exactly. So could you please elaborate on that? 

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 46687 points

    Hi Itoh-San,

    Adding a DC Bias is the idea of signal summation. A quick example can be shown below:

    Signal S has a range of V- to V+; V- < 0V .

    The DC Biased Signal S'  has a range of 0V to V+'. Where V+' = (V+) + | V- | 

    To add a DC bias using a non-inverting summing amplifier can be used:

    the DC bias should satisfy  the following equation, (V- = Signal S Minimum Voltage w.r.t. Ground  0V = DC_Bias + (V-).

    That being said - This part can only accept voltages in-between 0V and VCC; Where VCC is between 3V and 3.6V.

    If the signal with a DC bias goes beyond VCC then this part will not be the best solution.

    If the signals do not go less than 0V and does not go above VCC then no DC bias is needed.

    If you could - please confirm signal  Minimum Voltage and  Maximum Voltage so I can confirm if this part will work - or if a different IC or solution may be required. 

    Please let me know!

    Best,

    Parker Dodson

  • 0 in reply to Parker Dodson
    TI__Genius 9865 points

    Parker-san,

    Thank you so much for your kind explanation!

    Just a quick check, do you think the switch and DC-bias and amplifier power should be generated from isolated power supply in this system?

    Regards,

    Itoh

  • 0 in reply to Kazuki Itoh
    TI__Mastermind 46687 points

    Itoh-san,

    You're welcome, I am glad I could help!

    I think using Isolated power supplies for the amp and switch would be a good choice in this application, since the signal is coming from an isolated source.

    If you have any other questions please let me know!

    Best,

    Parker Dodson