TV output current requirements

Philldapill

Using a standard TV output adapter(1.1K, 570, 220 ohm resistors), about how much current is used? I am prototyping a project and need my power requirements pretty low. I will sometimes connect a tv to the device to check certain things, but don't want the TV object to run all the time if it is using power. Since the TV output·components form a DAC, some pins will source while others sink current. I don't want to have to code in extra features if it's not needed.

So, how much current does the TV output take(not counting extra cog)?

Mike Green

I don't think anyone has measured it. If the 570 Ohm pin is low and the 220 Ohm pin is high, you'd have 790 Ohms between 3.3V and ground which takes roughly 4mA. The actual amount of current would depend highly on the images being displayed and the drive requirements of the video line. The video cog itself would probably take the majority of the current. Best thing is to stop the TV driver when it's not needed and that will make the I/O pins high impedance.

Philldapill

That's what I was thinking. I suppose, theoretically that is, that the average current draw from the TV DAC would be the average of the currents for each state of the pins. i.e. The average for 000, 001, 010, 011, 100, 101, 110, 111.
Cooresponding currents of - 0, 5.54, 4.38, 2.62, 5.54, 4.38, 2.62, and 0.(220ohm---(570||1100), 570ohm---(220||1100), etc.)


I just calculated it in an excel sheet, and it seems the average current would be... 3.136mA... WHOA! That's way more than I expected. I would have imagined somewhere in the 100uA's range. I guess only a proper REAL measurement will be the final say, but wow. Thanks Mike.

Ken Peterson

@Phil: Are you figuring in the 75 ohm load?

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Philldapill

Eww, no I didn't... even more current. I guess I'll have to write in some code to power up/down the TV when needed.

hippy

I think I determined the max current would be when the 220R was driven high, so through that then into 75R paralleled with the 570R and 1K1 to 0V. You'll have to do the math, my brain's seized up.

Ken Peterson

Damn...hate when that happens

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Philldapill

Me too. I mean when it's Hippy's in particular.

Doing the math, it should be about 11.68mA peak, but the average(over a period of time) should be 3.136mA. Still, a lot of current just for a signal.

Ken Peterson

this is a very intuitive answer, but I'm guessing the maximum would be when the output voltage is close to half the supply.

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Philldapill

Seems to be so, Ken. Why does the maximum current really matter in all this? If we are talking about current consumption, shouldn't it be the average current, or am I wrong?

Ken Peterson

I would assume the maximum for your average. I think your average voltage for a video signal is half maximum anyway, unless your picture is especially bright or dark.

If you really want to get into the math, calculate a curve of the statistical distribution of brightness, and use the power dissipation for each brightness value, then integrate. In my job as an engineer , I don't integrate. I design around worst-case unless cost pressures force me to be more precise.

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Post Edited (Ken Peterson) : 6/28/2008 4:24:46 AM GMT

Philldapill

Good idea. Worst case scenario can't get much worse. I'm trying to add up all my power requirements since my application is monitoring battery voltage of a 7AH battery. I'd like to keep them low as possible, but I guess I should plan for the worst. Thanks all!

hippy

Brain unfrozen now and I also make it just short of 11.7mA ( using 560R and 1K2 ). I checked it when there was the misunderstanding that sucking more than 10mA out of an I/O could damage it.

That's only for one specific colour ( or control level ) so the average, unless you're unlucky would be less than that.