Value of Pulldown Resistor?

[Krumlink]

So today I finally started messing around with my SN754410, and I get it configured in a basic configuration (VDD1 5VDC, VDD2 3.6VDC) with the enables high and the grounds grounded. I have a LED with resistor on outputs 1 and 2 and without having anything on the inputs it is being pulled high. I know that a pulldown resistor will work but I was unsure of what the value would be, to prevent damage to the SN754410 if it was too low. I was thinking a 10K-100k?

 

[Roff]

You can connect the input directly to ground. The input voltage can be as high as 0.8V for a logic zero, and the input current is -10uA, so a pulldown resistor must be no larger than 80k. That would give you little or no noise immunity, with a worst-case part.

 

[Krumlink]

EDIT: Ok I see what you meant

I will use a 10k pulldown.

 

[Roff]

EDIT: Ok I see what you meant

I will use a 10k pulldown.

Damn strange. When I look at your post, I see this:

with no input (no connection, floating in air) the output is high.
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MechTronics
Last edited by Krumlink; Today at 12:07 PM.

When I hit the Quote button to reply, I get what you see in the first line, above.

 

[Boncuk]

Hi Krumlink,

according to my experience TTL ICs require a very distinctive pullup or -down resistor.

As TTL chips generally work with negative logic an output pulls itself high (zero) with a floating input.

Pullup and pulldown resistors should be between 1K and 4K7, in extreme cases also 470Ohm may be used.

I had a malfunction with an D/A converter one day, always using the MSB when converting. The cause was trace resistance between the pad and the IC socket.

Hans

 

[Boncuk]

You can connect the input directly to ground. The input voltage can be as high as 0.8V for a logic zero, and the input current is -10uA, so a pulldown resistor must be no larger than 80k. That would give you little or no noise immunity, with a worst-case part.

Hi Ron,

I'm afraid a TTL chip with a pulldown resistor of 80K will do - nothing. TTLs require power, power and power.

Hans

 

[crutschow]

I'm afraid a TTL chip with a pulldown resistor of 80K will do - nothing. TTLs require power, power and power.

The SN754410 is not a TTL chip. It's input requires only 10 microamps max. to maintain either a high or low input, so 80k ohms should work fine for a pulldown resistor.

 

[Krumlink]

I used 10k ohm resistors and it eliminated the problem.

 

[Dean Huster]

The major reason to want to use a pulldown resistor is if you have to momentarily pull the input HIGH with a switch.

I don't much like pulldown resistors. In TTL, you have a wide window (or target) to shoot for when pulling up, i.e., there's a broad range of voltage that TTL considers to be a logical HIGH. On the other hand, when pulling LOW you have a very narrow window as a target -- only 0.8v as Ron mentioned. It takes a much lower value of pulldown resistor to get that LOW low enough that you have a guaranteed LOW that won't be susceptible to noise at the input or on the power rails. Where I can use a 1K or 10K resistor to pull LS TTL to a HIGH, it may take less than 100 ohms to get it to a LOW.

If you have to have pulldown, you're better off driving that input with an inverter and pulling the inverter's input to a HIGH.

Dean

 

[Krumlink]

Dean, that would over complicate things and I am connecting directly to ground, and logical high is 5VDC for this project.