# Circuit ideas for undervoltage protection?

Discussion in 'Electronic Components' started by Dirk Leber, Aug 15, 2005.

1. ### Dirk LeberGuest

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

2. ### Tilmann RehGuest

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.
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 sheet, so I can't tell you if that spec is

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 sheet (if not already in the KB):
<http://www-k.ext.ti.com/sc/technical-support/email-tech-support.asp?AAP>

4. ### leggGuest

The spec sheet for this series gives voltage compliance limits for the
four pins - VCC, SENSE, EN, OUT- of -0.3 and +100V.

In normal operation, the OUT pin is fairly well protected by power
components natural to the application, being clamped to the source
or output ground, as well as these rails can be, by standard power
supply components.

Large filter capacitors allow both input and output rails to exhibit
low impedance to pulse disturbances.

Revere polarity on the input side can obviously be defeated by
conventional means, if hardware is designed to allow the condition to
be physically possible.

On the output side, the whole load takes the hit - and there are
likely some pretty hefty reverse conduction paths on any power supply
rail - but you have to ask yourself if this is a likely user-generated
fault. Is the node even accessible via an unkeyed interconnection?

This is not a good example of specific IC pin protection
requirements.

In all cases where the VCC +/- x volts limitation apply, the actual
threat has to be evaluated and dealt with.

RL