On Tue, 13 Nov 2001, Byron A Jeff wrote:

> On Tue, Nov 13, 2001 at 11:26:53AM +0200, Vasile Surducan wrote:
> > what load current do you have and from what supply  ?
> > Example: in  5V, out 3.3V supply current 100mA.
> > Answer: yes  R = ( 5 - 3.3 )/ 0.1 = 17 ohm, choose R=15ohm/0.5W
> > dissipation on zenner without load : 0.3W choose a 1W zenner
> > don't forget 10uF(tantal) or 10uF (paper dielectric) + 100nF parallel with
> > zenner, probably you have to choose a zenner to have the right 3.3V value
> > ( read data spec about absolute supply values for your chip )
> > Vasile
> >
>
> I can see very little advantage of the zener over a true regulator. I read
> the chip's specs, an it requires a +-9% regulated supply:
>
> 1) What's the max overshoot with the zener?
  ???

> 2) What's the ripple voltage?

  does not matter, the input is stabilised +5V

> 3) What's the regulation tolerance?

  idem, you said +/- 9% , that means approx 3.0 to 3.6V, perfectly for a
3v3 zenner

> 4) What's the current draw in the quiescent state?

  almost all current flow through zenner, no problem.
>
> All of these items are listed in the LM317 data sheet, because it's a self
> contained regulator. It consists of exactly one part more than the zener
> solution and can make some guarantees.
>
 guarantees same as a zenner...


> In this instance I'm just wonder why the zener is a better choice. Even in
> singles the price difference is only 14 cents US (0.50 vs. 0.36 on DigiKey).
>
  no one say that, but is possible...and there was someone who said it
isn't. Sometime classic devices are still working. That's all.

> I'm just wondering....

  fell free...

regards, Vasile
>
> BAJ







>
> > On Mon, 12 Nov 2001, Chetan Bhargava wrote:
> >
> > > I just need to power an EZ-USB chip that uses 3.3v. Would the zener approach
> > > be ok?
> > >
> > >
> > > ----- Original Message -----
> > > From: "Vasile Surducan" <vasile@L30.ITIM-CJ.RO>
> > > To: <PICLIST@MITVMA.MIT.EDU>
> > > Sent: Sunday, November 11, 2001 11:42 PM
> > > Subject: Re: [EE]: ~ 3.3V from 7805
> > >
> > >
> > > > Well, if you don't want only 50mA why don't you use a simple 3.3V zenner ?
> > > > regards, Vasile
> > > >
> > > >
> > > >
> > > > On Sat, 10 Nov 2001, Richard Sloan wrote:
> > > >
> > > > > Unless you really need the 5V!
> > > > >
> > > > > I just did exactly this on a project and for me it worked out fine.
> > > > >
> > > > > My 3.3V is only at 50ma
> > > > >
> > > > > R.
> > > > >
> > > > >  >>  On Fri, Nov 09, 2001 at 02:59:15PM -0800, Chetan Bhargava wrote:
> > > > >  >>  > Hello list,
> > > > >  >>  >
> > > > >  >>  > Is it possible to achieve near about 3.3 volts out of a 7805 plus
> > > a
> > > > >  >>  couple
> > > > >  >>  > of IN4007 (1 volt drop) on the output pin?
> > > > >
> > > > >  >>  It'll have very poor regulation characteristics dependant on the
> > > diodes,
> > > > >  >>  temperature, and current draw.
> > > > >
> > > > >  >>  A much better idea is to replace the 7805 with a LM317 and a couple
> > > of
> > > > >  >>  precision resistors.
> > > > >
> > > > >  >>  BAJ
>
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