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SN75453B: Does it have an internal pull-up resistor on each input pin?

Yoshikazu Kawasaki
Intellectual 751 points
Part Number: SN75453B
Other Parts Discussed in Thread: SN75451B

Hello,

My customer works on SN75453BD with VCC=5V and connected pull-down resistor(22kOhm) on each input pin to expect low level on the output.  However the output was Hi with the input pin voltage level of around 1.5V.  Would you please tell me the reason why it is?  Does it have an internal pull-up resistor on each pin even though the datasheet doesn't show such information?  I don't think it would, but does it have dependency on package?

Best regards,

Yoshikazu Kawasaki

  • 0
    TI__Mastermind 23640 points

    Hi Kawasaki-san,

    When transitioning from low to high, the SN75453B has a VIH of 2V min, and when transitioning from high to low, the device has a VIL of 0.8V. What voltage is the pin being driven high with? And, are you seeing this condition directly after one/both inputs are driven high?

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Intellectual 751 points

    Hello Pat,

    Thank you very much for your comments.

    Yes, they saw the table.  It seems both inputs are around 1.5V even with external pull down resistors of 22kOhm which made the output high level.

    Best Regards,

    Yoshikazu Kawasaki

  • 0 in reply to Yoshikazu Kawasaki
    TI__Mastermind 23640 points

    Hi Kawasaki-san,

    Understood - please see in the data sheet the Iil parameter, which is 1.6mA sourcing current max. In order to keep the pulldown voltage under 0.8V, the pulldown resistance would need to be 0.8V / 0.0016A = 500Ohm instead of 22kOhm.

    If the customer wants to have a higher input resistance, they can use SN75451B, the NAND version of this device, and use pullup resistors/active low input signals since Iih is lower than Iil.

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Intellectual 751 points

    Hello Pat-san,

    Thank you for your comments.  I missed the parameter, but it shouldn't be such simple to calculate the appropriate pull down resistor value since the internal resistors would have impact.  The external pull down resistor with the IIL just doesn't make it 1.5V.  I think the 1st pnp is ON from the VCC=5V and the node A will be boosted from the IIL and the 2nd pnp will be ON to make the resistor combination which made the node A 1.5V.

    Best Regards,

    Yoshikazu Kawasaki

  • 0 in reply to Yoshikazu Kawasaki
    Intellectual 751 points

    Sorry, the transistor is npn.

  • +1 in reply to Yoshikazu Kawasaki
    TI__Mastermind 23640 points

    Hi Kawasaki-san,

    Yes, I agree with your analysis. However, since the device is specified for -1.6mA maximum on the input pins, my understanding is that regardless of the internal schematic, as long as the current out of the in pins stays below 1.6mA, a 500Ohm resistor would keep the input below VIL. I would recommend that the customer tries changing their 22kOhm resistor to 500Ohm and probing the voltage and current on the pin, and see if they find that they have additional room to increase the pulldown resistance if they choose to.

    This device is old, and was made for direct TTL/CMOS input connections, so pulldown resistors need to be stronger than what would usually suffice.

    Please let me know if I can help with anything more.

    Thanks,

    Patrick

  • 0 in reply to Patrick Shalton
    Intellectual 751 points

    Hello Pat-san,

    I agree.  If the external pull down resistor is below 500Ohm, the internal resistors wouldn't have impact.

    Best Regards,

    Yoshikazu Kawasaki