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THVD1452: Power dissipation

Gertjan Maenhout
Prodigy 50 points
Part Number: THVD1452


Tool/software:

Dear TI,

Paragraph 7.6 outlines the power dissipation of the THVD145x devices at 5Vcc and 50Mbps signaling rate. However, I am wondering if there is more information available on the power dissipation at 3.3Vcc and for lower signaling rates. Or can I use assumptions that the power will scale with the voltage and the switching frequency for a specific line capacitance?

Kind regards,

Gertjan

  • 0
    TI__Guru* 77651 points

    Hi Gertjan,

    Power estimations are kind of system dependent. 

    I don't think the power numbers in the datasheet can be used for 3.3V as the FET resistance (Ron) will shift a bit compared to 5V.

    For lower signaling rates, the power will likely be lower as well (this would be a linear relationship).

    When I do power estimates I typically calculate 3 sources.

    1) Supply power: P = Icc x Vcc

    This one has the least impact in the system since the other two sources are generally much larger.

    2) DC power from drivers: P = Vcc x I_load

    I_load = VoD / R_term_eq

    VoD is in the datasheet for 5V but for 3.3V I would estimate maybe around 2.4V as a max

    R_term_eq is the equavalent termination resistance. In most systems this is two 120 ohm resistors in parallel or 60 ohm equivalent. You can use 54 ohm as the worst case value if we assume resistance tolerance of 10%. 

    3) AC power from switching losses:  VoD^2 x C_bus x frequency

    Cbus will depend on the cables you use but in general I usually assume 50pF/m (this is cat5 standard for cables)

    Note that frequency will be the datarate divided by 2 (example 2Mbps = 1Mhz)

    This is the power that I believe is more system dependent since C_bus and frequency vary more in each system/design.

    You can add the three power sources together to get an estimate on the total power of an RS485 system/bus.

    If you want to calculate the power across the device, you will need to go back to equation 2 and do Vcc-VoD to get the power across the RS485 driver (instead of the overall system). 

    -Bobby