Author Topic: Does the ST version of the NE555 have a dead time on the outputs ?  (Read 2502 times)

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Offline Simon

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I need to drive a "push pull" type setup with an NE555 made by ST. but the datasheet say's nothing about any dead period between high and low to stop shoot through. Do i therefore assume there is none and that shoot through will occur unless i do something myself ?
 

Offline edavid

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Re: Does the ST version of the NE555 have a dead time on the outputs ?
« Reply #1 on: June 08, 2013, 03:09:38 pm »
Are you talking about shoot through internal to the 555, causing supply current spikes, or shoot through in your external circuit?

Anyway, all standard bipolar 555s are the same, and they have no dead time.  CMOS 555s don't have dead time either, but they have less internal shoot through.
 

Online SeanB

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Re: Does the ST version of the NE555 have a dead time on the outputs ?
« Reply #2 on: June 08, 2013, 03:51:35 pm »
That is why there is the recommendation in the datasheet for the big 470uF or higher value capacitor connected to pins 1 and 8 right by the IC, as the output stages do have a lot of shoot through when switching. The CMOS versions suffer from this as well, but as the output devices are much faster the current spike is a lot smaller, and as well the larger threshold voltage on the devices and the quicker transition time of the output reduces it to a small value. The regular bipolar can have a 100mA plus pulse on the supply during switching, which often causes funny things if the supply impedance is high and the chip draws enough current to go into the indeterminate regions during the transition, often oscillating happily during switching or burning out.
 

Offline edavid

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Re: Does the ST version of the NE555 have a dead time on the outputs ?
« Reply #3 on: June 08, 2013, 04:00:51 pm »
That is why there is the recommendation in the datasheet for the big 470uF or higher value capacitor connected to pins 1 and 8 right by the IC, as the output stages do have a lot of shoot through when switching.

What datasheet recommends 470uF?  The TI datasheet shows 0.01uF.
 

Online SeanB

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Re: Does the ST version of the NE555 have a dead time on the outputs ?
« Reply #4 on: June 08, 2013, 04:31:03 pm »
Look at http://www.doctronics.co.uk/pdf_files/555an.pdf page 5 and you will see from 100n to 10uF recommended. Right next to the chip as well, and often the designers use 10 times bigger caps than needed.
 

Offline Simon

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Re: Does the ST version of the NE555 have a dead time on the outputs ?
« Reply #5 on: June 08, 2013, 08:47:07 pm »
I was refering to dead times on the output to stop shoot through on my external drive transistors. I'll use a driver IC or other method
 


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