NOTE: I am unable to "Insert Files" referenced in this. I are also company-confidential so I can't post them anyway. How do I get you these diagrams?
We are using a ATL431 shunt-regulator part in a crowbar circuit in "open loop" mode (i.e., when a voltage gets high enough it enables the regulator output which causes a triacto turn on). The diagram of or circuit is in the PDF attached "Crowbar_Diagram.pdf".
When we apply power across the PWRIN_POS and PWRIN_NEG inputs the triac fires at about a 20V level due to a droop on the SHUNT_CATHODE. At this point the shunt gate (SHUNT_GATE) is well below the 2.5V reference level so the output should be off. I have attached an oscilloscope shot in a ".png" file format called "Blue_PWRIN_POS_Yellow_SHUNT_CATHODE_NoTriacConnection.png" showing the input voltage (in Blue) and the SHUNT_CATHODE voltage (in Yellow). There is a droop from about 20-25V but it is fine at lower and higher voltages. It almost looks like the regulator is turning on some during this period. Note that we disconnected the triac gate to take these waveforms since the triac was firing due to this droop and the input voltage was dropping due to supply current limiting so the triac is not really involved at all and can be ignored in the diagram ... which just leaves the ATL431 circuit.
The almost 3.3V difference between the input voltage and SHUNT_CATHODE indicates there is a fairly high current draw into the cathode (approx 100uA) which is way higher than any maximum voltage specified for the ATL431. The data sheet is not very clear on how this type of open-loop circuit operates or the currents involved.
Do you know what is causing the "droop"?
If the "REF" input on the the ATL431 is tied to PWRIN_NEG (essentially grounding it) then the waveform looks good as shown in the scope shot "Blue_PWRIN_POS_Yellow_SHUNT_CATHODE_REFGrounded_NoTriacConnection.png" (the Blue and Yellow are right on top of each other).
Thanks
