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THVD1406: Power Dissipation

Jason Hu27
Intellectual 910 points
Part Number: THVD1406

Tool/software:

Dear Experts, please refer to below spec.  I use LDO to provide power to THVD1406.  LDO input  voltage is 24V and LDO output voltage is 3.3V. If THVD1406 will consume 220mW, Buck is better choice than LDO. 
When RS485 is working, the power dissipation of THVD1406 will reach 220mW (500 kbps)? If I use 3.3V to power the THVD1406, what's current rating of 3.3V power source is suitable?   Best regards.

  • 0
    Guru 280200 points

    The driver's power supply must supply the current that flows through the bus, typically 37 mA, plus about 20 mA for the transceiver itself at the highest speed. To be safe, the supply should be capable of 100 mA.

    When the output is shorted, more current can flow (up to 250 mA). If you do not want the supply to drop out in this case, it must be stronger.

  • 0
    TI__Guru 59030 points

    Hi Jason,

    Clemens is correct. If you want to take in the worst case scenario then you would assume 250mA for a short circuit. This would have about 825mW of power during this case where you wouldn't want the power rail to collapse during the short. 

    If you want to assume no short would ever happen, then I would look at the bottom row of the table you reference. That one shows a terminated line with 54 ohm loading. This would be about 220mW, if you wanted to add margin then multiply it by an extra 25% or whatever margin of safety you want to consider.

    -Bobby

  • 0 in reply to BOBBY
    Intellectual 910 points

    Dear Bobby, If bus I/O port A and bus I/O port B are short together or only bus I/O port A short to GND, THVD1406 will restrict the output short current to 250mA? Best regards.

  • 0 in reply to Jason Hu27
    Guru 280200 points

    Yes: the |IOS| specification guarantees that the short-circuit current is no larger than 250 mA.

  • 0 in reply to Jason Hu27
    TI__Guru 59030 points

    Hi Jason,

    Clemens is correct. 

    -Bobby