Society of Robots - Robot Forum
Electronics => Electronics => Topic started by: Admin on April 06, 2008, 09:06:14 PM
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I've strangely noticed that my voltage regulator doesn't seem to work under low current loads . . . Meaning, if I don't connect a few LED's to the power line, my microcontroller just doesn't work . . . so confusing!
I design my circuit to be ultra efficient, and it rebels at me!
Anyway, I always thought regulators just 'always worked' but apparently not . . .
This is what I am using: L4940V5
http://www.st.com/stonline/books/pdf/docs/2141.pdf
So, I'm guessing I should throw a resistor or LED between output and ground to fix it . . . so how do I calculate this resistor value?
Looking at the datasheet, I see something called ''quiescent current" . . . google and wikipedia don't know what it is, but I found something that said:
"The operating point of a device, also known as bias point or quiescent point (or simply Q-point), is the DC voltage and/or current which, when applied to a device, causes it to operate in a certain desired fashion."
The datasheet says 90mA is required . . . what a waste of power! If this is correct, I would need 5V/.09A= 55ohms . . . hmmm feels like I'm shorting something . . .
Am I on the mark here or totally confused?
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I could be wrong on this, but I believe the quiescent current is the current used to "run the regulator." And I didn't see the 90mA figure anywhere in the datasheet... in fact, on page 7/23 it lists the quiescent current as 8mA max under test conditions of Vi=6.5 and Io=5mA, so you certainly don't need 90mA of current dumping just to have the thing work--the test was carried out at 5mA.
I don't know where I read this but I've definitely heard that some current draw is necessary for certain regulators.
I've read that some regulators are more sensitive than others to the value and type of capacitance on their inputs and outputs. I'm definitely foggy on this, but it had something to do with the feedback loop on the regulator going unstable if either too much or too little capacitance was used on the output, and if there was too much or too little equivalent series resistance of that cap... I don't know too how often this actually causes problems, but here's a link to one article about capacitor selection for regulators: http://www.national.com/nationaledge/jul02/article.html (http://www.national.com/nationaledge/jul02/article.html) It also mentions something about LDO (low drop out) regulators being more unstable.
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Oops . . . I was using Fig 13 on page 12/23 for quiescent current. But I was reading it wrong for 5V instead of my input 6V . . . and I was reading it at the wrong current draw, too. So I shouldn't need more than like 5mA in normal operating conditions.
That article was a bit over my head, but basically I think it was saying that LDO's are inherently unstable and that good capacitance selection is important. That selection was also dependent on load resistance, too. But my load resistance can widely vary . . .
The LDO datasheet suggests a .1uF for input voltage, and 22uF for output. I have a .1uF ceramic and 3300uF electrolytic on the input, and .4uF ceramic on the output. I guess I should reconsider changing the output capacitance . . . but at the moment my oscope is happy enough with the output.
I'll play around and see if I can waste a few mA somewhere to fix the problem.
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maybe you could utilise high leakage current from ceramic caps :D
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but I believe the quiescent current is the current used to "run the regulator."
EXACTLY , you dont have control over this current, it is used to bias the internal circuit of the regulator,,to operate at the desired point.
so it is NOT a current to waste or anything, means , even if you dont connect anything at the reg out ,this current is running in the inside of the chip.
regulators should operate at any load..try changin the reg or check the caps values yea it is important,
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by the way , if you check with the ponyprog serial programmer schematic you will see a LOW quiescent current regulator(LM2936Z-5)
,this thing is considered a better regulator as it doesnt waste as much power as the high quiescent current ones(7805).
thats a reason for being more expensive ;D