EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Leopoldo on January 26, 2016, 08:09:27 pm
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Hi everybody,
I'm going through a project highly integrated where I have very limited space for extra components. The problem that I have is that I can't seem to find a low cost, ultra low-dropout current regulator that can supply or sink 20mA constant through an LED, having 3.6 to 4.2V of power supply and the LED being powered at 3.21 with said current (I tested the voltage myself, you can see the graph I've drawn). As you can probably tell, I'm powering everything with a Lipo and I cannot have a cutoff voltage for the LED lower than 3.6V, which is still quite high but it's already getting quite complicated as it is. I was wondering if any of you know about a particular low cost IC that can do the regulation; and I'm talking about CURRENT regulators, because I can only accept up to a 1mA error on the LED (so voltage regulators with resistors are banned) :-[
I know that it's probably impossible, but I highly believe in your knowledge :-+ :-+ :-+
Thank you so much! Byeee ;D
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Do you really need constant current?
There are many constant-current boost regulators, some in very small packages, designed for backlight applications, that would solve the low-dropout issue by adding enough series resistance (most would be the sense resistor) to keep in regulation at the highest battery voltage.
Another approach would be an op-amp, with a 10R current sense resistor (=0.2v dropout at 20mA), and an 0.2v reference (derived by dividing down from a higher, easier to obtain reference voltage, e.g. a 3.3v rail if you have one). You can probably find an opamp that will drive 20mA directly, so the solution would be the opamp (SOT23, maybe smaller) and 3 resistors, if you have an available reference voltage.
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If this is just an LED why not use PWM solution of some sorts? You'll need to sense RMS current for feedback. Quick search returned ... LT1241, comes in SOIC8, TLC5971 - multi channel comes in tiny 3x3mm packages.
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thanks for the answer!
Assuming that I use an op amp, how can I precisely regulate the reference voltage of the input?Because not having a fixed voltage rail, if I use a diode it will probably drift its forward voltage as the input voltage increases or decreases...and we're talking about millivolts, that would screw up many things. On the other hand, can you suggest me some switching current regulators? Because, just by looking at ebay, the lowest price I can find is almost 7$ for just one piece.
It looks like a stupid circuit, and indeed it is, except for the fact that it requires such a low voltage drop...
I thought about using a step-up voltage regulators, in fact, I've ordered one from china already 2 weeks ago, but I later realized I may barely have the space to fit that and nothing else, so if the voltage is unstable, the only thing that could prevent everything from drifting is a pathetic series resistor.
I cannot use a PWM signal, unless I highly filter it, because the light is needed for a very sensitive sensor and fluctuation can interfere with the readings. That's why the current MUST be precise and constant
If this is just an LED why not use PWM solution of some sorts? You'll need to sense RMS current for feedback. Quick search returned ... LT1241, comes in SOIC8, TLC5971 - multi channel comes in tiny 3x3mm packages.
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There are plenty of voltage references available.
The TL431 can give a stable 2.5V reference which can be divided down using a couple of resistors.
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thanks for the answer!
Assuming that I use an op amp, how can I precisely regulate the reference voltage of the input?Because not having a fixed voltage rail,
If you really need accuracy, use a voltage reference. But if you need more accuracy than you'd get from a diode,you should probably be measuring light output and regulating that, rather than relying on constant current.
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Well I'm not THAT concerned about the light output ;D ....I think that knowing the current would be more than enough to get an accurate reading.
I think I may try the suggestion of using the TL431 voltage reference for a simple op-amp, hoping that I can find a SMT one that can deliver enough current.
Thank you all for your suggestions, if I can find a solution I will let you know.
Byeee :D :D :D
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LM4041 is about the same price as the TL431 and has better specs (tempco)