Transistor Power Switch
One inexpensive and practical method of reducing dissipation in a digital circuit is to power the active components only as they perform a function, as for instance when operating briefly during a low duty cycle. Power is saved by clocking on the power for the IC's only when necessary, so that the IC's draw no energy during periods of inactivity.
A solid-state switch/low-pass pifilter has been designed to source these components. The filter employs a storage capacitor that is trickle-charged to present a constant load to the main power supply. Other filter components are used to smooth transients as the IC's are switched on. The circuit may be useful in battery-powered equipments.
A schematic of the switch/filter is shown above. Power is supplied to the switching transistors Q1 and Q2 via the filter network comprised of C1, C2, R1, and R2. As Q1 and Q2 turn on, current is drawn from C1, a capacitor that is charged to a voltage equal to the IC supply voltage VCC (plus the collector-emitter drop across Q2).
The V+ supply therefore is slightly greater than 5 volts for TTL devices but is less than 5.5 volts (the design maximum specified for these devices). The values of R1 and R2 are chosen to permit C1 to charge to the required voltage during the off cycle. Components C2 and R2 are optional; they provide additional circuit filtering.