Nonvolatile standby/on switch remembers its state

You can use the standby/on switch in Figure 1 for industrial or telecom applications in which the circuitry must somehow “remember” its state—standby or on—after a power failure that occurs when no operator is present. An alternative approach uses a battery or a supercapacitor and a flip-flop. This approach is less reliable, however, because the circuit can lose its state if leakage current drains the battery. Another alternative involves the use of a microcontroller and EEPROM, but that approach requires software plus a provision for start-up time. Also, a stand-alone EEPROM for this application has an awkward interface.

You can use an electronically programmable voltage reference, IC₄, as a single-bit nonvolatile-memory cell. To remember the state of the standby/on switch, this circuit programs IC₄’s output voltage high or low and can reprogram it at least 50,000 times. IC₁ is a low-dropout linear regulator with reset output and a wide input-voltage range that extends to 72V. A microprocessor supervisor, IC₂, debounces the standby/on pushbutton and supports the programming of IC₄ by increasing the pause length between pulses. IC₄’s output drives IC₅, an inverter with Schmitt-trigger input, which in turn drives the gate of transistor Q₂ to control the main power supply.

Flip-flop IC₃ helps to change the standby/on state with each press of the control button. At the end of IC₄’s programming cycle, a low-to-high edge at IC₃’s clock input sets the flip-flop to its opposite state, thanks to the feedback from the inverter. IC₂’s reset triggers this action at power-up to ensure that the switch is ready to change state. Transistor Q_1B and IC₁’s reset output prevent the programming of incorrect states by blocking IC₄’s adjust input during start-up and power-fail conditions.

You must block the effect of IC₂’s power-up or -down reset pulse on IC₄’s adjust input; C₂ therefore sets IC₁’s reset time-out to be longer than IC₂’s reset time-out. The threshold voltage of IC₂, 2.9V, is also lower than that of IC₁, 4.6V. The worst-case 1.32V input-threshold voltage of IC₅ guarantees the standby position at first power-on because the factory-preset output for IC₄ is only 1.2V.