Circuit ideas for undervoltage protection?

Maybe a simple answer... If you use the SL44 to clamp a given input to VCC and GND, then each diode would never see 28V reverse bias, because the 'other' diode connected to the 'other' rail will clamp the voltage to .3V of the rail. Unless you're in a system where VCC>=28V, the diodes won't see a reverse voltage larger than VCC+.3V.

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w2aew
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No. This SPEC is derived from requiring that ESD structures NOT be forward biased other than by ESD events.

Nothing comes for free. The "impressing" forward voltage comes at a cost of slowed speed.

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
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Jim Thompson

You need to bear in mind that the current flowing through the protection diodes may not go directly to the supply rails, and may provoke the integrated circuit into deviating from the data sheet.

I ran into this when developing a quadruple ramp A/D converter back in

1979 - it was good to about 14-bits once I clamped the ramp with a dscrete Schotty diode, rather than relying on the free protection diodes on an IC to do the job - and the current involved was much less than a milliamp.

Using an external diode and a current limiting resistor is an excellent scheme, but I wouldn't like to rely on the behaviour of the circuit being protected while it is being protected - it won't blow up, but it may behave oddly.

------------ Bill Sloman, Nijmegen

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bill.sloman

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Reply to
John Fields

This voltage limit is when from a low impedance source. Provided current is adequately limited, you can violate this voltage limit vith whatever voltage causes a small amount of current to flow through internal diodes.

This current limit is typically 1 milliamp. Exceeding 1 milliamp can be problematic since internal diodes are often parasitic ones that are parts of unintentional parasitic SCRs, and if one of those SCRs turns on then you usually get bonding wires ehaving as fuses.

This current limit may not be in the datasheet, but in some sort of supplemental notes for the IC's family.

A way to get protection: Use external diodes for clamping, and then add a resistor in series with the IC pin after the point the external diode does its clamping. The resistor value would be the worst case external diode voltage drop minus .3 volt, divided by whatever the maximum safe input current is. If the external diode drops 1 volt and the max. safe current is 1 mA, then this would indicate a 700 ohm resistor - next higher common value is 820 ohms. Maybe use 1K ohms to be safe.

- Don Klipstein ( snipped-for-privacy@misty.com)

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Don Klipstein

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

Reply to
Dirk Leber

Hello Dirk,

Mfgs usually spec two limits and not just that 300mV limit: The voltage applied to a pin from a source with supposedly infinitely small source resistance, then the maximum current into a pin.

If you source or sink 2mA into a pin and the manufacturer states under abs max that you should stay below 10mA then the voltage of the pin will go above that 300mV limit. Unless there are Schottkys inside, something that would be pretty unusual these days.

There recently was a lengthy discussion started by Chris Carlen here in the group. That is also where ESD measures were discussed. Excessive current into a substrate diode is what causes grief so that needs to be limited.

Regards, Joerg

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Reply to
Joerg

Dirk,

You should contact the manufacturer. They will provide you with some application support. I think the out pin is a high impeadant pin used for regulating the current. Shorting out to battery should be no prolem for the pin, it will cause you to loose current limitation. Shorting to gnd should also be no problem. Talk to an application engineer.

Josh

Dirk Leber wrote:

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josh lawton

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