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SE555-SP: Temperature drift of TRIG/THRES voltages

Benjamin Green
Prodigy 10 points
Part Number: SE555-SP

The SE555-SP datasheet shows that the device has trigger and threshold voltage characterized at several supply voltages and temperatures, but there is no information on what is driving these variations. If possible, I would like the following info:

- The contribution of the Trigger/Threshold voltage range due to static errors (e.g. resistor tolerance or comparator tolerance)

- The contribution of the Trigger/Threshold voltage range due to the effects of drift (e.g. temperature or aging effects)

My operating voltage is ~8.2V, which, looking at the tables, would give me around +/-10% bound on the Trigger voltage, and ~+/-5% on the Threshold voltage. A little more insight into where these values come from would be helpful for determining whether this IC can be used in our application. 

Thanks,

Ben

  • +1
    TI__Expert 8275 points

    Hi Ben,

    I'll do my best to address your questions here.

    1.) The contribution of the Trigger/Threshold voltage range due to static errors (e.g. resistor tolerance or comparator tolerance)

    I believe by "static errors" you mean errors at 25°C which do not include the effects of drift, as the drift errors will be addressed in your second question. You are correct, at 25°C the Trigger/Threshold tolerance is dominated by factors such as resistor tolerance, voltage offset of internal comparators, and bias current of internal comparator inputs. These errors are all accounted for in the min/max specifications at 25°C. 

    2.) The contribution of the Trigger/Threshold voltage range due to the effects of drift (e.g. temperature or aging effects)

    Each of the individual parameters listed above will have some variation over temperature, which is referred to as drift or temperature coefficient. All of these drift errors will combine and are added to the room temperature error terms, resulting in a wider min/max specification as seen in the table.

    Notice the Trigger voltage min/max specifications are different, depending on if the device is operated at the extreme high or extreme low temperature. The Threshold voltage min/max has the same specification for both high and low temperature operation, and this is a relatively moderate increase to the overall error.

    Keep in mind that these are the maximum specifications, which typically represent the worst-case over six standard deviations (6-sigma) of the gaussian distribution. The typical device will operate much closer to the 1/3 Vdd and 2/3 Vdd values.

    The effects of aging (sometimes referred to as long-term drift or long-term parametric shift) are not accounted for in the datasheet specification table. For absolute value specifications such as Trigger Voltage and Threshold Voltage, it is possible for the values to shift an additional 10% of the min/max datasheet value over the lifetime of the device. Once again, this 10% shift represents another 6-sigma event.

    The initial error distribution compared to the distribution after long-term shift will look something like this:

    I hope this helps.

    Regards,

    Zach

  • 0 in reply to Zach Olson
    Prodigy 10 points

    Thanks for the insight,

    While only having datapoints for the min/max supply rails is a bit limiting, I think we can make some inferences to get the data we need.