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TXB0108: Input driver requirements

K.Hamamoto
Guru 12155 points
Part Number: TXB0108

Hi

I read the datasheet which is mentioned as below,

What happen to output and input if the input current is lower than 2mA?

Also I read the follwing post. However I do not understand why this device need the input current more than 2mA.

( I need some a simple mechanism.)

Our customer asked us that they wonder why you are not specified this current to electrical specification if this is very critical.

We need to talk with our customer about this. So please give some concerns why and what will be happen if the current is lower than 2mA.

 

Regards,

Koji Hamamoto

  • 0
    TI__Mastermind 24975 points
    Hello,

    The appropriate applications contact has been notified of this question, please note today is a US holiday and responses may be delayed through the weekend.

    Best,
    Michael
  • 0 in reply to Michael J Schultis
    TI__Guru 62060 points
    Hi Koji san ,

    The input requirement of 2mA is necessary to overdrive the 4k internal buffer for a transition. The worst case is for a 5.5V supply and multi bit operation. Having lower than 2mA may work depending on lower voltage of operation .
  • 0 in reply to ShreyasRao
    Guru 12155 points
    Hi Micheal-san, Shreyas-san,

    Thank you for your support.

    Shreyas-san,

    As you mentioned above, I understood that the input current is needed to drive the 4k internal buffer.

    On the other hands, I would like to know that concerns (behavior) if that current is lower than 2mA. (e.g. VCC=5.5V)
    Could you provide the any concern if it is lower drive current?

    Regards,
    Koji Hamamoto
  • 0 in reply to K.Hamamoto
    Guru 316690 points

    The input current flows through the 4 kΩ resistor and results in a voltage drop over it. One end of the resistor is held at VCC or GND by the output buffer; the voltage drop must be large enough so that the other end of the resistor goes beyond the switching threshold of the input.

    For example, if the state is low, and if there is a current of 0.5 mA, the voltage at the input is 0.5 mA × 4 kΩ = 2 V, which is not enough to switch high (at VCC = 5 V).

    In theory, a current of 2 mA would allow a voltage drop of 8 V; in practice, the resistor has very loose tolerances, and there is a safety factor.

  • 0 in reply to Clemens Ladisch
    TI__Guru 62060 points
    Thank you , Clemens, I appreciate your inputs.
  • 0 in reply to ShreyasRao
    Guru 12155 points

    Hi Clemens-san,

    Thank you for your support.

    I understood. 

    On the other hands, how much is the current if VCC is 1.8V?

    (e.g. Can we estimate as 1.8/5.5*2mA=0.654mA?)

    Our customer is using TXB0108 on their board. However they found that on some portion of circuit the current was smaller than 2mA at VCC=1.8V.

    So, they would like to know how much the current should be at least at VCC=1.8V. 

    Regards,

    Koji Hamamoto

  • 0 in reply to K.Hamamoto
    Guru 316690 points

    The 2 mA recommendation is for the worst case (i.e., VCC = 5.5 V, over temperatures, and with a safety factor).

    Figure 6 of the datasheet shows typical values, and implies that the switching threshold and the needed current scale linearly with the supply voltage.

    Please note that 5.5 V is a nominal 5 V plus 10 % error, so you should include the 10 % margin in your 1.8 V value, or scale with the nominal 5 V:

        drive strength at 1.8 V = 2 mA × (1.8 V / 5 V) = 2 mA × (1.98 V / 5.5 V) = 0.72 mA

  • 0 in reply to Clemens Ladisch
    Guru 12155 points
    Hi Clemens-san,

    Thank you! This is very useful advice!

    Regards,
    Koji Hamamoto