Digital systems that are not very demanding about their clock frequency, like multiplexed panel indicators and display sequencers, often require a square-wave (clock) generator for driving digital or AC-powered subsystems. Some sensors also require the use of an AC drive. This application note presents a square-wave oscillator that requires no external components except the bypass capacitor. This circuit generates a square-wave that is useful as a clock signal or AC drive for the excitation of sensors.
This design idea appeared in the March 10, 2006 issue of EE Times.
Systems often require a square-wave (clock) generator for driving digital or AC-powered subsystems. These same systems may not be very restrictive about the frequency of the signal. Such frequency-tolerant digital systems include multiplexed panel indicators and display sequencers.
Some sensors also require the use of an AC drive. Electro-chemical sensors (e.g., for monitoring tilt, conductivity, and moisture) cannot tolerate DC current, because it degrades the materials of which they are made. Capacitive sensors operate on a principle that calls for AC drive. With piezoresistive sensors, an AC drive cancels DC errors and low-frequency noise. Fluxgate sensors also need an AC drive.
The circuit of Figure 1 requires no external components except the bypass capacitor. This circuit generates a square wave that is useful as a clock signal or as an AC drive for the excitation of sensors. The design produces a powerful 3V to 5V square-wave output with very low source impedance (less than 10Ω). It also provides rail-to-rail excursions in a small SOT23 package. The output frequency is reasonably stable with variations in supply voltage and temperature (Figure 2).
The IC shown is an inverting charge pump, the MAX1697, which was originally designed to supply a negative voltage in positive single-supply systems. Its rail-to-rail capability means that the output-amplitude precision depends directly on the level of VIN. Duty-cycle error is a fraction of 1%.
The circuit has a dependable start response, even with supply voltages below 1V (depending on the MAX1697 version used). Note that the tradeoff for simplicity in this circuit is a lack of flexibility in output frequency. As shown in Figure 1, four nominal frequencies are available according to the IC version chosen (i.e., 12kHz, 35kHz, 125kHz, and 250kHz). Figure 3 is a scope shot of the output waveform.