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SN74LVC1G17: Output operation when input is floating

Jade Han
Intellectual 880 points
Part Number: SN74LVC1G17

Tool/software:

Hello experts,

My customer hopes to use SN74LVC1G17, however because they are designing 1 standardized board for several different models of refrigerator, for some models, they hope to keep the input floating.

For example, if they were to use total 3 ICs per board, they want to use 2 out of 3 for model A, all 3 for model B, only 1 for model C, etc. so this would leave them to leave 1 IC input floating for model A, etc. 

In this case, I know the datasheet says to never design the input pin short..May I ask,

1. How does the output operate when input is left floating?

2. If input should never be left floating, could we maybe add a pull-down resistor to GND between the input pin and GND, and for the models that hopes to leave the input pin floating pull it down to GND? If the model does not leave the input pin floating we can simply not place the resistor on board and just leave the space for this..Would this affect the communication speed in any way?

Thank you in advance for your support.

  • 0
    TI__Mastermind 23155 points

    Hi Jade,

    If an input is left floating in a logic device, this may cause the output to oscillate and high frequency which results in high current draw. The best practice is to short unused inputs to GND (or Vcc) depending on your system.

    https://e2e.ti.com/support/logic-group/logic/f/logic-forum/737694/faq-how-does-a-slow-or-floating-input-affect-a-cmos-device 

  • 0 in reply to Albert Xu1
    TI__Intellectual 880 points

    Hi Albert,

    Thank you so much for your feedback.

    Customer hopes to use our logic IC as below image,

    For some of their refrigerator models load will not be connected, so there is no input at the IC input.

    There will only be RC filter and TVS diode on the input side in this case, could this cause any problem?

    Would it be ok because it is connected to GND?

  • 0 in reply to Jade Han
    Guru 267410 points

    The input is not connected to GND.

    You need to add a pull-up or pull-down resistor at the input.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 880 points

    Hi Clemens,

    Thank you for your feedback.

    I have some follow up questions,

    1. Would it be ok then if we added pull-down resistor here (marked in blue)?

    2. What is the recommended pull-down resistor value for this case? 

    3. For PBA boards or models where we will have input for this IC, would it be ok to put 0ohm resistor where I marked in blue? Would this affect the signal or communication in any way?

    This is because customer wants to use 1 standardized board for various refrigerator models, however some models will need this logic IC but some will not, so only some boards will have input for the logic IC here. (They will still need to solder on our logic IC for all boards)

    Thank you for all of your support.

    Best regards,
    Jade

  • 0 in reply to Jade Han
    Guru 267410 points

    1. No; that place is not connected to the input, and the resistor would make the TVS ineffective.

    2. When you do not have specific requirements for speed or power consumption, it does not really matter. Typically, it's 10 kΩ.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 880 points

    Hi Albert, Clemens, 

    Thank you for your feedback.

    I had additional question from customer,

    1. Customer just wants to keep the output at logic low, while keeping the input floating, because of the MICOM load that is connected at the output of the IC. With RC filter and TVS diode, they intend to block off any noise that may come through and make the logic output high. In this case, with customer’s circuit, is there a possibility where the output could become high while input is floating?

    2. I understand that with noise the output may become high instead of low, when our intended input is low, but could there be cases where the output is still high even after the noise is no longer coming through the input is at low? 

    Thank you so much for your help and support. 

  • 0 in reply to Jade Han
    Guru 267410 points

    1. A floating input might be read as low, or high. And it increases power consumption; see the FAQ linked above.

    2. Due to the input capacitance, the voltage will drift slowly (this is why it's called "floating"). But the capacitance is small enough that it will definitely drift.

    You must not let the input float.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 880 points

    Hi Clemens,

    Thank you for your feedback.

    I had 2 follow up questions:

    1. As you suggested, would this work: instead of RC filter & TVS diode, put 10kohm pull down resistor?
    Could it affect the signal or communication in any way when the input is there? (90% of the time there will be an input, but other times there will be no input (unknown)) 
    (Please find the circuit image below.)

    2. In case of external noise to the input, does the output stay in high state?
    Because it is a Schmitt trigger IC, customer's understanding is that once the output comes out, it stays that way, in other words if noise causes output to be high, it would stay in high even when the noise is removed..Is this the case?

    To add some background on the application, the IC's input is connected to the download connector, and most times it would be connected but for some models where the connector would not be connected, there would be no input signals and in this case, they expect the output to stay in low. They want to keep the output in low, because when output becomes high, the TR turns on and MICOM is not able to be recognized. However, they were afraid the noise to the input would make the output high. So, they used RC filter & TVS diode on the input only to protect the input from any noise. 

    Thank you for your support.

  • 0 in reply to Jade Han
    Guru 267410 points

    1. When another device drives the input high, it must also supply the current through the pull-down resistor (0.5 mA at 5 V, which usually is negligible).

    2. A floating input tends to stay at the same value for a short time. But with the pull-down resistor, even if a voltage spike happens to force the input high, it will immediately be pulled down again.

    The RC filter and TVS diode do not enforce any specific voltage. The RC filter removes high-frequency noise, and the TVS clamps voltages that would damage the logic device otherwise.

  • 0 in reply to Clemens Ladisch
    TI__Intellectual 880 points

    Hi Clemens,

    Thank you for your response. I had one quick follow-up question,

    so you mentioned "A floating input tends to stay at the same value for a short time.",

    So you mean that when the input is floating, even if noise comes through and make the output high for a short period, the output will soon become low once the noise is gone, is this correct?

    Customer wanted just the confirm on this statement.

    Thank you for your support.

    Best regards,
    Jade

  • 0 in reply to Jade Han
    Guru 267410 points

    No; a floating input is not held to any specific voltage. When noise makes a floating input switch, then the new voltage will stay.