Re: Lithium cell on Virtex2 Pro

Aug 28, 2003 2 Replies
K


I notice that the maximum recommended Virtex2pro Vbatt is 2.63V.


> Lithium cells put out more than that.

Many chips don't go to sleep mode until the VDD has dropped quite a bit.


See:

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The 2V63 value might be the ideal value on their VDD/IDD graph, with most headroom for battery voltage droop, but lower voltages (e.g. 2V4) might work just as well.


Is the solution as simple as a series Si diode on the Vbatt line?

Textbooks show zero current up to Vf but I suspect Vf drop depends on current, even before then.


It should be a quick experiment to see what happens.

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A

Is this relevant to Virtex2pro?

The data sheet indicates voltages down to 1.0V are fine.

You might like to Google for the "diode equation". It will confirm your suspicion. BTW, you need better textbooks ;)

Extrapolating a curve in the 1N914 datasheet gives about 120 - 130mV forward voltage at 100nA forward current. (This diode has an ideality factor of about 2.)

The V2P data sheet indicates 100nA typical Ibatt over the recommended operating conditions. They don't specify a maximum current - I hope it's not 10uA or something. ;)

Regards, Allan.

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K

The bits about chips often having definite thresholds for low-power modes kicking in.

Great, a 2V4 batt should be fine then.

No insult to your intelligence intended. It's just that on most graphs of Vf/If, it does look flat below Vf threshold.

Even with 700 mV drop, a 2V4 batt should be fine. By the time it drops to 1V7, it is pretty much out of juice anyway.

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