2-comparator sawtooth generators?

[Rick Jansen]

Gentlemen,

A few times the topic of a 2-comparator sawtooth generator has 
come up, whereby the voltage over a capacitor swings between
two levels guarded by comparators. A capacitor is charged with
a current until the treshold level of one comparator is reached.
Then the capacitor is discharged with some kind of electronic
switch, until the treshold of the second comparator is reached.

The first comparator will work ok, but I wonder if the second
comparator is functional at all due to the relatively large 
reaction time of a comparator. The LM311 has a reaction time of
ca. 200 ns. The (dis)charge time for a capacitor via a resistor
is:  
   t=RC ln( (Vaim-Vstart) / (Vaim-Vtr) )

where Vaim is the voltage the capacitor will ultimately charge
to, Vstart the initial voltage, and Vtr the voltage we're 
interested in.
For a typical application where a capacitor of 2.2nF is discharged
through a 10 ohm switch, initial voltage say 1V, charge voltage 5V
and a Vtr of 4V, t would be : 10 x 2.2n x ln(4) = 30 ns

So, the discharge time of the capacitor is far far shorter than
the reaction time of a comparator. So does this work at all in 
practice??

(There are faster comparators than the LM311 (200ns), but not
fast enough for this example. National Semi has ultra-fast
comparators that are listed as 15ns devices, but even that 
would be too slow.)

Rick Jansen
__
rick at sara.nl   http://www.sara.nl/Rick.Jansen
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