I am designing a piece of test equipment that has a DAC output. The test equipment might be used to troubleshoot UUTs that are damaged or malfunctioning. Therefore I want to add a protection circuit to the DAC output. The DAC circuit itself is already designed.
The question is what's the cheapest protection circuit for this test equipment?
- I already looked at the Bourns TBUs but the smallest ones they had had a current limit of 100mA (which is too large). They also had rather large output impedance.
- I already looked at PTC poly-fuses, but I couldn't find any that were rated for > 100V, had a hold current > 3mA, and a trip current < 10mA. Also poly-fuses with very low trip current have very high output impedance.
- I came up with this circuit, using two depletion mode MOSFETs to form a bi-directional current limiter. I then reduced the apparent output impedance using an op-amp. This seems to work but it costs several dollars and I was hoping to do better price-wise.
These are the design goals:
1) The output range of the DAC is 0.0V to 5.0V.
2) The DAC output bandwidth is 1KHz max.
3) The protection circuit sits between the DAC output and the connector pin on the test box.
4) The voltage present on the output of the protection circuit should track the input to within ±2.5mV when the output current is within the range of ±3mA and the input voltage is between 0.1V to 4.9V.
5) When the input voltage is outside the range 0.1V to 4.9V then the protection circuit output voltage is not required to track the input voltage.
6) When the output current is more than ±3mA the output voltage is not required to track the input voltage.
7) The DAC output should survive a direct short to +33V DC indefinitely.
8) The DAC and protection circuits should survive intermittent voltage spike of ±100V occurring for less than 1 second not more than once every 10s at the UUT pins.
9) During a fault the protection circuit shall not allow more than 10mA to pass through the connector pin on the tester for shorts up to ±100V.
10) After an over-voltage or over-current fault is removed from the pin the protection circuit should be un-damaged and revert to normal operation within 10 seconds (faster is better).
11) The box is sealed and the user cannot open it to replace any parts or disposable fuses in the protection circuit.
12) ESD is handled separately and not considered here.
13) Protection circuit should not occupy an area larger than 20mm x 20mm on the PCB, and it shall not be more than 20mm tall.
I already have supplies 3.3V, +5.0V, and ±15V available for the protection circuit to use. For cost and space reasons I would like to avoid having to add extra power supply voltages in the tester.