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TXS0108E: drive optocoupler

Shai Berman
Mastermind 7400 points
Part Number: TXS0108E
Other Parts Discussed in Thread: SN74LVC8T245, SN74HC244, SN74AHC244, TXB0108, LSF0108

Hi Team,

I try to drive optocoupler directly from device output B.

The optocoupler connected to 5V and 270 Ohm resistor to the  TXS0108EPWR output.

When I try to drive '0' to the TXS0108EPWR output the output low level is ~2.5V instead of ~0.5V as expected.

The current I expected to sink into the TXS0108EPWR output is -12mA.

 

Please advise.

Thanks in advance,

Shai.

  • 0
    TI__Guru 56100 points
    Hi Shai,

    The TXS0108 (or TXS family) is not a active buffered device with current drive. It has one-shot edge accelerator which provides AC drive during the rise/fall edges, however , it cannot provide a constant DC drive as your application needs currently (12ma).
    If a true buffer is needed, i would suggest using SN74LVC8T245 which can sink in upto 32mA at 5V.
    SN74AHC244 or SN74HC244 are unidirectional buffers for 5V operation but provides much lesser current drive of ~8mA.
  • 0 in reply to ShreyasRao
    Mastermind 7400 points
    Hi Shreyas,

    Thanks for the information but it can't help because I already have board and thus products are not pin compatible.
    However I found TXB0108 that is pin compatible to the TXS0108 but I did not get any information about the current drive capability of the TXB0108.

    Please advise.
    Thanks in advance.
    Shai
  • 0 in reply to Shai Berman
    TI__Guru 56100 points
    Shai,

    TXB0108 can provide about 20uA of drive only which is not sufficient for your application.
  • 0 in reply to ShreyasRao
    Mastermind 7400 points
    Shreyas,

    Can you please confirm if the LSF0108 (www.ti.com/.../lsf0108.pdf) can sink 10mA from optocoupler?

    Thanks
    Shai
  • 0 in reply to Shai Berman
    TI__Guru 56100 points
    Hi Shai,

    LSF is also passive translation device with internal FET just like the TXS.
    The LSF is capable of passing through up to 64mA current, however,the sinking current capability is more of a question to the host device driving it. If the host is capable of sinking the 10mA of current and still be able to maintain the output Vol below the spec threshold, then you can sink in 10mA through the LSF.