SN74CB3Q3245: Bidirectional communication and short circuit risk

Part Number: SN74CB3Q3245

Hi All,

Customer has an application that uses SN74CB3Q3245, the block diagram is as below. Two slot board, each has a SN74CB3Q3245, and they have a back board, the output of the SN74CB3Q3245 is connected together to control the hard disk power, normally one is as control signal, another is for reading back. Each board can be read board or control board.

So questions are as below,

1) can SN74CB3Q3245 support bi-directional communication?

2) If two board are both configured as control board, one is sending high, and one is sending low, that means one SN74CB3Q3245 is outputing high level, another is SN74CB3Q3245 is outputing low level, and they are connected directly, will this cause damage to the two devices.

Best Regards

Charles Zhang

  • Charles,

    1) can SN74CB3Q3245 support bi-directional communication?

    Yes, all passive signal switches are bi-directional.  You can see more information in the switches and multiplexers FAQ page.

     

    2) If two board are both configured as control board, one is sending high, and one is sending low, that means one SN74CB3Q3245 is outputing high level, another is SN74CB3Q3245 is outputing low level, and they are connected directly, will this cause damage to the two devices.

    The SN74CB3Q3245 is a passive FET switch that can be ideally modeled as a 0 ohm wire.  If you set the OE pin "low" the A port will be connected to the B port like a wire.  If you set the OE pin "high"  the A and B port will be high impedance Hi-Z.  If there is a voltage present on the signal path when the device is Hi-Z there will be a tiny amount of leakage current (<1uA) that is characterized in the datasheet. 


     


    Thank you,

    Adam

     

  • In reply to Adam Torma:

    Hi Adam,

    Thank you for your reply, for the second question, i am not talking about the case by enable of disable OE pin, instead, OE is always enable. In normal operation, one SN74CB3Q3245 is for sending signal from A0 to B0, another is for read back signal from B0 to A0, in this case FPGA pin connected to A0 is input status, which should be high-Z status. But in some cases, both SN74CB3Q3245 are in sending signal from A0 to B0 status, and one is sending high, another is sending low, this works a little like high voltage is shorted directly to low voltage, so we should not deem it as ideal 0ohm resistor, we should consider as a resistor with Rdson value to limit the current. So do you see some risk for the latter applicaiotn? Thanks

    Charles Zhang
  • In reply to Charles-Zhang:

    Charles,

    Thank you for more details to help me understand.  If both switches signal paths are enabled and you place high voltage on one path and low voltage on the other path will not be an issue as long as the current passing through the switches is <64mA. 64mA is the maximum amount of current that can pass through the signal path with damage or reliability risk.   

    Thank you,

    Adam

  • In reply to Adam Torma:

    Hi Adam,

    Thank you for reply.

    According to the datasheet, the Rdson value is about 4ohm, if VDD=3.3v, the current for two device is about 3.3/(4+4)=412.5mA, i think this is overspec. So if we put a series 33ohm resistor for each board before connecting them together, then the current is 3.3/(4+4+33+33)=45mA,  this will overcome the problem, do you see any other risks? Thanks.

    Charles.

  • In reply to Charles-Zhang:

    Charles,

    Placing series resistors to limit the current is a valid solution to keep from violating the abs max current rating for the SN74CB3Q3245

    Thank you,

    Adam