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SN75178B: Current Consumption

SS
Prodigy 230 points
Part Number: SN75178B
Other Parts Discussed in Thread: SN75176B, THVD1500, THVD1551, , THVD1550

Hi,

We are planning to add a buffer for our Motor Encoder RS-422 signals. I am calculating maximum current consumption by this IC on the board. It's mentioned in the datasheet that supply current (No Load) when the output is enabled max current would be 70mA. In our application output of this buffer would go to a controller with 120 ohms termination resistor. I am assuming output current would be roughly 20-25 mA. 

My question is: should I add the output load current to the supply current for total consumption? In my case, would it be around 70+25 = 95 mA?

Thanks.

--Saumya

  • 0
    TI__Guru 62920 points
    Hi Saumya,

    Yes, that is correct - you would add the expected load current to the "no load" ICC specification of the device to get the total current drawn from the supply.

    Note that newer devices like THVD1500 are footprint-compatible to SN75176B but have substantially lower supply currents. (There is nothing wrong with SN75176B and we do not have plans to obsolete it, but it was released 34 years ago and so I wanted to point out that some improvements have been made in the meantime.)

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 230 points

    Thanks Max for your response. I referred to part number THVD1500, but it's a transceiver to in order to make a buffer I would need to use 2 of these ICs right? I have space constraint on the board that's why I picked SN75176B.

    Also, I checked on the datasheet but just wanted to confirm that SN75176B only comes in PDIP package right? The surface mount option looks like discontinued by TI.

    Regards.
    --Saumya

  • 0 in reply to SS
    TI__Guru 62920 points
    Hi Saumya,

    I believe SN75176B should still be available in an SOIC (surface mount) package. Where are you seeing that it is obsolete?

    Both SN75176B and THVD1500 are transceiver devices, and so in either case multiple chips would be needed to realize a repeater function.

    Does your application only involve communication of data in a single direction (i.e., a "simplex" link)? If so, then a simpler solution may be to use a full-duplex transceiver like THVD1551. You could then use a single IC, receiving the differential signal on the A/B lines, outputting it on R, tie R to D externally, and then transmit the same received signal out on the Y/Z lines.

    If this is an application involving two-way communication, the same THVD1551-based solution could be used as long as separate differential pairs are used for transmit and receive functions. If these are shared across a single differential pair (i.e., a half-duplex link), then the design of a repeater would get a little more complicated since you would need a timing circuit to help control the driver/receiver enable lines properly to avoid having multiple transmitters active on a common bus simultaneously.

    Please let me know if this is unclear or if you have any further questions.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 230 points
    Hi Max,

    Sorry I meant to say SN75178B surface mount option.

    Yes my application only involves unidirectional communication: encoder > buffer > controller. That's a good suggestion, I can definitely use THVD1551 for my application and tie R to D together to make a buffer. Plus it's much more less power hungry compare to SN75178B.

    My only other concern is what would be the maximum differential output voltage? I can see in the datasheet that minimum is 2V, I am guessing max would be equal to my supply voltage.

    Thank you so much.
    --Saumya
  • 0 in reply to SS
    TI__Guru 62920 points
    Saumya,

    Yes, you are right then that SN75178B is only available in PDIP at the moment. The maximum differential output voltage would indeed be equal to VCC (the device doesn't have the ability to generate larger voltages).

    Max
  • 0 in reply to Miguel Robertson
    Prodigy 230 points
    Thanks Max for your help.
  • 0 in reply to SS
    Prodigy 230 points

    Hello,

    For part THVD1551, is there a way to adjust the driver differential output voltage (VOD) magnitude? Currently it's mentioned as 2V minimum and 3V typical. I have 120 ohms fixed termination resistor for RS-422 lines on the controller side of the circuit. 

    Thanks.

    --Saumya

  • 0 in reply to SS
    TI__Guru 62920 points
    Saumya,

    The VOD will be a function of the output load current, which will vary based on the load resistance. You can reference Figure 2 in the datasheet to see how these parameters relate to one another. Note that this is the typical behavior - there will be some variation with temperature/supply voltage/etc.

    Regards,
    Max
  • 0 in reply to Miguel Robertson
    Prodigy 230 points

    Hi Max,

    If I use THVD1551 as a buffer as we discussed earlier by connecting R and D pins externally, what would be the maximum current consumption(supply current+load current) by the IC? 

    Thanks,

    Saumya

  • 0 in reply to SS
    TI__Guru 62920 points
    Saumya,

    It will be similar to what is shown in Figure 9 of the datasheet (THVD1551 and THVD1550 are based on the same design). Note that the majority of this current would be due to the output loading; the device's "unloaded" supply current would be at most 1 mA.

    Max
  • 0 in reply to Miguel Robertson
    Prodigy 230 points
    Thanks Max.

    I have a load resistor of 120 ohms on the controller side, how would it affect the supply current graph shown in figure 9? I am assuming current consumption is going to reduce by some amount.

    Regards.
    Saumya
  • 0 in reply to SS
    TI__Guru 62920 points
    Saumya,

    That's right, higher load resistance would mean lower load current and therefore lower supply current. You can get an idea of the effect of load resistance on current by referencing Figure 2. It gives a curve of output voltage versus output current, but this can be translated to output current versus load resistance using Ohm's law. In this case a 120-Ohm load would give a load current around 25 mA and a 54-Ohm load would give a load current around 50 mA. Therefore, you could expect the curve given in Figure 9 to shift down by about 25 mA. The slope of the curve will remain similar, though, since the increased current at higher data rates is more a function of load capacitance than resistance. You can find a good summary of the factors influencing transceiver power consumption here:

    e2e.ti.com/.../how-to-calculate-the-power-dissipation-of-high-speed-rs-485-transceivers

    Best regards,
    Max