This application note (AN895) describes how the TSM101 integrated circuit can be used to realize a voltage supervisor, and gives an example of a working design. The TSM101 combines a high stability series band gap voltage reference, two ORed operational amplifiers and a current source.
The note explains the principle of operation, in which the two operational amplifiers act as comparators against the internal 1.24V reference, and gives the equations for calculating the resistor network. A worked example supervises a 12V ±5% power supply and lists the resulting component values, with the complete schematic shown in Figure 2.
| Voltage reference | 1.24 V internal reference |
|---|---|
| Operational amplifiers | Two ORed operational amplifiers |
| Minimum operating voltage | 5 V |
| Output pin | Pin 6 |
| Zener diode in example | 4.7 V |
The minimum operating voltage of the TSM101 is 5V.
When the input voltage is out of the limits, the output on pin 6 is close to zero.
With a 4.7V Zener diode, Q1 will never be saturated if the supply voltage is lower than 5V, avoiding erratic behavior below the operating voltage.
The optocoupler is ON only if the input voltage is inside the specified range.