Interface RGB LED with nRF52840 in Low Power Mode

[Dhara]

Hi,

I want to interface RGB LED- APHF1608LSEEQBDZGKC (Kingbright make) with nRF52840 operating at low power mode on 1.8V (@Vdd). The 1.8V external source is capable to drive the RGB LED but is there need of connecting the external transistor circuit to interface the LEDs on GPIO of nRF52840.?

Is the sink current capacity of nRF52840 sufficient enough in low power mode (1.8V) to sustain the switching of RGB LED with If, forward current = 20mA??

Please refer the attached snapshot of the schematic file.

Thanks.

[Jørgen Holmefjord]

Hi,

Please have a look at the GPIO Electrical Specifications in the nRF52840 PS. With VDD > 2.7 V, the maximum sink current is 15 mA, with a minimum value of 6 mA. With VDD @ 1.8 V, maximum sink current will be lower. It is not possible to sink 20 mA at this voltage.

Best regards,
Jørgen

[Nguyen Hoan Hoang]

You won't be able to turn the LED on with 1.8V.  

[awneil]

Dhara said:

The 1.8V external source is capable to drive the RGB LED

Really?!

As Nguyen Hoan Hoang says, it is not going to be enough to light the LEDs at all - look at the forward voltage specs in the datasheet ...

[Dhara]

Hi,

Actually, the forward voltage of Red is 1.8V typ. and max is 2.3V. So in this case I guess Red shall be able to glow but green and blue shall not turned ON at 1.8V.

Actually, the supply on my board will toggle from 3V and 5V supply. I am dropping those to 1.8V and other to 2.5V. So, I am confused whether to operate LED at 1.8V or 5V or 2.5V or if this direct 3V/ 5V toggle supply can be routed to LED.

Due to space constraints I am not preferring to add one more LDO or converter. Same time, the DC forward current rating of this LED is 20mA which is not matching the sink current specs. of nRF52840 @Vdd =1.8V.

Check the shared image for power supply on my board. Share your suggestions and opinion for status LED interface with nRF.

FYI- the LED part number is the same one in the nRF52830 PDK board.

Thanks,

Dhara

[awneil]

Dhara said:

the forward voltage of Red is 1.8V typ. and max is 2.3V. So in this case I guess Red shall be able to glow

But if the LED forward voltage is 1.8V, and your supply voltage is only 1.8V, that means you have no margin available for the voltage drop across the driver - let alone a current-limit resistor!

Dhara said:

I am confused whether to operate LED at 1.8V or 5V or 2.5V

LEDs operate on current - not voltage.

Dhara said:

the DC forward current rating of this LED is 20mA

That is a maximum limit - you can run it at less than that ! 

Especially as your title says, "Low Power" !

These are really basic electronic circuit design questions - nothing specifically to do with Nordic.