The MAX975 is ideal for making a window detector (under-voltage/over-voltage detector). The schematic shown uses a 1.2V reference and component values selected for a 2.0V under-voltage threshold and a 2.5V over-voltage threshold. Choose different thresholds by changing the values of R1, R2, and R3. The output of U1 provides an active-low under-voltage indication, and the output of U2 gives an active-low over-voltage indication. AND-ing the two outputs provides an active-high, power-good signal. The transient analysis of the circuit shows the the varying input voltage and the corresponding outputs from U1 and U2.