S7V8A Not Shutting Down When SHDN at 0V

I am using a MAX931 comparator to generate a 0v output when the voltage of a LiPoly battery pack falls below 3.2v. I have connected the output of the comparator to the SHDN pin on the S7V8A. I have confirmed (with a voltage meter) that the voltage on the SHDN pin is 0v when the battery is low and 3.2-4.2v when the battery is charged. I have not actually connected it to the microcontroller it will be powering but the output of the S7V8A does not change from the 3.3v I have it adjusted to when the SHDN pin is at 0v. Is it not shutting down? Does the voltage not drop when it is shutdown? Thanks.

I had an additional thought today about a voltage I did not think to test. I realized that I have the battery connected to the Vin and GND of the S7V8A and a different bench-top voltage source connected to the comparator circuit. My intention with this was to test my circuit worked before connecting it to the battery as well (I could control the cut-off and cut-in voltage with the bench-top source). What I did not test was the voltage between Gnd on the S7V8A and SHDN. I discovered it was significantly above the 0.4 v required to shut it down (it was about 4v when the battery was at 4.2v). So my testing process was obviously flawed. I now soldered the comparator to the same ground and positive as the S7V8A. So will it work now? Is my only way to test it to run the battery down to my cutoff then retest if the S7V8A shuts down?


You do not necessarily need both common GND and common VIN to test that setup. You just need common GND, so you could test using both the battery and the bench-top supply as long as both power supplies and all your devices have a common GND.


Great advice! I was able to do a quick resolder and connected the grounds together. I then connected the positive source for the comparator back to the benchtop supply and was easily able to test the circuit and it worked great. Obviously it was just a misunderstanding of potential (voltage) from the beginning. Thanks.