I want to driver 3 power LEDs and depending on the solution I choose I find something strange.
If I have this situation :
I want 330 mA flowing in my LEDs. I choose a 10 V DC-DC converter. The forward voltage of the LEDs is 3 V. I set the current through the resistor with the remaining 1 V by choosing R = 3 ohms. 330 mA under 10 V equals to 275 mA under 12 V, so the input current of the DC-DC converter is 275 mA. So far so good.
Now let's consider this situation, where I use this LED driver : http://www.ti.com/lit/ds/symlink/tl4242-q1.pdf and choose its resistor so that it outputs a constant 330 mA :
I want to know the input current of the DC-DC converter. To do so I need to know the output current of the DC-DC, which is the input current of the LED driver. In the datasheet they say the LED driver consumes around 12 mA when its output voltage is 6.6 V. First : How does this value change when the output voltage changes ? Why don't they give that information ? Finally if the output current of the DC-DC converter is 12 mA, then the input current of the DC-DC converter is 10 mA.
In the first situation the system draws 275 mA, in the second situation is draws 10 mA.
Is there something I am doing wrong ?