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Re: gEDA-user: TL431

Hi Gene,

As has already been mentioned, the problem will be what happens as the
supply turns on. In theory, as VCC ramps up, the 431 will start to
regulate and so limit its own cathode voltage. However, there may be a
delay before this starts to happen (due to the devices internal
startup and any external capacitances). If VCC exceeds 36V before the
431 has started drawing current then all bets are off.

Crude but you could try putting a zener in series with R3/NPN_B and
431 cathode to drop the excess voltage so that VCC - Vzen < 36V.

Another caution with 431's: check the specs of them carefully as they
vary a lot from one manufacturers to another.

And another: they are notoriously unstable (and the spice models don't
always show it) with certain cathode loads.

I think it's either National or TI that if you look closely at the
curves on the datasheet and some of the small print notes it hints
that this happens but is unclear about where and how.

That said, I have used them to make an adjustable 75V - 35V series
regulator running from 80V.

If you use an NPN cascode like this you can limit the max cathode
voltage by setting Vref < 36V + Vbe where Vbe is the base emitter drop
of the NPN cascode.


            (npn)

VCC ------ C       E ------
     |      \   /        |
     /      -----        /
 R3  \        | B        \
     /        |          /   R1
     |        |          \
      --------|          /
              |          |
            |/ C         |
  Vref------|   NPN      |
            |\ E         |
              |          |
              |          |
           ------/       |
          /  / \         |
  TL431     /   \---------
            -----        |
              |          /
              |          \  R2
              |          /
              |          |
              |          |
              ------------
              |
           --------
          /   /   /

Vref can be derived from a resistive divider or a zener reference.

This can be made to work but is tricky to compensate (see above). Even
though it should be OK as drawn, in practice, it will probably need a
resistor in series with the cascode emitter and a cap from the 431
cathode to its adjustment pin.

All in all, may be more trouble than it's worth.

Cheers,

         Andy.

Signality Solutions
t: +44 (0) 5601 720 580
m: +44 (0) 7796 538 192
skype: andyfierman
www.signality.co.uk



On 21 February 2010 01:54, gene glick <carzrgr8@xxxxxxxxxxxxx> wrote:
> Wojciech Kazubski wrote:
>>
>> -> Anyone use these shunt regulators?  I'm wondering about the max
>> voltage.
>
>>> Or set up your mailer to use a fixed-width font..
> Yes, that was it - thanks.
>
> I suspect this TL431 isn't a good device for my app.  I have an LM317 to
> give me around +12V for bootstap voltage to a high-side mosfet driver. The
> average current is very low, but the peaks can get pretty high.  The LM317
> circuit is solid, just uses a lot of parts.  I was trying to replace the
> LM317 with TL431 - but simulation does not look promising.
>
>
> _______________________________________________
> geda-user mailing list
> geda-user@xxxxxxxxxxxxxx
> http://www.seul.org/cgi-bin/mailman/listinfo/geda-user
>


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