STCS2 Power Supply

I'm not sure this is a good answer. Since ST's STCS2 is a true CS, a current sink, and not a voltage source like most other power supplies, multiple STCS2 units can be paralleled, but note, only on their "Drain" pin, so each IC keeps its own sense resistor.

In fact, if an STCS2 is to be used for say 3A, many of them in parallel, working at reduced currents, would be advisable. The STCS2 is a linear current regulator, which means it internally dissipates the LED current times the unused supply voltage,

Pd = I_Drain ( V_Supply - V_LED ),

which at I_Drain currents above even 0.5A can quickly amount to many watts of internal power dissipation. The STCS2 comes in a 10-pin power SOIC package, with a metal back to solder to the PCB, and ST intends that you conduct the dissipated heat into the epoxy-glass PCB. This scheme is rarely good for more than a few watts, even if you dedicate several square inches of PCB to the dissipation. Hah, it's a shame ST doesn't make these in TO-220 packages, so we could attach a real heat sink.

Andy Germany suggested using a switched-mode current source, that's a good idea, because it uses an inductor to match the supply voltage to the LED operating voltage. I thought RAMConsult was wrong when he implied that paralleling STCS2 devices is not allowed, but his suggestion of using an LED2000 was good, because this is a switched-mode device. However it has limited voltage capability, only 18V, and at 3A would be used to its rated limit without any safety margin. Moreover, I don't see a way to parallel this circuit, because of the way ST places the current-sense resistor on the far side of the LED string (each paralleled unit would need access to its own private sense resistor to independently regulate its share of the total current). But conntaxman may find it sufficient for his 12V 3A requirement. Maybe he can work at 2.5 amps.

I am impressed by the array of serious LED-driver ICs offered by Linear Technology. LTC has a selection page to help you choose which of their many capable ICs to use. A part like their internal-switch LT3478, rated to 36V and 4.2A, may work well. LTC's most interesting control ICs use external MOSFET switches, which means they can work to very high currents, and many can work up to 100V. They're especially well suited to series strings with many LEDs. Their LT3954, or LT3756, or LT3755 (block diagram at left), or LT3796, or LT3761, or LT3763 parts are attractive to me.

The latter part can switch a pwm-regulated 20A current into a 25-volt LED string in just a few microseconds. When they say PWM dimming, they mean PWM modulation of the PWM smps current-source circuit. The LT3763 waveform shows PWM-switched current-source regulation at 500kHz, with a fast rise and fall time to the regulated current. This means you can do your own light dimming PWM at say 10kHz. Amazing. We have used these for fast high-current LED pulsing in biological fluorescence experiments, to replace expensive lasers. They show a 25us pulse in the waveform, which is just about right for many high-speed camera experiments.

I checked the STC2 datasheets and there is no provision for paralleling. There is however a 3 amp driver known as LED2000

Once you are done, you want dimming. Good PWM allows that.

maybe http://www.ieeta.pt/~alex/docs/ApplicationNotes/DC%20Power%20Supply%20Handbook.pdf pg.96

What is the background to this question, are you trying to learn electronics or just light some LEDs.

If the latter, you will find many really good power supplies on the web, that will accept between 100 - 240 volts AC and supply 12 volts at good decent currents. I recently bought two, one for 5 amps and one for 6 amps. Together they cost about US$30.00 from ebay!

Also, using say a 1.5 amp PS at 1.5 amps will usually reduce its life span dramatically. Buy one that can supply double your needs, it will run cooler and safer, also make sure that it is a Switched Mode (S.M.P.S.) type as they use far less power to achieve their outputs, often being around 90% efficient.

If you are learning electronics, don't start by trying to parallel power supplies, you could start a fire....... Pain being another possibility.....

Look here as an example:-

LED Power Supplies on ebay UK