| ??? 12/01/09 16:56 Read: times |
#171348 - What definition? CC/CV both have line regulation tolerances Responding to: ???'s previous message |
Richard Erlacher said:
Just about the first thing it says is
"Ireg(SS) = 20 mA@ Vak = 7.5 V" Doesn't that tell you that the supply voltage is critically involved in setting the current? Not at all. There is a diagram showing the current as function of voltage. No much different from the knee for a zener diode. And if you look at a processor that can work between 2.7V and 6V, you will notice that the datasheet will normally mention power consumption, input/output levels, max frequency and switch times for a number of typical supply voltages as complement to graphs. Giving typical values at specific loads/supply voltages/temperatues/... is normal, and will in no way mean that the component is only usable with that specific value. Erik's assertion is that it's possible to generate a constant current without any input supply regulation. He implied that there's a current source that will provide a constant current despite the fact its input supply ranges randomly between 3 and 24 volts. That's where I challenged him to show us how it works. Of course you can have a constant current - within tolerances for line regulation - with varying input voltage. Just as a voltage regulator can give constant voltage - within tolerances for line regulation - with varying input voltage. A CC generator is very similar to a voltage regulator, which is the reason why a voltage regulator can be used to design a CC regulator. But a CC regulator is better optimized for the task since it only needs one drop-out - a voltage regulator needs the specified dropout in addition to the nominal voltage. I'm not sure if Erik mentioned the correct component name - or if I have seen the wrong datasheet - but the ST2221A doesn't seem to manage variations of 3-24 on it's own. But there are a number of CC driver chips that can supply a constant current for a large input voltage range. With a switching regulator, the power loss in the regulator will be quite low even for very large input/output voltage differences. For a non-switching CC regulator, you will normally have to settle for low currents or a low voltage drop over the regulator. The component I linked to can manage about 3-45V voltage loss. At 1.8V dropout, you will only get 85% current. There are switching regulators where you can have much higher currents at similar voltage drop. So with a couple of percent of line regulation precision, you can use a rectifier bridge and an mF-class capacitor and feed the CC regulator - no voltage regulator involved. A completely separate issue is that most CC drivers for LED use are not just CC generators but also latches and/or shift registers which means that they may require a separate, regulated, supply voltage for the logic section. |
