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DS25BR440 - unused input and output pins

Kawai
Guru 19775 points
Other Parts Discussed in Thread: DS25BR440

Hi Team,

What should we do with the DS25BR440 unused input and output pins ?

I believe you could leave both input and output pins floating, however, I am wondering if this device have failsafe circuit or not.

What is your recommendation ?

Best Regards,

Kawai

  • 0
    TI__Mastermind 29635 points

    Hi Kawai-san,

    Please see the following related post: 

    The device does not appear to have an internal fail safe circuit, so an external failsafe bias circuit is recommended to prevent the output from oscillating unexpectedly. Please reference the following recommendation from Lee:

    "To provide for some external failsafe it is possible to use a 4.7K pullup and a 2.2K pulldown.  This will setup a 50 mV offset on the input pins.  This offset will hold the DS25BR440 in a known state if the input signal is disconnected or muted."

    Thanks,

    Michael

  • 0
    TI__Expert 3175 points

    Hi Kawai-san,

    Lee is on PTO, he will know the detail on the DS25BR440.

    Looking at the DS, I did not see referernce to fail-safe, so I have to assume it does not have this feature. With this, I recommend to have a R to VCC at +input, and a R to GNd at the - input for un-used LVDS input.

    The LVDS output can be left unconnected if they are not used.

    Regards,

    TK Chin

  • 0 in reply to T.K. Chin
    Guru 19775 points
    Hello Michael-san, TK-san,

    We greatly appreciate your cooperation. I apologize that I had missed Lee-san's post.
    I will ask customer to use 4.7k pull up and 2.2k pull down.

    It would be nice to have recommendation how to treat the unused pins on the datasheet.

    Best Regards,
    Kawai
  • 0 in reply to Kawai
    TI__Expert 3175 points
    Kawai-san,
    Good suggestion. For new DS or upgrading DS, we will put in more info on how to handle un-used pins.
    Regards,TK Chin
  • 0 in reply to T.K. Chin
    Guru 11235 points
    Hi Michael-san, TK-san,

    About this question and answer, I add a question.
    Could you please answer me?

    Add Q1: My customer's situation is VDD=3.3V. But He wants to pull-up 5V.
    Can we use a R to 5V at +input, and a R to GND at -input for un-used LVDS input pin at VDD=3.3V?
    How much value of resistor can we use as pull up/down at 5V?
    * GND of 5V is the same as GND of VDD(=3.3V).

    Add Q2: I think it is not important resistor value and the voltage of pull-up, it is important a 50 mV offset on the input pins.
    is it correct?

    Best regards,
    Shimizu
  • 0 in reply to masahiko shimizu
    TI__Mastermind 43795 points

    Hi Shimizu-san,

    It is possible to use a pull-up to +5V on the DS25BR440 IN+ as long as the IN- is also connected to GND with a similar resistor.  This will limit the maximum positive voltage at the input pin to < 3.6V.

    The DS25BR440 input current specification has a maximum leakage current of 10uA for input voltages less than 3.6V.  I would choose a resistor combination which can easily overcome this level of input leakage current and create a 50mV offset across the input differential pair.  Using 4.7K pull-up and down would give just over 50uA of current and bias the input with ~ 50mV of offset.

    I agree with your Q2, the bias needs to be enough to achieve a stable output condition.

    Regards,

    Lee

  • 0 in reply to Lee Sledjeski
    Guru 11235 points
    Hi Lee-san,

    Thank you for your support.
    I understand it.

    Thanks,

    Best regards,
    Shimizu