Re: Circuit ideas for undervoltage protection?

Dirk Leber schrieb:

> For most standard integrated circuits manufacturers rate the min/max
> voltage levels at the pins with something like -0.3V to Vcc + 0.3V,
> which is probably derived from the idea of having schottky diodes,
> either internal or external, clamping the pin to the above levels.

I think it's rather derived from the internal structures of the chips.
These specs are to avoid triggering of internal parasitic SCRs, which
would lead to latch-up and possibly destruction of the chip.

> [TPS 249x example, OUT pin]

> The question now is, which schottky diode to choose:
> For example Vishay's SS2H10 that is able to stand a reverse voltage of
> up to 100V would have a forward voltage drop of approx. 0.68 Volt. The
> OUT pin would be driven far out of spec.
> The opposite direction would be something like Vishay's SL44 whith a
> impressing forward voltage of only approx. 0.31 Volt at 1.3 ampere. The
> problem here is the low reverse voltage of only 28 Volt.
>
> Did anybody solve this quest in the past? Any alternative protective
> circuit ideas?

Most probably there also is a spec about the maximum *current* at the
OUT pin to prevent latchup, and most probably you can safely stay within
these specs by using a reasonable schottky diode *and* an additional
resistor between that diode and the OUT pin.
I didn't look at the data ***, so I can't tell you if that spec is
contained therein, but I'm pretty sure TI *knows* about this. :-)

Did you already ask TI about this detail? I guess their "PIC" should be
able to provide detailed application information, as well as more
detailed information about how to understand the voltage and current
limits provided in the data *** (if not already in the KB):
<http://www-k.ext.ti.com/sc/technical-support/email-tech-support.asp?AAP>

--
Dipl.-Ing. Tilmann Reh
http://www.autometer.de - Elektronik nach Maß.
.