Figure 6-5 shows that adding a second resistor, R_B, to the circuit of Figure 6-2 and connecting the trigger input to the threshold input causes the timer to self-trigger and run as a multivibrator. Capacitor C charges through R_A and R_B and then discharges through R_B only. Therefore, the duty cycle is controlled by the values of R_A and R_B.

Figure 6-5 Circuit for Astable Operation

Figure 6-6 Typical Astable Waveforms

This astable connection results in capacitor C charging and discharging between the threshold-voltage level (? 0.67 × V_CC) and the trigger-voltage level (? 0.33 × V_CC). As in the mono-stable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage. To reduce distortion, use at maximum frequency of 100kHz or below. If higher-frequency operation is required, consider using the TLC555 LinCMOS? Timer instead.

Figure 6-6 shows typical waveforms generated during astable operation. The output high-level duration t_H and low-level duration t_Lare calculated as follows:

Other useful relationships for period, frequency, and driver-referred and waveform-referred duty cycle are calculated as follows:

Figure 6-7 Free-Running Frequency