Choosing pull down resistor

  • #1

Main Question or Discussion Point

Hi, I have this IC
Datasheet:
http://www.ti.com/lit/ds/schs015c/schs015c.pdf

I need a pull down resistor at one of the input pins but I am not sure what value to use. How can I calculate what value resistor I need ?, can someone give me a specific range ?
 

Answers and Replies

  • #2
I figured a 10K should be fine ?
 
  • #3
rbelli1
Gold Member
923
340
10K is usually the default selection for pullup and pulldown resistors. If you don't care how much power it takes then you don't really need to go farther. For most applications it is negligible power.

If you are using batteries to power it and wan very long run times you should go higher. 500K would be usable over all conditions in the datasheet. Look at Iin and Vil Max. (Other current paths on the input + Lin) * R must be less than Vil Max for all of the conditions you expect. At room temperature and 18V operation this could be hundreds of Meg.

BoB
 
  • #4
Svein
Science Advisor
Insights Author
2,027
651
It depends on what you use the pull-down resistor for. If you use it to pull down an input after something else has pulled the input high, you need to look at the time constant defined by your resistance and the input capacitance of the gate (which is ≤7.5pF from the spec sheet). A resistor of 10kΩ gives a time constant of 75ns, which is acceptable. A resistor of 1MΩ gives a time constant of 7.5μs, which will keep the gate in the linear region for quite a long time (means higher current draw, increased noise sensitivity and possible instability). If you need a high resistance, consider putting it in front of a schmitt-trigger input.
 
  • #5
755
78
Boy, that datasheet brings back memories, back when the IC mask was actually (for some reason) included.

Those early CMOS parts had a tendency to burn up if the inputs of unused sections were left floating. If the logic included inversion you could get positive feedback from the toggling output back to the floating input, and the frequency, and corresponding power supply current, would race way beyond the parts spec.

Is this a hobby project?

Edit --> I just noticed that Berkeman discussed this on a different thread from same OP.
 
  • #6
Boy, that datasheet brings back memories, back when the IC mask was actually (for some reason) included.

Those early CMOS parts had a tendency to burn up if the inputs of unused sections were left floating. If the logic included inversion you could get positive feedback from the toggling output back to the floating input, and the frequency, and corresponding power supply current, would race way beyond the parts spec.

Is this a hobby project?

Edit --> I just noticed that Berkeman discussed this on a different thread from same OP.
Yes, this is a project. I did tie all the unused inputs to ground
 

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