Design Ideas: March 3, 1994

Shutting down circuitry not in use is a common method of saving power in portable and battery-powered equipment. Although many ADCs have the ability to power down, most other analog components, such as op amps and sensors, do not. The conventional approach to shutting down analog signal-conditioning circuits is to power them using a voltage regulator that has shutdown capability, and to control this regulator using a dedicated signal line from the microcontroller. Often, how ever, this additional I/O line isn't available.

The circuit in Fig 1 uses an alternative approach by tying the bandgap V_REFOUT pin of an ADC to the shut-down pin of the voltage regulator. When the ADC shuts down, so does the regulator and the rest of the analog circuitry. Thus, a single command line can control power to the ADC and, through the ADC, control power to the entire data-acquisition subsystem.

When the ADC is in power-down mode, Fig 1's supply current—including all circuits in the signal-conditioning block—typically drops to 3.4 µA (3.5 µA for the ADC and 300 µA for the regulator). The ADC requires 55 msec to complete a conversion on all four inputs. If the ADC samples the analog inputs once per second, the average current drain will be approximately 315 µA. EDN BBS /DI_SIG #1381