Re: Schmitt trigger for feedback SSTCs

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jan Wagner <jwagner-at-cc.hut.fi> 

Hi,

On Thu, 29 Apr 2004, Tesla list wrote:
 > Original poster: Mr Gregory Peters <s371034-at-student.uq.edu.au>
 >
 > Been looking at common, cheap schmitt triggers used for feedback SSTCs, and
 > have noted that they:
 >
 > a) generally only run on 5v
 > b) Can only supply a few tens of mA output

And in what way are those a problem? ;-)

If you need to run the last power stage (e.g. gate drivers) from 15V, but
have a 5V or 3.3V or 2.8V logic supplt, you could always level-shift the
logic signal to 15V. Either open-collector / open-drain level shifters, or
one of those "digital transistors" (those small xistors packaged with
internal resistances) with logic input to gate, source to ground, pull-up
resistor on drain to +15V and drain to the last power stage logic input.
With the transistor the signal will be inverted, though.

 > Anyway I found this circuit at:
 >
 > http://www.uoguelph.ca/~antoon/gadgets/555/555.html
 >
 > Go to the "example circuits" section, figure 9.
 >
 > It is a 555 timer being used as a schmitt trigger to clean up dirty wave
 > inputs. Seems this would be useful to us SSTCers, as it is cheap, can run
 > off 12v, and can supply up to 200mA output current.
 >
 > Is there any reason why this couldn't be used in a feedback SSTC system?

The trigger propagation delay is a bit long
   http://cache.national-dot-com/ds/LM/LMC555.pdf
(probably even worse for the non-CMOS 555 version)

The delay introduces phase shift, IIRC it was 100 ns or something like
that for the CMOS, and 1 us for the bipolar 555. For a very low frequency
SSTC this shouldn't matter. If you want a "real" 3..18V Vdd schmitt
trigger, there's for example the CD4093 NAND with schmitt trigger inputs
(though 80ns typical propagation delay at 15V). MC14584 is similar, 40ns
typ -at- 15V. High voltage logic ICs just tend to be slow...

On the 3.3V/5V side you'd get a lot of 74HC/HCT/LS series fast logic with
schmitt trigger inputs, and a bunch of ultrafast analog comparators
(for example from Maxim-IC-dot-com) that you can wire to have more hysteresis
(just a few extra resistors).

regards,
  - Jan

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