• State TI Thinks Resolved
  • Locked Locked
  • Replies 2 replies
  • Answers 1 answer
  • Subscribers 20 subscribers
  • Views 613 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

NA556 / 555 issues

Michael Lumetta
Prodigy 20 points
Other Parts Discussed in Thread: NA556, LM555, SE555

I am attempting to use an NA556 as an oscillator and as an inverting buffer. The circuit is simple enough, but it doesn't work..

1464.556 support 2.pdf

Supply is regulated 12 volts. Bypass capacitor is about 1/2" away.

First issue: From my calculations the oscillator portion of the circuit (output pin 5) should cycle at 220KHz, however it only runs at 156KHz with about a 55% duty cycle. I have measured the capacitance in circuit and it is 335pF. The resistors are 9.53K. Has anyone had this experience trying to oscillate a 555 or 556 in this "high" frequency range?

Second issue: The second half of the 556 is configured to be an inverting buffer driven from the oscillator. It should oscillate 180 degrees out of phase with the first half of the 556.. It doesn't at this frequency. The output stays high and the discharge pin stays off. As I increase capacitance C9 to slow the oscillator, The buffer will start to cycle.. At 100KHz the waveform is about 95% duty cycle. But as I slow the oscillator further to 50KHz the buffer is now working perfectly. It appears that the 556 is operating with a huge delay, but why? I don't see in the datasheet where a several microsecond delay should be expected.

 

Thanks in advance,

Michael

 

 

  • 0
    Prodigy 20 points

    I've answered my own question. After reviewing a data-sheet provided by National Semiconductor for the LM555, I found a note on the last page stating that the delay can be up to 10uS if the trigger pin is driven all the way to ground!

    The Texas Instruments data-sheets for NA, SA, NE, and SE555 and 556 make NO mention of this issue.. I am the victim of a poorly written data-sheet.

     

     

  • 0 in reply to Michael Lumetta
    TI__Guru* 97046 points
    This thread is closed due to inactivity. If you have a question or comment on this topic, please feel free to continue the conversation here -- just don't expect the OP to respond!