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SN74ABT244A: The tolerance of the Undershoot and Overshoot

Abed Meguenni
Prodigy 10 points

Part Number: SN74ABT244A

Hi,

 

Recently we changed the component upstream our octal buffers. To validate this modification, we decided to perform some functional test and unitary test.

We had no problem on functional test nor unitary test. However, we decided to compare the input signal of our octal buffers to see the difference between the old reference and the new one.

We noticed that the new ref got an Overshoot and an Undershoot more important than the previous one.

 

Now my question is: 

How can i be sure that this new value of Overshoot and Undershoot (generate by the new component upstream) don't harm my octal buffers? 

 

About the measure and value: 

The measure has been done on the octal buffer with the paperclip probe method to minimize the loop and to have the most accurate measure.

SN74ABT244ADW

Vcc = 5V 

Overshoot: 5.9V for around 5 ns

Undershoot: - 1.5V for around 2 ns

 

From the datasheet: 

 

It indicates clamp value. So, the device has negative (and positive?) clamping diodes. 

This clamp value is given for VCC=4.5V but we have VCC=5V, so does it change?

The Absolute maximum rating for the input voltage range is -0.5V to 7V, why the clamp value has a lower voltage?

 

In our case, we shouldn't have any problem from the overshoot but the undershoot can be dangerous for the octal buffers ?

 

Regards,

Abed

  • 0
    TI__Guru* 97376 points

    Hi Abed,

    Abed Meguenni said:
    This clamp value is given for VCC=4.5V but we have VCC=5V, so does it change?

    No.

    Abed Meguenni said:
    The Absolute maximum rating for the input voltage range is -0.5V to 7V, why the clamp value has a lower voltage?

    The abs max table is generic and repeats through the majority of our parts -- it's likely that the generic limits were included here (31 years ago) as a reminder that voltages should be between -0.5V and +7V for inputs. I should also mention that the abs max table is generally based on the technology and design and is not measured.

    The electrical characteristics table includes characterized values that were measured. In this case the negative clamp has a much lower voltage as shown in the electrical characteristics table.

  • 0 in reply to Emrys Maier
    Prodigy 10 points

    Ok , so we should be able to exceed maximum ratings if we are below clamp value ?  But which value, voltage or current ?

    For exemple : If our voltage undershoot is around -1.4V with a current lower than 5mA  does it mean we are good or not ? 

    If you have a chart with the Voltage clamping tolerated depeding on the clamping current it could be nice !

    Regards,

    Abed

  • +1 in reply to Abed Meguenni
    TI__Guru* 97376 points

    Hi Abed,

    Abed Meguenni said:
    Ok , so we should be able to exceed maximum ratings if we are below clamp value ?  But which value, voltage or current ?

    The input negative-voltage ratings may be exceeded if the input clamp-current rating is observed.

    In other words, you can apply a negative voltage below -0.5V (for example, -2V) so long as you do not exceed the clamp diode current listed in the datasheet, which is 18 mA.