TS339I

[omarvario]

Hi, Team

   We are facing issues with test process performed at ICT test station. We are testing driving current to detect Zener Clamp diode 8.2V, the units fail with 2.64688 volts, but this line is connected to comparator TS339I pin 5 that is not power up. When disconnect the pint 5 of this comparator, TS339I, our Zener Diode start works and the voltage 8.2V is present. 

    Our concern is related with this TS339I that something is affecting Zener's test process when the V+/V- of the comparator are absent, probably the current used for the test, partially is going into the comparator. Do you have information about this behavior and recommendations when this device is tested? Or are there some test recommendations or critical aspects that need to be careful when is under test for these comparators?

[Peter BENSCH]

Welcome @omarvario, to the community!

If you want to test the zener voltage of CR402 in your schematics, you must inevitably apply a higher positive voltage to its cathode, where pin 5 of the comparator is also connected. Now please take a look at the data sheet, fig 1 of the comparator, where you will find two internal diodes on each input pin: one from VCC- to the input and one from that input to VCC+. And it is precisely via the latter diode that you connect the zener voltage to VCC+ of the comparator, so that depending on the current limitation of the supply, the voltage you measured results.

I don't see any way of measuring the zener voltage with the input connected in an ICT. But maybe you can measure the forward voltage of CR402, which at least gives an indication of its presence.

Hope that helps?

Regards
/Peter

[omarvario]

Hi, Peter

  Thanks for your recommendation, we will change the method to detect this Zener Diode CR402 in the circuit, our concern at this moment is with the performance of the comparators, if they were damaged. At this moment we removed all components (TS339I) to use other supplier of comparators that it's not affecting the current test. However, all comparators (TS339I) were taken out when the unit under test failed on ICT because we don't know if this condition affected internally its performance or malfunction on the near future. What do you think?

Regards  

[Peter BENSCH]

Depending on the zener voltage, an ICT at zener diodes can indeed be dangerous for the surrounding circuit. In this respect, I would always work with the designers in advance to avoid such possible damage. A series resistor between the CR402 cathode and U502, for example, would limit the current flowing into the input of U502.

But to come back to your question: I am currently not worried that the applied test voltage will damage U502, as the test voltage supplies the comparator with current via the upper ESD protection diode, so to speak. However, your current test current does not currently manage to supply the entire circuit connected to +12V with enough current, which is why it collapses to approx. 3.3V, which you can measure at U502.5 reduced by the forward voltage of the ESD diode (approx. 0.65V). So as long as the test voltage is below 18V (data sheet, table 2, VCC+) and the test current is below 50mA (table 2, IF), TLC339 will neither be damaged nor destroyed.

Regards
/Peter