I'm designing a circuit using an LED driver that is intended for RGB LEDs and uses a resistor to set the current for the constant current supply.

On the LED datasheet, each color is tested at different forward currents

  • Red @ 20mA (max 25mA)
  • Green @ 15mA (max 25mA)
  • Blue @ 10mA (max 20mA)

It would be wise to stay away from the absolute maximum ratings. One priority is designing for longevity / lifetime of LEDs.

If I set the current to 10mA per channel, with 2 LEDs per channel, how do I calculate the ballast resistors (high side)?


simulate this circuit – Schematic created using CircuitLab

LED datasheet specs

example application circuit

  • 4
    \$\begingroup\$ If your LED driver can take the (range of) forward voltage as is, you don’t need any resistor. By the way, the LED driver is a sink, not a source, but that’s beside the point here. \$\endgroup\$
    – winny
    Jun 29, 2022 at 21:16
  • 1
    \$\begingroup\$ The resistors on REXTA, REXTB & REXTC on your driver IC are responsible for setting the LED current. Series resistors with the LEDs will (or at least should) have little to no effect other than dissipating unnecessary heat. \$\endgroup\$
    – brhans
    Jun 29, 2022 at 21:40
  • \$\begingroup\$ Read the datasheet on driver, it may have the answers. \$\endgroup\$
    – user263983
    Jun 30, 2022 at 2:03

1 Answer 1


The point of using an adjustable constant current led driver is that you don't need ballast resistors (most of the time, varies by IC).

Use the R Ext resistors as calculated by the formula in the datasheet, to set whatever current you want for your leds, not the nominal max or absolute max.


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