This is a constant current source using a FET. This is the most simple replacement to series resistor to limit current. The N-Channel FET BF256C can give 15mA current.
Before you get to use chips, experiment with some methods, which will help you learn about the LEDs better. The first is just One Resistor in series. This is to Limit the max current in a Series LED Chain. If you have a Regulated Supply with a Fixed Voltage, then you can use this method.
Let us take a 12V SMPS, Each HB White LED has a drop of around 3.2 (please see datasheet). If you put 3 LEDs in series it is a drop of 3 X 3.2 = 9.6 V.
12V - 9.6V = 2.4 V. This is the drop across the Resistor, let us keep the current at 20mA for a Long life for LED. Some LEDs will get damaged at 30mA some take more that that. We now have LED Modules which can take even 1 or 2 A.
V/I = R as per OHM. 2.4V/20mA = 120 E or Ohms.
How Hot? W = VI Power in Watts. 20mA x 2.4V = 48mW. This is where you lose the Money. Keep it low, else the Green Goblin will frown. Unless you want LED Lighting to double up as a Room Heater, Nice idea if you are in the Artic.
Now you have a chain of 3 LED with one R, make many such chains and put it in parallel to around 70% of SMPS capacity. If you have 20 Strips of 3 LEDs each, 20 X 20mA = 400mA. You will need a 12V 600mA SMPS .
MOSFET Drive for LED Constant Current
Let us assume, you have a supply that is varying and not stable. Then use a SMPS and Resistor as shown above. Closely matching the LED Chain to the SMPS voltage to keep the heat loss minimum. At Low voltages the above idea may not work. So you can try a MOSFET circuit shown.
You can use Transistors too but The Heat is more, as the Drop is more. When you use batteries, you cannot afford to lose even 0.5V. So the MOSFET is the answer.
This circuit is a nice design idea about LED drive with low voltage and watts burden. In combination with a Joule Thief - and PWM you can make many White LED utilities like Lanterns and Flashlights. PWM is to modulate brightness and also Extend LED Life.
The Essence is The LED has to have a long life, constant current is the answer. The Efficiency has to be High, Switching MOSFET is the answer. For just a LED or Two, you do not need to bother about Efficiency, but Constant Current, No Compromise. Why? Because it is in the Absolute Maximum Rating.
"You do not cross the road, when the light is RED. You do not Drive Faster, than the Speed Limit. You do not Eat, more than you can Digest.You do not Stress the Absolute Max in the Ratings."
The Elektrik Jedi