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Hi Dennis,
The official answer is what is stated in the datasheet, the abs max rating for any pin is VA + 0.3.
Here is more information on what is happening on the SYNC pin. There is a internal diode to ground. If the voltage on this pin goes more than about 0.3V beneath ground it will start conducting. There is no diode between this pin and VA, which is why you are not seeing the current go up when the voltage is at 3.3V and the power is off. The voltage on the SYNC pin is probably going into a gate of a transistor so the current will be low. The danger is that when the voltage becomes too high you can damage this transistor. 3.3V probably will not damage it but this is not certain.
For best reliability you could install an external Schottky diode between this pin and VA. Then when the voltage is more than 0.3V above VA the Schottky diode will start to conduct and protect the SYNC pin. You would also be good to increase the series resistor so that the current stays below 1mA when the diode is conducting.
Mike
Thanks for the official reply, Mike.
Please confirm that the diode to ground has it's anode connected to ground, and is reverse biased when we apply +3.3V to the input.
To minimize the rework on our spaceflight board, we'd prefer to only increase the series resistor to 499 Ohms, but not install the diode you describe.
However, this would result in 3.3V still appearing at the SYNC pin if no current flows, exceeding the VA+0.3V maximum requirement. Would this still pose an unacceptable risk to the input transistor? In that case we would have to install retrofitted diodes in many places on our flight board, which could be difficult.
Sorry, I have to ask. Retrofitting diodes on a flight board is a serious step and I need to be sure it's absolutely necessary.
Thanks.
Thanks, Mike. I'll pass on to our review board that TI makes no guarantees or exceptions to the datasheet.
Dennis
Mike,
The Schottky diode we have in stock and approved for flight has a 0.4V to 0.5V drop across our operating temperature range.
Therefore the voltage between -SYNC and VA could be as high as 0.2V over the "absolute maximum" 0.3V spec.
Would you be able to authorize this, since the process has a 6.5V reverse breakdown rating? Or will we have to identify a Schottky that will have </= 0.3V forward drop across our temperature range?
Thanks again,
Dennis
Hi Dennis,
I can't authorize anything that contradicts the datasheet since the datasheet is our contract with the customer. However, as you mention, with the 6.5V breakdown rating, I don't think it will be a problem being 0.2V over, especially as no current is being drawn into this pin, you have a series resistor, and the time that this will occur is of a very short duration.
Mike