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NE555: NE555 50% duty application issue

Zhengxing Li
Prodigy 120 points
Part Number: NE555
Other Parts Discussed in Thread: TLC555

Hi ,

My customer is using NE555 for 50% duty generation.

The circuit is attached. ---Vs=12V, Reset pin is pulled up to 12V, control is floating, 33k frequency(R=12k, C=1.8nF)

The disch pin output is as expected, while the output pin has 53% duty (3% ahead the disch voltage jump high).

You can check the waveform as below.

---The first question: Could you let me know what the possible reason for the unexpected 3% duty and how to remove it?

And, even for the disch pin, the duty is not 49% (not 50%). In our customer's application, we need a precise 50% duty cycle(49.75%-50.25% duty range) . 

---The second question: The RC charging and discharging time should be the same, why the duty is not a good "50%"?

---The third question: What can we do to improve it basing on NE555 circuit? we can re-design the circuit?

---The fourth question: If 555 circuit can't make sure the good "50%", can you suggest a simple/low cost circuit if you have?

Thanks a lot.

  • 0
    TI__Guru**** 152796 points
    Zhengxing Li,

    The output of NE555 uses NPN transistors. For VOL the saturation voltage is close to 0V. The VOH is a base emitter diode drop lower than VCC. Because of this drop the duty cycle won't be 50%. The discharge output transistor is heavily driven with base current so there is a small delay to come out of saturation. However the time shift is more than I would expect.

    It is not practical to get exactly 50% duty cycle output. Can you double the frequency then use a flip-flop to get a 50% duty cycle?

    I do see one very odd effect in the waveform. In the highlighted red ellipse area, Vc continues to fall even after output goes high.
  • 0 in reply to Ron Michallick
    TI__Guru**** 152796 points
    Zhengxing Li,

    I tested NE555 and the 800nS delay between output and discharge was seen. The capacitor voltage slope was synchronized to output as I expected.

    TLC555 does not have a large delay between output and discharge. Alos VOH and VOL are both rail to rail so output will be much closer to 50%, however, I cannot be confident that it will be with .25% of 50%