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SN74LVC4245A: Relationship between overshoot/undershoot and power supply voltage

Shinichi Inoue
Guru 21686 points
Part Number: SN74LVC4245A

Tool/software:

Dear Specialists,

My customer is evaluating SN74LVC5245 and has a question.

I would be grateful if you could advise.?

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I am using SN74LVC4245APW, and there is a problem where a bit is misread somewhere on the bus. (It is not an abnormality of this element.)

I changed the damping resistor of the previous stage buffer and reduced the overshoot/undershoot input to SN74LVC4245APW, and the problem disappeared.

Therefore, I would like to ask two questions regarding the operation of the element.

(Overshoot Waveform)

(Undershoot Waveform)

CH3:A1 input

VCC: 4.94V

CH1,CH3 and CH4 are  signal of different devices.

1. For example, if a rising overshoot waveform that exceeds5.88V at  the power supply voltage of 4.94V or a falling undershoot that falls below -1.2V is input from the A1 side, is it possible that the output propagation delay time on the B1 side will exceed the specified value?

2. Also, when the power supply voltage was increased from 5V to 5.5V from the state where the abnormality occurred (without the damping resistor), the abnormality disappeared.

The idea is that if the magnitude of the overshoot/undershoot at 5V and 5.5V is the same, increasing the power supply voltage will make it relatively less likely for malfunctions due to overshoot/undershoot to occur.

Is this understanding correct?

3．Is there  a relationship a latch-up resistance of 250mA.

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I appreciate your great help in advance.

Best regards,

Shinichi

  • 0
    Guru 316860 points

    Any input voltage below 0 V or above VCCA exceeds the recommended operating conditions, so the electrical characteristics are no longer guaranteed.

    There are internal clamping diodes from GND and to VCCA, so when an overshoot/undershoot happens, a current flows into VCCA or out of GND.

    I'd guess that external clamping diodes would make the current flow directly into the decoupling capacitor and not affect the transceiver's internal power supply as much. Schottky diodes with a lower forward voltage would be even better.

    A latch-up can happen only when you exceed the absolute maximum ratings. This is probably not the case here (but I do not know the actual currents).

  • 0 in reply to Clemens Ladisch
    Guru 21686 points

    Hi Clemens,

    Thank you for your reply.

    I'll share your answer with the customer.

    I appreciate your great help and cooperation.

    Best regards,

    Shinichi