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SN74LVC245A: Undershoot Range on data lines.

Sahil Bhat
Prodigy 70 points
Part Number: SN74LVC245A

I am using buffer SN74LVC245APWR (U5 in the attachment) with 3.3V as Vcc. When measuring the voltages on the I/O lines, I see a undershoot (on the incoming data to the buffer) going to -0.8V while the data sheet (section 7.1) says the minimum voltage applied on the I/O ports can be -0.5V. However the datasheet also says (in note (2)) that input/output negative voltage can be exceeded.

Can you please tell me if -0.8V on the incoming I/O ports can be accepted as an undershoot for this device without impacting the buffer's operation and reliability?

SN74LVC245APWR Undershoot.pdf

Please look at the attached document for more details.

  • 0
    TI__Mastermind 26100 points

    Hi Sahil,

    You can exceed the voltage if you do not exceed the current rating on the input, in this case, -50mA.

    The scope shot says something about a 33 ohm resistor. If you have a 33ohm resistor connected to the input then you are only seeing -0.8/33 = -24 mA of current across the pin and you are ok. Otherwise consider adding a resistor to the input to reduce the amount of current.

    Also, if you aren't already, make sure you are measuring the voltage with a ground spring attached to your probe instead of a clip; it's possible your measurement is introducing capacitance to the input which is causing the undershoot, and the undershoot is not actually being seen by the device.

    Best,

    Malcolm

  • 0 in reply to Malcolm Lyn
    Prodigy 70 points

    Thanks for your reply Malcolm. 

    Based on the datasheet and your reply, it looks like we might be okay with the undershoot. But you said we might have a -24mA current across the pin (based on the waveform) and in the datasheet (section 7.5) its mentioned that the input current won't be higher than +/- 20uA. An engineer from TI told me that these CMOS buffers have high input impedance (even on the I/O ports) and so input current is limited to 10uA. If that's true then how can we say that there is -10mA current across the buffer pins.

    Please share your thoughts and correct me if I am wrong.

    Thanks

  • 0 in reply to Sahil Bhat
    Guru 316780 points

    The inputs have a high impedance only of you stay inside the recommended operating conditions (between 0 V and 5.5 V).

    There is a clamping diode from GND to the input. It's a silicon diode, so no noticeable current will flow for a voltage above −0.5 V. For lower voltages, the device will not be damaged as long as the current through the diode is smaller than 50 mA.