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LM25066A: Applying external current/voltage to TIMER pin

Christopher Arntzen
Intellectual 650 points

Part Number: LM25066A

Hello,

I am implementing an additional circuit that provides external current (~80uA) to the TIMER capacitor on my LM25066A to speed up the insertion time, but not affect the fault timing.  When my circuit is powered on and the LM25066A does not have any input voltage (LM25066A is powered off), my circuit applies ~1.5V to the TIMER pin.  Is this safe for the LM25066A?  As soon as the input voltage reaches the POR, my circuit works as expected.

Please let me know if it is safe to have external voltage applied at the TIMER pin when the LM25066A is powered off.  If so, how high can the voltage be?

Thanks,

Chris

  • 0
    TI__Expert 5940 points
    Chris,

    I wouldn't add any voltage or current source to the timer pin. The 80uA / 1.5v will not hurt the pin with no Vin (i.e. below IC POR), at least as far as I can see. I'm sure there is ESD protection somewhere above 1.7v, most likely in the 6v+ range, but would have to pull the IC design files and cannot access for this. If you wanted to measure, using a limiting resistor, increase the voltage on the timer until something triggers or limits futher increase.

    But, with what you propose, you will impact a the actual fault timer and completely swamp the discharge current (2.8uA) shown in figure 32 of the DS. 1.5v is very close to the 1.7v timer threshold also. So not recommended.

    Brian
  • 0 in reply to Brian Daugherty
    Intellectual 650 points
    Hi Brian,

    Thanks for the info. I have specifically designed the external circuit not to interfere with the fault timer currents, as it disengages (latches off) the first time that the TIMER voltage reaches 1.52V (max) which is below the 1.54V (min) TIMER threshold. It works as I expect it to in the lab, and do not see an issue implementing it.

    -Chris