06-22-10
Open Source Hardware Is Messy
Just started working with an STM1061N31WX6F 3.1V supervisor. These kinds of devices come in lots of different flavors from many companies. More on that when I get to the battery charging circuit. This one has some potential to be a useful Supply Voltage Sensor. The attribute that makes it useful is that the output is ‘open drain’. That’s a MOS term. When the output is off, it is connected internally to GND. When the output is on, it is in high Z (effectively connected to nothing). By simply attaching an external pull-up to this pin we can get a HIGH signal when the output is off. Why this is better than other styles of output circuitry will be gotten to in a later post.
The pros of using this device or one similar are that it is small (SOT23-3) but fairly easy to prototype. It costs less than the TL7700 as well (~$0.50). Also, it tracks a very linear response:
The bummer so far is that the stated trigger voltage of the part number I’m using is 3.1V, and the lowest I can go and still get a useful output is 3.24V. In any case, the system would have to be calibrated, and as long as the factory is consistent this may be a better part than the TL770. Here’s the circuit I’m using:
The next test for this one is to use a supervisor with a lower threshold. They range from 1.6V to 3.1V in 0.1V steps and Mouser has a pretty good selection.
