# How do I know the input current of a LED driver?

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 ?

Thanks.

• why are you using a DC-DC converter with the LED driver? Feb 13, 2020 at 16:26
• The datasheet is telling you how much current the driver itself consumes internally in addition to whatever it's pushing thru the LEDs. Feb 13, 2020 at 16:34
• I will need to light 1 branch of 3 LEDs, 1 branch of 2 LEDs, and 1 branch of 1 LED. Given that my power supply is 12 V, I am afraid the voltage dropout for the branch of 1 LED might cause too much power dissipation (12 V to 4 V) Feb 13, 2020 at 16:35
• That DCDC converter serves no purpose, so you can remove it. Feb 13, 2020 at 17:16
• Wouldn't it be useful if I want to light only one LED from a 12V power supply to first reduce the voltage to 4 V with a DC-DC converter to handle power dissipation ? Feb 13, 2020 at 22:12