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TLC555: Why TLC555 CONT capacitor will cause the output segmentation in astable operation

Zhen Meng
Prodigy 530 points
Part Number: TLC555
Other Parts Discussed in Thread: UCC27624

Hi Friends,

I have some output question about TLC555.

The customer use the output of TLC555 as the input of UCC27624. The TLC555 works in astable operation just like the datasheet.

But there are several segmentations in output waveform of TLC555. It happened in customer’s lab.

TLC555 outputs square wave for a period of time, but suddenly the output becomes low voltage.

After about 1ms, the output becomes square again. This time ,the square output last more time, but still drop to a low voltage.

This happened three or four times, and the output finally becomes square waveform and never drop again. Every time it becomes square waveform, the duration will be longer than last time.

 

We have found that the CONT CAPACITOR is the reason.

If we connect a 0.01uF capacitor to CONT, there are no segmentations. But when we use 4.7uF capacitor to CONT, the segmentations will appear.

 

Why this happen?

 

Here are the schematic and the oscilloscope picture. By the way the schematic is drawn by myself, the RL in customer schematic should be the the UCC27624 part. 

OUT means output port of TLC555, and 2-6 means the THRES and TRIG port(Because they are connected together)

If we zoom in this picture, it will be like this:

The purple line is 2-6, it is an triangle waveform, and green line for output, which is square waveform. 

  • 0
    TI__Mastermind 25915 points

    Hey Zhen, 

    The cont pin is tied to the input of the comparator as shown in the simplified schematic below. We recommend adding a capacitor that is 0.01 uF. Adding a large capacitor creates an RC time constant and will take a long time to charge up completely. The trip point of the threshold comparator is changing over time because of the RC time constant and giving unpredictable results.

    Best Regards, 

    Chris Featherstone

  • 0 in reply to Chris Featherstone
    TI__Prodigy 530 points

    Hi Chris,

     

    I’m still confused about the trip point of the comparator.

     

    I know that with larger cont capacitor,  will be larger. The trip point of the comparator will also change.

    But I think it will cause the duty time but not create a zero voltage area.

    Just like my simulation, the upper plot is output square, the lower plot is the TRIG-THRES waveform. Red line is voltage of CONT.

     

     

    There is no output signal at the beginning. After capacitor charging, device start to output square form, but duty cycle is abnormal.

    With the cont capacitor charging to proper value, the duty cycle becomes proper value.

     

    There is NO zero voltage zone in my simulation. But there is zero voltage segmentation in customer’s oscilloscope.

    Why the voltage of TRIG-THRES will drop to zero? Does it means C5 discharge? But why?

  • 0 in reply to Zhen Meng
    TI__Prodigy 530 points
    Zhen Meng said:
    I know that with larger cont capacitor,  will be larger. The trip point of the comparator will also change.

    I miss a equation, it should be T=RC. With larger cont capacitor, the charging time T will be larger.