# How Do You Choose the Right Pull-Down Resistor Value for an IC Input Pin?

• TheRedDevil18
In summary, the conversation is about determining the appropriate value for a pull-down resistor on an IC input pin. It is suggested that a 10K resistor is usually suitable, but a higher value may be needed for longer run times or if using batteries. The conversation also mentions the importance of considering the time constant and input capacitance, and using a schmitt-trigger input if a high resistance is needed. The discussion also touches on the tendency for early CMOS parts to burn up if unused inputs are left floating.
TheRedDevil18
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 ?

I figured a 10K should be fine ?

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

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.

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.

the_emi_guy said:
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

## 1. What is a pull down resistor and what does it do?

A pull down resistor is a type of resistor that is connected between a signal and ground in a circuit. It is used to ensure that the signal remains at a low voltage when no input is present, preventing it from floating or picking up noise.

## 2. When should I use a pull down resistor?

A pull down resistor is typically used in digital circuits to ensure that a signal remains at a known state, especially when using open drain or open collector outputs. It is also commonly used in combination with push buttons, switches, and digital inputs to reduce noise and stabilize the signal.

## 3. How do I choose the value of a pull down resistor?

The value of a pull down resistor depends on the characteristics of the circuit it is being used in. It should be chosen to be significantly smaller than the input impedance of the circuit, but large enough to avoid excessive current flow. A common rule of thumb is to choose a value that is at least 10 times smaller than the input impedance.

## 4. Can I use any type of resistor as a pull down resistor?

While any type of resistor can technically be used as a pull down resistor, it is recommended to use a carbon film resistor or a metal film resistor due to their stable and predictable resistance values. Additionally, they have low noise levels and are less susceptible to temperature changes compared to other types of resistors.

## 5. Are there any alternatives to using a pull down resistor?

Yes, there are alternatives to using a pull down resistor such as using a pull up resistor or using a dedicated IC (integrated circuit) with built-in pull down resistors. However, using a pull down resistor is often the simplest and most cost-effective solution for stabilizing and controlling signals in a circuit.

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