SLFS043I September 1983 – July 2019 TLC555
As shown in Figure 12, adding a second resistor, RB, to the circuit of Figure 9 and connecting the trigger input to the threshold input causes the timer to self-trigger and run as a multi-vibrator. The capacitor C charges through RA and RB and then discharges through RB only. Therefore, the duty cycle is controlled by the values of RA and RB.
This astable connection results in capacitor C charging and discharging between the threshold-voltage level
(≈ 0.67 × VCC) and the trigger-voltage level (≈ 0.33 × VCC). As in the monostable circuit, charge and discharge times (and, therefore, the frequency and duty cycle) are independent of the supply voltage.
Figure 13 shows typical waveforms generated during astable operation. The output high-level duration tH and low-level duration tL for frequencies below 100 kHz can be calculated as follows:
Other useful relationships are shown below:
The formulas (1-7) do not account for any propagation delay times from the TRIG and THRES inputs to DISCH output. These delay times add directly to the period and overcharge the capacitor which creates differences between calculated and actual values that increase with frequency. In addition, the internal on-state resistance ron during discharge adds to RB to provide another source of timing error in the calculation when RB is very low. The equations below provide better agreement with measured values. The formulas Equation 8 represent the actual low and high times when used at higher frequencies because propagation delay and discharge on resistance is added to the formulas. Because the formulas are complex, a calculation tool, TLC555 Design Calculator can be used to calculate the component values.