Decision circuit for switching to back-up power supply e.g. for sequence or run-off control unit, counter etc

摘要

The decision circuit (100) has two voltage signal inputs from two power supplies. The circuit includes a comparator (26), one input of which is connected to the main voltage (22), and which provides a control signal according to which of the voltage signals is greater. The circuit also includes a switch (28,30,32) controlled by the control signal to connect the appropriate power supply (34). A current source, which generates a current proportional to the temperature, contains a current generating stage with a transistor and operates in the region lying below the threshold of the transistor. A voltage regulator (102) is provided which regulates the back up battery voltage signal (24) to generate a reference voltage (104) which forms the second input to the comparator. The voltage range of the reference voltage does not overlap that of the of the main supply. The voltage regulator has two voltage drop elements, which, depending on the current generated by the current source and generates a voltage component with a negative or a positive temperature coefficient