A previous Design Idea “Gated 555 astable hits the ground running” (Ref. 1) offered a fix for the problem of the excessively long first pulse that’s generated by traditional topology 555 astable circuits on start up when gated by the RESET pin from oscillation-off to oscillation-on. See Figure 1 and Figure 2.
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| Figure 1. | The problem – the first oscillation cycle has a too-long first pulse on start-up, when gated by the RESET pin from oscillation-off to oscillation-on. |
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| Figure 2. | The fix via C2 charge injection on oscillation startup to equalize pulse length. |
However, unaddressed in this design idea is the fact that less traditional 555 astable topologies also suffer from the same long-first-pulse malady. Important examples of such circuits are oscillators that generate 50:50 symmetrical square waves, such as Figure 3.
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| Figure 3. | The long first-pulse problem also occurs in a 50:50 square wave topology popular for CMOS 555s. |
Happily, the same fix from “Gated 555 astable hits the ground running” works in this genre of oscillators too, as illustrated in Figure 4.
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| Figure 4. | C2 charge injection fix applied to CMOS 50:50 square wave oscillator. |
So, the problem is solved for CMOS 555 square wave generators. But what about their bipolar kin?
Despite their age, bipolar 555s still get designed into contemporary applications. The reasons for the choice include advantages like higher supply voltage rating (18 V vs 15 V) and greater output current capability (hundreds vs tens of mA) than CMOS types. But they do need to be wired up somewhat differently – for example with an extra resistor (as described in a previous Design Idea (Ref. 2) – when a 50:50 square wave output is required. See Figure 5.
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| Figure 5. | Bipolar 555 in gated 50:50 square wave configuration. |
The C2 charge injection trick will still work to correct Figure 5’s first pulse, but there’s a complication. When held reset, Figure 5’s circuit doesn’t discharge the timing capacitor all the way to zero, but only to VZ where:
Therefore, our old friend C2 = C1/2 won’t work. What’s needed is a smaller charge injection from a smaller C2 = 0.175•C1 as Figure 6 shows.
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| Figure 6. | C2 charge injection first-pulse fix modified for bipolar 555 square wave generation. |
References
- Woodward, Stephen. "Gated 555 astable hits the ground running."
- Woodward, Stephen. "Add one resistor to give bipolar LM555 oscillator a 50:50 duty cycle."
