> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On
Behalf
> Of SME
> Sent: 27 October 2009 10:42
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Cheap, low power, isolated DC-DC converter design
> 
> An LM339 quad comparator may be easier to use in that application. The
> circuit would need revising - especially so as the comparator is open
> collector. Offset voltages are lower and the IC design is optimised
> for switching and effective GBP is higher. Quiescent current drain may
> be slightly lower. With some work you can probably get by with the
> LM393 dual, but both are cheap. Vcc for the comparator is from 2V (!)
> compared to 3V min for the LM324 - but more is better.
> 
> If using an LM324:            Note that across temperature the LM324
> has a common mode range of ground to Vcc-2V and at 25C it's Vcc-1.5V,
> so for full temperature and Vcc = 5V, using Vbias = half-supply is
> "unwise" as you have as little as 0.5V input "headroom".  (Yes -
> inverting input will be driven to Vbias, but it's awfully close.
> 
> Also, Vout max is Vcc-1.5V, so for a 5V Vcc it is probably useful to
> lower Vbias to say ~= (Vcc-1.5)/2 = 1.75V or so.  All that said, the
> original circuit probably often works OK as shown.

Thanks for the comments!  The original circuit was modelled using some
pretty decent (and not particularly cheap) rail-rail input and output
op-amps from LT, the LM324 was the only thing I could find in a DIP
package whilst making the stripboard version, so I put this in the
LTSpice model to see if it works...and it does appear to, but your
points are well noted regarding the limitations of this part.

It would be interesting to see how well an open collector comparator
would work as a transimpedance amp, I'll have a go at modelling this
since this would make a very low cost and low parts count overall
solution.

Do you have any comments on the comparator section?  I had to keep
hysteresis fairly low to prevent pulse width distortion.  Would using a
balanced comparator design (i.e. with hysteresis on both inputs) be
worthwhile?

Thanks again.

Regards

Mike

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