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I have recently been having issues where the buffer of the circuit below has been damaged. The output pin (2) does not change with changes to the input pin (1). I sent 2 of these ICs to Fairchild for FA analysis, and their report indicates that there was an Electrical Overstress (EOS) condition at the ESD circuitry of pin 1 which caused metallization.

Could this be due to too much current going into pin 1? I have checked the circuit many times, and do not see anyway for the input pin to be damaged.

Circuit

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  • \$\begingroup\$ What if IN1 receives an input pulse which is more than six volts below ground. Would anything prevent the voltage on U1.1 from going beyond the Absolute Maximum Rating? \$\endgroup\$
    – supercat
    Feb 20, 2012 at 21:54
  • \$\begingroup\$ @Supercat: Yes, that could damage U1, but due to the optical isolation should not hurt U2. \$\endgroup\$
    – Olin Lathrop
    Feb 22, 2012 at 0:35

1 Answer 1

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First, you left off the bypass cap on U2. Second, you have the ground tied together on both sides of the opto-isolator, thereby defeating its purpose. You might as well connect IN directly to U2 pin 1 via the 220 Ω resistor and be done with it.

Separate the grounds and put a 1 kΩ resistor in series with U2 pin 1. If the isolated circuit on the left has high dV/dt, then the capacitive coupling could cause momentary high currents into U2. The 1 kΩ resistor will help limit that to hopefully survivable levels.

Another possibility is that you're zapping the parts in handling. Make sure to observe the usual ESD precautions, like non-static work surface, grounded wrist strap, proper packaging, etc.

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  • \$\begingroup\$ What scenario could cause the isolation circuit to have high dV/dt? \$\endgroup\$
    – dla59
    Feb 21, 2012 at 18:16
  • \$\begingroup\$ @Daniel: Lots of things can cause spikes. Without knowing what the isolated circuit is connected to, it's hard to say. It also depends on what it is referenced to and what part of that circuit is driving the opto-coupler. For example, if the opto input is sitting on the high side of a ignition coil, there can certainly be large spikes. We can only assume here that the circuits were opto-isolated for a reason. \$\endgroup\$
    – Olin Lathrop
    Feb 21, 2012 at 20:10

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