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SN65HVD234: External Biasing at Vdd/2

Antonio Fadhel
Genius 9410 points
Part Number: SN65HVD234
Other Parts Discussed in Thread: TCAN334

Hi Team,

My customer is using the SN65HVD234 on a 3.3V-only application and running into EMI issues due to the high common-mode voltage. They would like to know if they can re-bias to a lower voltage (closer to Vdd/2). Can you please share any suggestions and/or provide details of the internal "bias unit" that we show on the device diagram? 

They can't use the TCAN334 because the +/-14V fault protection is too low.

Thanks,

Antonio

TCAN334

  • 0
    TI__Guru 62910 points
    Hi Antonio,

    Yes, this is possible. The recessive voltage level is weakly biased by the receiver, and you can get an idea of what the circuitry looks like by referencing the CANH/CANL input diagrams in Figure 28 of the datasheet. The resistor network that connects to each CANH/CANL input is what sets the nominal common-mode bias (and also helps to attenuate signals at the pins to a range that is more manageable by the differential receiver). Since the effective impedance of this bias network is fairly high (in the ~30-35 kOhm range), it can be overridden by a stronger external bias.

    Let me know if this isn't clear or if you have any further questions.

    Best regards,
    Max
  • 0 in reply to Miguel Robertson
    TI__Genius 9410 points

    Hi Max,

    Great, thanks for the feedback. They have PCB placeholders for pulldowns on the CAN lines. Do you have a rough idea of the tolerance of the internal bias resistors? Just want to double-check if the absolute value is fairly accurate rather than them just being closely matched ratiometrically.

    Thanks,

    Antonio 

  • 0 in reply to Antonio Fadhel
    TI__Guru 62910 points
    Antonio,

    There's actually a fairly wide range on these. This is encompassed by the "RIN" specification in the datasheet, so you can reference that to get an idea of the worst-case variation. The best strategy would likely be to use the minimum value as a reference and make sure that external biasing is suitably "stronger" (lower impedance) than this minimum.

    Regards,
    Max