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THVD1449: Clarity required on the application curves

Anway Das
Prodigy 40 points

Part Number: THVD1449

What is the peak to peak amplitude of the differential voltage for the differential output of the RS485 driver ? Do I follow C2 or C4 in the attached image ? If I follow C2 , I see the drive strength is directly proportional to supply voltage . But on the text , it shows that Channel 4 is the differential output voltage.

Could you please help with this.

Regards..

image.png

  • 0
    TI__Mastermind 30542 points

    Hi Anway,

    Channel 4 is the VOD. You can tell this because the voltage swings positive and negative. Channel 2 is the D input, where it goes either 0-5V or 0-3.3V depending on the transceiver supply voltage. Channel 1 is the bus output read back to the RXD pin.

    With VCC=3.3V, there will be a slight decrease in the VOD output compared to when VCC=5V. However, I would not rely on these graphs that much as they are under ideal testing conditions (no cable harness, ideal termination, no other RS-485 nodes, etc). The difference between 5V and 3.3V VOD does show up as much under ideal conditions. When designing with RS-485, we always recommend testing a transceiver in a real system to understand its performance. 

    Best,

    Ethan

  • 0 in reply to Ethan Sempsrott
    Prodigy 40 points

    Hi Ethan,

    Thanks for the reply , but here is my concern :

    As per Fig 6.3  Driver Output Current vs Supply Voltage curves , with 54 ohm loading at Vcc = 5V , I should expect a drive current of around 57mA .

    As per Fig 6-2. Driver Differential Output voltage vs Driver Output Current , at 57mA of loading I should expect a output differential voltage of around 3.3V

    Now in the application waveforms I see that the output differential for 54 ohms is just 2.168V.

    Could you help me understand ....

    I have referred the figures from THVD1449 datasheet.

    Regards,

  • 0 in reply to Anway Das
    TI__Mastermind 30542 points

    Hi Anway,

    Please refer to this other E2E thread:  SN75176B Driver Output Capability of 60mA max 

    Best,

    Ethan