Level shifter SN74AVC16T245, SN74AVC4T774 - Leak voltage
Part Number: SN74AVC4T774
I am using the SN74AVC4T774 to buffer several GPIO from a processor to a terminal block connector to the outside world. The B port is connected to the processor and the A port is connected to the terminal block. Both VCCA and VCCB are 3.3V, but supplied from different regulators. VCCA is always on, but VCCB can be off depending on the power state of the system.
Because there is a potential for misapplication of voltage to the terminal block, I want the GPIO to be tolerant to +/-20V. Is it sufficient to place a 1kΩ between the SN74AVC4T774 IO pin and the terminal block? From the Absolute Maximum Ratings table in the datasheet, it seems like this would be acceptable for the -20V case, since this would result in about -19mA through the low-side clamping diode, which is within the allowable -50mA. But it looks like there is no clamp diode on the high-side, or a back-to-back diode as indicated in the linked forum thread. Would I need to add an external diode to VCCA and a Zener from VCCA to ground to protect against 20V? I also considered a Zener to ground on the IO pin, but the several hundred picofarad capacitance of the Zener combined with the 1kΩ resistor would slow down the signal slew rate, which would violate the 5ns/V input rise/fall rate requirement.
Thanks for any assistance.
I do not recommend using that high of a voltage on the inputs of the parts. Although there are diodes to protect against high voltages and lower currents, those diodes take time to turn on. In this same time, there may be a current strike which will completely damage the part at that high of a voltage.
You are correct, there is no positive clamp diode on the input structure of the device.
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In reply to Karan Kotadia:
Thanks for the feedback. I'm afraid I don't understand your comment about diode turn-on time. The datasheet states that this part is rated to ±8000V HBM and ±1500V CDM ESD transients. Surely if the turn-on time of the protection diodes is sufficient to handle these transients, it would be sufficient for any transients associated with application of ±20V?
If you have a suggestion for an alternate part, please let me know. Meanwhile, I would like to keep the SN74AVC4T774 if possible, as it is already in my design. Would it be sufficient to add an external low-capacitance steering diode to VCCA to protect against the +20V scenario? VCCA is normally 3.3V supplied by an LDO regulator, but would be clamped to ~3.6V via TLVH431 (or similar) shunt regulator to dump the excess current from the steering diode.
In reply to Eric Peters:
I do recommend you to switch the SN74AXC4T774. It is a newer part with a better design that helps get rid of power up glitches. It is also a drop in replacement.
As for applying 20V, as I mentioned earlier, I do not recommend you use such a large voltage on the input. You will need an external diode if you want to apply such a large voltage. I have seen customers use Zeners with a series resistor maintain the input voltage at the level they need it.
The SN74AXC4T774 has an absolute max VCCA/input voltage of 4.2V, versus 4.6V for the AVC version. It seems that the newer version is less robust and I would not want to use it for this application.
Just to clarify, I want to protect the input against accidental application of ±20V. The normal use-case is 3.3V logic signaling. The problems with using a Zener on the data line are 1) excess capacitive loading (hundreds of picofarads) and 2) very high dynamic resistance for the package size I want to use. I will move forward with the steering diode approach. Please let me know if you have any other suggestions.
I don't have anymore suggestions. Your solution should work for you. I am just not able to guarantee operation as the use-case is outside the datasheet operating condition.
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