At times, it may be necessary to power a solenoid-type pulsed actuator from a low-voltage battery in portable equipment. The circuit in Fig 1 accomplishes this task using a standby current of 10 µA. The design takes advantage of the fact that solenoids like being actuated by capacitive discharge at much higher levels than their nominal voltage. This mode of operation results in a good, crisp response by the solenoid plunger.IC1 is a micropower boost converter powered directly from a 3V lithium battery. As long as the ACTUATE line is low, IC1 is in standby mode and consumes little current. When ACTUATE goes high, IC1's pin 3 goes low. C1 couples this low-going edge into the feedback divider comprising R1, R2, and R3, which pulls the LBI pin of IC1 below the IC's internal reference. LBO then goes low, tying the bottom of the feedback divider to ground.
IC1 then begins to switch at approximately 150 kHz, gradually charging C2. When C2's voltage reaches 21V (which takes approximately 0.15 sec), the voltage at LBI crosses the 1.24V reference threshold, forcing LBO's open-collector output high. IC2A and IC2B buffer this signal and drive MOSFET Q1's gate high, allowing the energy stored in C2 to dump into the solenoid coil. Because LBO is now high, the voltage at LBI is also high and stays that way. Thus, the FET stays energized until nearly all of C2's energy dumps into the solenoid.
The FIRED signal reports to the system that the solenoid has fired and that the circuit should now be disabled. D1 clamps the feedback to VIN. The feedback divider sets the voltage limit on C2 to about 23V. This setting ensures that overvoltage won't destroy IC1's internal power switch should the solenoid coil open or become disconnected.
The size of C2, which you choose empirically, depends on the particular solenoid. To choose C2, charge up larger and larger capacitors to approximately 18V, and connect the solenoid to the charged capacitor until you find a size that reliably fires the solenoid. Using the next larger capacitor and 21V to fire the solenoid provides some design margin.