Re: Simple but precise ramp/triangle generator

From: Spehro Pefhany (speffSNIP_at_interlogDOTyou.knowwhat)
Date: 09/15/04 

Date: Wed, 15 Sep 2004 21:09:17 GMT

On Wed, 15 Sep 2004 21:02:56 GMT, the renowned Rich Grise
<null@example.net> wrote:

>On Monday 13 September 2004 12:22 pm, Spehro Pefhany did deign to grace us
>with the following:
>
>> On Mon, 13 Sep 2004 19:01:39 GMT, the renowned Joerg
>> <notthisjoergsch@removethispacbell.net> wrote:
>>
>>>Hi Tilmann,
>>>
>>>The only pretty sawtooths that I designed were all consisting of a
>>>current source and a very good film capacitor. For the current source a
>>>simple two-transistor setup would work.
>>>
>>
>> I'd use a BJT current source and a capacitor and a comparator for a
>> 50kHz sawtooth. The amplitude can be nice and stable, and I assume for
>> this application you don't care if the frequency varies a bit, so a
>> ceramic cap would work.
>>
>Just a nitpick, but does a ceramic cap actually vary, or is it just
>that with their wide tolerance, your absolute freq. could be off
>from design freq, which, of course would have some "variance" from
>unit to unit.

Possibly both in this case. Economy of words, y'know.

>Or is an individual ceramic cap really unstable? I know they're leaky,
>and have crap TC, and +70-20 tolerances, but that's what makes them
>good bypasses.[0] :-)

The X7R type would probably be appropriate for this. NPO would be
pretty good stability wise depending on the value.
BTW, none of them are inherently leaky, despite their other dubious
chararacteristics.

Okay, let's say the charging current is a fairly robust 100uA, and the
sawtooth or triangle is 20usec long, with a 5V or 10V swing. Then the
cap is ~400pF, so an NPO would be ideal and it shouldn't vary much
either (though the simple-minded current source I had in mind would
vary noticably with temperature).

>Thanks,
>Rich
>
>[0] or, of course, decoupling caps, depending on the phase of the moon. ;-)

Best regards,
Spehro Pefhany 

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