Re: Circuit ideas for undervoltage protection?

Look in the link below for "Circuit Protects FPGAs From killer Spikes", plus
other circuit ideas.

http://www.discovercircuits.com/S/safety.htm


"Dirk Leber" <amidumb@xxxxxxxxx> wrote in message
news:ddqgqa$74s$1@xxxxxxxxxxxxxxxxxxxxxx
>I recently became aware of a problem which I believe is seriously
>underestimated in a lot of designs:
> 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. Which seems so
> obvious proves problematic when taking a closer look at various schottky
> diode datasheets. The forward voltage drop is heavily depending on the
> forward current which can easily result in voltage drops of more than 1
> volt. Inacceptable with the above mentioned absolute maximum ratings for
> the "protected" integrated circuit.
> Only very few schottky diodes guarantee Vfw values of less than 0.3 Volt
> even at very low forward current. Apparently there seem to be two
> qualities of a schottky diode that seem to be incompatible. Low forward
> voltage drop values are apparently not achievable together with high
> maximum reverse voltages, which resembles a problem anywhere close to
> things like driver lines. How about an example:
>
> TI's hot swap controller TPS249x can safely drive a FET as high side
> switch by survising the drain-source current via a shunt resistor and
> additionally doing something like a power calculation for the FET by
> multiplying the current by the drain-source voltage drop. For measuring
> this voltage drop it uses an additional pin OUT which has to be connected
> to source. So far for the background.
> The pin in question (OUT) since being connected directly to "the world
> outside" will have to withstand all the nasty things like surge/burst
> pulses, reversed polarity caused by users etc. As a little help, you might
> want to add maybe a suppressor diode together with a small serial
> resistance to limit currents. Not to forget the must-have schottky diode
> to ground against erroneous negative voltages applied to the drivers
> outside.
> Still, since you cannot spend too much voltage drop over that serial
> resistor, the resulting current through the schottky diode will grow to
> something like 1.3 ampere when applying negative voltages up to the
> suppressor diodes clamping voltage (let's say 40 volt), limited by a
> serial resistance of 30 ohm.
> 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?
>
> Cheerio,
> Dirk Leber
> HEITEC AG


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