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SN74AHC1G125: Is there any absolute maximum rating about output current for transient condition?

Kazuya Nakai59
Guru 20885 points
Part Number: SN74AHC1G125

Hello guys,

One of my customers is evaluating SN74AHC1G125 for their new products.

They connect a few LC filter and a pull down resistor to Y output of the device.
Also they input 16MHz signal to A input.
At this moment, they have the following questions.
Could you please give me your reply?

Q1.
Is there any absolute maximum rating about output current for transient condition? 
They found that output current of Y terminal exceeded 25mA, absolute maximum of Io(continuous output current)
during about 5ns. (The peak current is about 35mA)
Is this any problem or damage for the device?

Q2.
In their evaluation, about -28mA current is following from Y terminal to the LC filter during "H" level output (about 30ns).
Also about +28mA current is following from the LC filter to Y terminal during "L" level output (about 30ns).
Considering the duty (50%), I think average current would be +14 or -14mA and I think these current is not problem
for SN74AHC1G125.
Is my thought correct?

Your reply would be much appreciate.

Best regards,
Kazuya.

  • 0
    Guru 286620 points

    Logic devices are designed to handle a capacitive load of at most 70 pF. For larger loads, the absolute maximum ratings must not be exceeded, not even for a short time. If your circuit tries to draw a larger current, insert a current-limiting series resistor, or use multiple outputs in parallel.

  • 0 in reply to Clemens Ladisch
    Guru 20885 points

    Hello Clemens,

    Thank you very much for your reply.

    Could I ask you an additional question as the below.

    Q.
    Several SN74AHC1G125 switching characteristics in the device datasheet were measured
    when 50pF capacitor was connected to "Y" output terminal directly.
    I think that Y terminal output current would exceed -25mA with very short time at Y output level is changed
    from L level to H level because it is similar to Y output shorting to GND.
    Is it no problem for the device?

    In other words, is 50pF direct connection case no problem in spite of exceeding -25mA of Y terminal output current?

    Thank you again and best regards,
    Kazuya.  

  • 0 in reply to Kazuya Nakai59
    Guru 286620 points

    50 pF is OK; the current spike for that is short enough. You need to care about the current only for larger loads (typically when you connect an actual capacitor).

  • 0 in reply to Clemens Ladisch
    Guru 10745 points

    Hi TI team,

    Does the SN74AHC1G125 have no problem with exceed -25mA with very short time?
    When 50 pF is connected, it momentarily exceeds 100 mA.

    BR,
    Kengo.

  • 0 in reply to Kengo.Y
    TI__Mastermind 20010 points

    Hi Kengo,

    100mA is too much current. The customer should limit this with a series resistor as Clemens suggested.

    Best,

    Malcolm

  • 0 in reply to Malcolm Lyn
    Guru 10745 points

    Hi Malcolm,

    Thank you for your reply.

    I understood that it has to limit of 20mA.
    But, CL is 50 pF for switching characteristic tests in the datasheet.
    I believe that the charge to this capacitor will flow 100 mA at the moment.
    What do you think about this?

    BR,
    Kengo.

  • 0 in reply to Kengo.Y
    Guru 286620 points

    When the load is a capacitance and when the total load (capacitor, traces, etc.) does not exceed about 70 pF, you do not need to care about the current.

    The LC filter does not have the same behaviour as the circuit in the CL = 50 pF test, so there is no guarantee. Just add a series resistor.