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PCA9548A: The effective pull-high resistor in cascade PCA9548A topology

Jason Dai
Expert 3720 points
Part Number: PCA9548A

Dear E2E,

I2C signal communicates between CPU and SFP transceiver through 5 pcs of PCA9548A in series as cascade.

They put pull-high circuit on each net from CPU to SFP transceiver.

The spec of SFP transceiver ask the minimum pull-high resistor.

our question is that what the effective pull-high resistor when we stands on the point of SFP transceiver?

i think the effective pull-high resistor is to parallel all of pull-high resistor from CPU to SFP. Am i right?

another question is that I see the internal topology of PCA9548A is passive FET. If so, why do we still need to put pull-high resistor on both input and output side? 

 

BR,

Jason

  • 0
    TI__Guru 60540 points

    Hey Jason,

    "our question is that what the effective pull-high resistor when we stands on the point of SFP transceiver?"

     It would be 1k//4.7k//4.7k//4.7k//4.7k = ~540 ohms when all the MUXs are enabled and the bus is driving low.

    "i think the effective pull-high resistor is to parallel all of pull-high resistor from CPU to SFP. Am i right?"

    When you are driving low, yes. If the MUX uses a pass FET architecture then once the Vgs is less than Vth it then becomes 1k again.

    "another question is that I see the internal topology of PCA9548A is passive FET. If so, why do we still need to put pull-high resistor on both input and output side? "

    When the Vgs is less than Vth of the pass FET, the pass FET will then be in the cut off region of operation. At this point the side which is not driving would just sit at Vgate-Vth so if you didn't have the pull up resistor populated then you may sit mid rail. For example if Vth is 1.3V and you are using a 3.3V Vcc. The secondary channel without a pull up resistor would then sit at around 2V.

    The last thing I notice is that if you keep the 33 ohm resistors, you'll introduce VoL shifts because each one will generate a voltage offset (recall V=I*R where I is the pull down current). This may make VoL larger than ViL if you are not careful.

    -Bobby