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I want to driver 3 power LEDs and depending on the solution I choose I find something strange.

If I have this situation :

enter image description here

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 :

enter image description here

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.

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  • \$\begingroup\$ why are you using a DC-DC converter with the LED driver? \$\endgroup\$ – jsotola Feb 13 at 16:26
  • \$\begingroup\$ The datasheet is telling you how much current the driver itself consumes internally in addition to whatever it's pushing thru the LEDs. \$\endgroup\$ – brhans Feb 13 at 16:34
  • \$\begingroup\$ 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) \$\endgroup\$ – JackOfHearts Feb 13 at 16:35
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    \$\begingroup\$ That DCDC converter serves no purpose, so you can remove it. \$\endgroup\$ – user1850479 Feb 13 at 17:16
  • \$\begingroup\$ 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 ? \$\endgroup\$ – JackOfHearts Feb 13 at 22:12
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In the first situation the system draws 275 mA, in the second situation it draws 10 mA.

Second situation current will be 285 mA.

The LED driver manufacturer can only tell how much the driver will consume itself (for the driver to work properly) and how much is the LED current it can support for a given set of condition.

The second part in the image is what the IC can support. You have to consider 330mA plus 12 mA as the output for your DC DC regulator.

enter image description here

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