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THVD1400: THVD1400 related issues

jeno.zhang
Genius 3205 points

Part Number: THVD1400
Other Parts Discussed in Thread: SN65HVD75

Tool/software:

HI,

The customer uses THVD1400 as the RS485 transceiver. Feedback power consumption is high and cannot meet the demand. Is there a recommended product with better power consumption？

Thanks!

  • 0
    Guru 320330 points

    The quiescent supply current of the THVD1400 is about 1.5 mA. When connected to a bus, the driver must supply the current that flows through the bus and the termination resistors; this cannot be changed by using a different transceiver.

    What exactly is the problem? What are the expected and measured power consumption?

  • 0 in reply to Clemens Ladisch
    TI__Guru 74302 points

    Jeno,

    As Clemens stated, please give more information on the customer's concern. The current consumption in quiescent state cannot be changed, and the bus termination in a operating state is what drives the increased current consumption, as Clemens said. If you need a device with a lower quiescent current, I'd recommend using the parametric search tool for RS485 transceivers on TI.com.

    Regards,

    Eric Hackett 

  • 0 in reply to Eric Hackett
    Genius 3205 points

    Hi，

    Operating status, with data transmission and reception, May I ask which products have lower current consumption?

    Thanks！

  • 0 in reply to jeno.zhang
    Guru 320330 points

    The pin-compatible transceiver with the lowest idle power consumption would be the SN65HVD75. But driving the RS-485 bus requires 40 mA for any transceiver; see figure 7 below:

    If you want a much lower power consumption, you need to use an entirely differect protocol, e.g., LVDS.

  • 0 in reply to jeno.zhang
    TI__Guru* 78111 points

    Hi Jeno,

    As Clemens pointed out, the RS485 standard states that the RS-485 driver must output atleast 1.5V across a 54 ohm load this is approximately 28mA. Most modern day RS 485 drivers will be spec'd be output much higher than 1.5V inorder to meet the standard with margin. Typical VoD values are closer to 2.2V so with a 54 ohm load this would be about 40mA (which is what Clemens' graph points to). The RS-485 transceiver consumption currents are no where near the ~40mA drive that you see when they are outputting. 

    This is why Clemens has pointed to using LVDS which uses much lower driving currents. (You're not going to save much power moving from a RS485 trasnceiver with 1.5mA to one that uses 1mA when the load currents are 40mA+ for each channel).

    -Bobby