MP3306EG-LF-P-Z : do I need series resistor with the LED?


I am modifying a design that uses the MP3306EG-LF-P-Z :

It is used with 2 LED in series (the forward voltage of each LED is 3 V). There is a 0.4R resistor in series with the LED (R20). The resistor is connected to a microcontroller, it was previously used to measure the current passing through the LED. The driver is controlled by a fixed PWM on the PWMI input, and another PWM on the EN input (5 ms on, 5 ms off, and so on).

I plan to remove the series resistor R20 because I do not need to measure the current anymore. I have several questions:

  • Is it advisable to have a series resistor with the LED (other than being able to measure the current) ? Do I need to put a 0 ohm resistor, or is it OK if I put no resistor at all ?
  • If I remove the 0.4R resistor, will there be a difference (even a tiny one) in the current passing through the LED, for the whole range of the input PWM ?
    I would expect that there is no difference given that it is a constant current driver, but I want to make sure. :slight_smile:

Thank you.

The whole point of this IC is to control current in an LED as efficiently as possible so minimal resistance to determine current. R20 doesn’t appear on the reference shcematics and you understand that is was put there to satisfy a MCU somewhere else in the system. Get rid of it, if you are nervous just use a jumper then you don’t have to relayout the board.