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LED driver that regulates its own input current with an error amplifier?

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treez

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Most LED drivers regulate their output current (= the LED current).

Though if a LED driver is designed to regulate its INPUT current, then is this likely to mean a more stable situation than a LED driver which regulates its output current?
 

dick_freebird

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Don't know about that (more stable or less) but there is
at least one company making a LED lighting controller that
feeds back primary current.

If by stability you mean LED current vs line, temp, etc.
I doubt primary regulation works as well because it has no
idea, other than maybe an initial cal, about the downstream
losses. But for a dimmable drive where absolute output is
set by the user, that's probably a "don't-care". For an app
requiring absolute output voltage / current accuracy, too
much uncomprehended slop I expect.
 
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betwixt

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Apart from a small amount used by the regulator itself, the output current and input current are the same. Are you thinking of a complete power supply or just the LED current regulator?

Brian.
 
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treez

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thinking of say, a sepic regulator....or flyback, a complete power supply.
 

mtwieg

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Most LED drivers regulate their output current (= the LED current).
What do you mean by "stability"? I have trouble how regulating something besides LED current could lead to more stable LED current.
 
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treez

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"Stable led current"...thats it....thats what i am after......i dont want it "dancing" so to speak....just nice flat dc with the switching pulses being regular.

i rekkon, if you are regulating the input current, its easier to get stable led current, as you are regulating what goes in before it gets there, rather than regulating the output current, where you are having to react to whats gone through the delay of the power stage................if you are regulalating the input current, you dont have any nasty power stage phase changes to think about, because its input current, it hasnt even got to the power stage.

LED driver with regulated input current:
http://i49.tinypic.com/2rw6az7.jpg


this one above has regulated input current.

So what do you think?......is "regulating the input current" the best way to easily get a stable led current in the output?
 

FvM

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There are usually reasons why you want a LED current to be regulated. Regulating the input current of a switched mode led supply isn't the same thing and in most cases won't serve the purpose of LED current regulation. It also might risk to exceed the rated LED current.

I also don't understand about the imagined stability issues.
 
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treez

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If the Vin is fixed, then regulating the input current will tend to regulate the led current.

The input current signal undergos no phase change traversing the supply wires, so its easier to regulate ...?
 

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If the Vin is fixed, then regulating the input current will tend to regulate the led current.
"Assume a spherical cow..."

What you're basically describing sounds like current mode control. Yes, having a feedback loop acting on the input current has benefits for bandwidth, and therefore stability. But strictly speaking, in a current mode DC-DC converter, input current isn't regulated, it's simply controlled as smaller part of the overall feedback loop, which in fact regulated output current/voltage.

edit: oh and also even if Vin is fixed and Iin is perfectly regulated, you're only regulating output power, not current. Remember that the Vf of LEDs can change significantly.
 
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