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