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HD3SS460: Common mode voltage and caps on sstx and ssrx pins

Part Number: HD3SS460

Hello. I can`t figure out should i place RC-network in SSRX and SSTX lines between MUX and HOST(on same pcb).  In my case host is IMX8M processor, in documentation they say it meets usb 3.0 rev 1 specification, how can i check that DC common-mode voltage does not out of range for your MUX? In USB specification rev. 1 i found only one table (page 6-21), that include parameter "Transmitter common-mode voltage" with range of 0 V (min) to 2,2 V (max).

  • Hi,

    If you have a topology of IMX8M -> HD3SS460 -> Connector, the AC coupling capacitor should be placed between the HD3SS460 and the connector with required 0.22uF on the SSTX and optional 0.33uF on the SSRX.

    You would then need to check the common mode voltage of IMX8M. If the common mode voltage of the IMX8M is within the HD3SS460 spec, then you just DC-couple between the IMX8M and the HD3SS460. 

    If the common mode voltage is outside the HD3SS460 spec, then you need to AC-couple between the IMX8M and the HD3SS460 and then use an external termination network, placed between the AC coupling capacitor and the HD3SS460 to set the HD3SS460 common mode voltage. The external termination network voltage must be within the HD3SS460 common mode voltage requirement.

    Thanks
    David

  • Hello. Why exactly this values? In usb spec, HD3SS460 datsheet, 75 nF (min) and 200 nF(max). And why i should set common-mode voltage between host tx line capacitor  and  HD3SS460, if it simply passive switch? Thanks!

  • HI,

    The AC coupling cap value came from the USB3.2 spec. 

    HD3SS460 does not provide common mode biasing for the channel. Therefore it is required that the device is biased from either side for all active channels.The HD3SS460 has a common mode voltage requirement of min 0V and max 2V. If it is AC coupled on one side, then its common mode voltage is biased on the other side assuming the bias voltage is within the spec. If it is AC coupled on both sides, then its common mode voltage is biased by the external bias network as shown in Figure 5 of the datasheet.

    Thanks

    David 

  • I`m very grateful for your help. I don`t clearly understand how biasing network in Figure 5 works. Can you please explain me? I thought to use network like in attached picture for 1.65 Vcm.

  • HI,

    You could use the resistor divide network to generate the 1.65V bias voltage and it is still within the HD3SS460 common mode voltage spec. You could also follow the Figure 5 and set the common mode voltage to 0V (pulled to GND).

    But before you add the AC caps and external bias network between the IMX8M and the HD3SS460, please check the IMX8M common mode voltage. You may not need the AC caps and the bias network if the IMX8M can satisfy the HD3SS460 common mode voltage requirement.

    Thanks

    David