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TPS3850: TPS3850 voltage overdrive 0-1% timing?

Michel Buchs
Prodigy 220 points
Part Number: TPS3850

Hello

We are using the TPS3850 in a critical system that needs to test the functionality of the TPS3850 during operation.

I am using the 4% type so the voltage supervisory tresholds are 3.3V*1.048 = 3.4585V for the upper and 3.3V*0.952 = 3.1416V for the lower.

I will pull the nominal 3.3V sense pin with two FETs and resistors down to GND or to 5V to be able to test if the supervisory does make a reset.

Now when calculating the resistors I would like to get values so for example for the lower treshold I get a voltage at the sense pin of 3.1387V. This is only about 0.1% below the threshold. I would like the watchdog to make a reset then to pass the test.

Now in the diagrams of the datasheet the 1% and higher overdrive are shown and the reaction time seesms to increase exponentially the lower the overdrive. What are the overdrive timings for values below 1% overdrive? Does that mean at 0.1% under treshold the device will take seconds to reset?

Do you have any values/hints as to what is possible?

Thank you

  • 0
    TI__Genius 12630 points

    Michel,

    Once you calculate the upper and lower thresholds using the 4%, these values become the typical thresholds.

    So V_IT(OV) typical and V_IT(UV) typical.

    But the device itself has 0.8% accuracy which means that the minimum for each threshold  V_IT can be V_IT typical * 0.92 and the maximum can be V_IT typical * 1.08. This is why the overdrive curves are shown at 1%. This is because if the device just happens to be at the minimum side of the spectrum of accuracy spread under some certain operating condition, the device won't even trigger at "0.1% below typical". To guarantee the device triggers, you must be below 0.8% of typical for UV or above 0.8% of typical for OV. This does not mean the device won't trigger at 0.1% below the typical threshold, since most will trigger at the typical threshold, but it does mean that to be absolutely sure in every case under all operating conditions, the overdrive should be more than 0.8%.

    Also, the Watchdog input (WDI) triggers the watchdog output (/WDO) and does not trigger /RESET unless /WDO and /RESET are tied together.

    Does this answer your question? Please let me know if you have any more questions or if anything is not clear. Thanks!

    -Michael

  • 0 in reply to Michael_DeSando
    Prodigy 220 points
    Hello Michael

    Ah ok, so this 1% is refered to the typ. values! I already am calculating with the 4% + 0.8% values, that's what I calculated 3.3V * 1.048 for the upper threshold for example. So my 0.1% I use to test the threshold is in that meaning 0.9% above the typ values and I can use the diagram from the datasheet!
    All clear now, thank you!