Interface RGB LED with nRF52840 in Low Power Mode

RE: Interface RGB LED with nRF52840 in Low Power Mode

awneil — Mon, 07 Jan 2019 16:49:41 GMT

😃

[quote userid="65515" url="~/f/nordic-q-a/41937/interface-rgb-led-with-nrf52840-in-low-power-mode/164358"]we just saved him 20mA[/quote]

or even 60mA ... ?!

😮

RE: Interface RGB LED with nRF52840 in Low Power Mode

hmolesworth — Mon, 07 Jan 2019 16:42:44 GMT

Hey - we just saved him 20mA so he already changed the design to a 3v coin cell :-)

RE: Interface RGB LED with nRF52840 in Low Power Mode

awneil — Mon, 07 Jan 2019 16:12:19 GMT

Dhara said the 1.8V is being regulated down from something higher - so it would be silly to then use a doubler to step that back up again!

RE: Interface RGB LED with nRF52840 in Low Power Mode

hmolesworth — Mon, 07 Jan 2019 16:04:41 GMT

First 20mA will drain the planet's resources, perhaps try at just a mA or so which is easily visible with current tri-colour LEDs .. second 1.8 volts is plenty enough if you use a voltage doubler at low current, which typically requires two capacitors and two diodes, but in this case you can throw away the transistors and use a single extra diode and capacitor. Google should provide plenty of answers, as a starting point and further explanation here is one: link

RE: Interface RGB LED with nRF52840 in Low Power Mode

Dhara — Mon, 07 Jan 2019 10:55:28 GMT

Thanks for your detailed explanations.

RE: Interface RGB LED with nRF52840 in Low Power Mode

awneil — Mon, 07 Jan 2019 09:06:34 GMT

[quote userid="75366" url="~/f/nordic-q-a/41937/interface-rgb-led-with-nrf52840-in-low-power-mode/164160"]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 [/quote]

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!

[quote userid="75366" url="~/f/nordic-q-a/41937/interface-rgb-led-with-nrf52840-in-low-power-mode/164160"]I am confused whether to operate LED at 1.8V or 5V or 2.5V[/quote]

LEDs operate on current - not voltage.

[quote userid="75366" url="~/f/nordic-q-a/41937/interface-rgb-led-with-nrf52840-in-low-power-mode/164160"]the DC forward current rating of this LED is 20mA [/quote]

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.

RE: Interface RGB LED with nRF52840 in Low Power Mode

Dhara — Mon, 07 Jan 2019 08:41:36 GMT

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

RE: Interface RGB LED with nRF52840 in Low Power Mode

awneil — Thu, 03 Jan 2019 19:55:24 GMT

[quote userid="75366" url="~/f/nordic-q-a/41937/interface-rgb-led-with-nrf52840-in-low-power-mode"]The 1.8V external source is capable to drive the RGB LED[/quote]

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 ...

RE: Interface RGB LED with nRF52840 in Low Power Mode

Nguyen Hoan Hoang — Thu, 03 Jan 2019 18:14:13 GMT

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

RE: Interface RGB LED with nRF52840 in Low Power Mode

Jørgen Holmefjord — Thu, 03 Jan 2019 13:31:49 GMT

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