Is this robust? NRF52840 1.8v low power mode driving RGB LED from 3.3v

[JustinL]

It would be great to get some reassurance on this design. Is it ok for production i.e. is robust and within limits?

I am successfully driving an smd rgb led by sinking 2ma current into the GPIO pins of an NRF52840 that is running at 1.8v.

The LEDs forward voltage drop range from 1.7-3.0v so I have the common anode tied to a 3.3v power line to achieve threshold.

Will the 3.3v damage the NRF52840? The datasheet says GPIO max voltage is VDD +0.3v (if I recall) 

 When the GPIO is set high the led voltage is is below threshold for red,green and blue and they don't light. As required.

When GPIO is set low, they light and the forward voltage drop of the LEDs should ensure that the pin does not see a voltage above 1.6v which is below vdd.

So as long as the total current sink is managed within limits this seems to be all ok?

Is that correct?

Thanks

 

[hmolesworth]

Very welcome. Since you are going to experiment, how about trying this minimalist single-LED driver:

//       VDD 1.8V--#--  Schottky
//                 |    Diode
//                 |    |\  (         |\  |LED 1
//                 |    | \ )         | \ |       +------+
//                 +----|  )----#-----|  )--------[ R1   ]---+
//                      | / (   |     | / |       +------+   |
//                      |/  )   |     |/  |                  |
//                              |                            |
//                            -----                          |
//                            -----                          |
//                              |Boost                 Drive |
//                              |                            |
//   LED 1 Drive/Boost ---------+----------------------------+

It saves an i/o pin, but might not work as well. For both schemes remember to sample the 3.3 volts if that is drawn from a battery where the voltage droops over the lifetime of the battery, for example a coin cell. The trick is to use a lookup table for the PWM adjustment as the battery droops from 3.4 volts down to 2.4 volts. If the voltage stays constant then no need for adjustment.

[JustinL]

I prototyped your original design this morning and it worked like a charm!!