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SN65LBC172A Drive Low

Adrian Soltero
Intellectual 1135 points
Other Parts Discussed in Thread: SN65LBC172A, AM26C31, SN74HC367

Hi,

 

I am using the SN65LBC172A in a single ended configuration to drive another logic device.  I have a 5k pullup resistor to pull the line up when it isn't being used.  When a logic low is provided to the SN65LBC172A it only drives the output down to 1.2V and not to 0V. 

Does the device not have enough of a current drive in single ended mode to pull down the output with the pullup resistor?

 

Thanks,

Adrian

  • 0
    Mastermind 7395 points

    This device is a differential driver, how did you connect in single ended configuration? What did you do with the negative output pin? And what did you do with the enable pin (G) ?

  • 0 in reply to Avi Chami1
    TI__Intellectual 1135 points

    The device is always enabled and the negative pin is floating.  Only the positive output pin is used.  It works without the pull up resistor, but with it, it cant pull all the way low which makes me think that it has something to do with the output current. 

    Is there a more ideal way to use the device in a single ended configuration?

  • 0 in reply to Adrian Soltero
    TI__Genius 14015 points

    Hi Adrian,

    I don't believe this is an output current issue. At the output stage, there is a voltage drop across the low-side BJT as well as a diode. The output won't ever get down to 0V. Since this is a differential device, that doesn't normally matter. All that is required is about 1.5V between the outputs, which leaves ~3.5V of headroom between the rails. 

    Is there a reason they are using an RS-485 driver in a single-ended, essentially buffer application? They might be better off with an actual TTL buffer, or even a simple single-transistor inverter!

    Regards,
    Jason Blackman 

  • 0 in reply to Adrian Soltero
    TI__Mastermind 28265 points

    A differential output should always be loaded symmetrically even if you use only one output.. Since you already connected the non-inverting output via 5k to Vcc, connected the "non-used" inverting output via 5k to GND.

    grretings, Thomas

  • 0 in reply to Thomas Kugelstadt
    TI__Intellectual 1135 points

    Thanks Jason and Thomas,

    Will loading the "non-used" output allow the non-inverting output to drive low to 0V?

     

  • 0 in reply to Adrian Soltero
    TI__Genius 14015 points

    Hi Adrian,

    I just spoke with Thomas, and the symmetric loading that he suggested is just for good analog practices.

    My thinking is that it may help get closer to 0V, but there is still that diode drop which will prevent the output from going lower than ~0.7V. 

    It is possible to use a differential receiver to receive single-ended data, which we have discussed previously through email. I think for transmitting single-ended data, however, it is probably best to use a device meant for single-ended transmission. That way, the levels will be guaranteed in terms of absolute voltage, rather than just differential voltage.

    Are you locked into using this device already? Or are you working on a new design and open to suggestion? 

    Regards,
    Jason Blackman 

  • 0 in reply to Jason Blackman
    TI__Intellectual 1135 points

    Jason,

     

    It seems like an output stage limitation, but do you have any other suggestions to get the SN65LBC172A to drive low with the pullup resistor on the line?

    Do you have any recommendations that can drive a single ended line with a pullup resistor and have the 16-DW package (it doesn't necessarily have to be a single ended driver)?  The AM26C31 was looked at, since it seems like the output stage can drive to 0 V, but the package is too narrow. 

     

    Thanks,

    Adrian 

  • 0 in reply to Adrian Soltero
    TI__Genius 14015 points

    Adrian,

    I don't see any differential parts that will meet your needs, because all differential drivers will have that output stage limitation. 

    After a quick search of our buffer portfolio, it looks like the SN74HC367 might fit the bill. That being said, you'd have to ask the Logic group any specific device questions relating to that part.

    Thanks,
    Jason Blackman