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# LTC2917 Min/max watchdog timeout calculation

capacitor CWT sets the watchdog timer per:

page 7  "set a watchdog upper boundary timeout time of 72ms/nF."

and  page 12:  "CWT = tWD • 13.8 [nF/s]"

my CWT is a 10nF 16V  5% X7R 0402

10nF +5% = 10.5nF  for a  tWD of 761ms  per the 13.8nF/s equation

10nF -5% = 9.5nF   for a tWD of 684ms

10.3nF cap yielded 608ms  watchdog timeout.  another 10nF cap yielded 650ms.   what is the best way to calculate the lower boundry WD timeout?

Thanks

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• Hello asw3000,

The lower boundary limit applies to the B version of the parts LTC2917/2918, and is for implementing a windowed watchdog function.  To get the lower boundary value, divide the upper boundary by 32.

Best regards,

• blovell,

Thank you for the reply. Maybe I should clarify my question. I'm using a non B version.  What is the best way to calculate the expected range of tWD?  I've been using the 72ms/nF conversion applied to the cap value and it's tolerance.

10nF-5% = 9.5nF  -->  684ms

10nF +5% = 10.5nF -->  756ms

Which doesn't take into account internal chip tolerances.  Now I see the spec on datasheet p.5 that says that a 2.2nF cap will result in min:130ms, typ: 160ms, max: 200ms.

It looks like I should be using the CWT = tWD • 13.8 [nF/s] conversion and then applying  a -18.75% / +25%  tolerance to that result

• blovell,

Thank you for the reply. Maybe I should clarify my question. I'm using a non B version.  What is the best way to calculate the expected range of tWD?  I've been using the 72ms/nF conversion applied to the cap value and it's tolerance.

10nF-5% = 9.5nF  -->  684ms

10nF +5% = 10.5nF -->  756ms

Which doesn't take into account internal chip tolerances.  Now I see the spec on datasheet p.5 that says that a 2.2nF cap will result in min:130ms, typ: 160ms, max: 200ms.

It looks like I should be using the CWT = tWD • 13.8 [nF/s] conversion and then applying  a -18.75% / +25%  tolerance to that result

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