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Powering NeoPixels from 3.3V - Wrong but everyone does it?

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sebmadgwick:
NeoPixels are prolific, used in countless hobby, industrial, and commercials products.  They have become synonymous with "RGB LED".  There a several variants but the datasheets all specify an operating voltage of 5V, more specifically, 3.5V to 5.5V.

Nevertheless, I have seen many designs power NeoPixels from 3.3V.  I do not do this myself and will always provide a 5V supply, using a charge pump or similar if necessary.  It just seems unacceptable to me to end up in a situation where a product I have designed starts misbehaving because I ignored clear engineering data.

I am also not satisfied assuming that empirical data overrides datasheet specifications.  Just because the first ten thousand units work, it does not mean that the next ten thousand will.  Manufacturers change components characteristics knowingly or unknowingly and only the datasheet specification can be relied upon.

So my question is, can NeoPixel 3.3V operation be verified analytically, such that a design engineer can take an informed risk?

For example, perhaps it is known that the logic is 3.3V compatible CMOS but the LED Vf is 3.5V which would mean reliable operation at 3.3V but with compromised brightness / colour mixing.

aliarifat794:
If the NeoPixel uses 3.3V compatible CMOS logic, it can generally accept logic signals at 3.3V without issues. This means that the control signals from a microcontroller operating at 3.3V should be compatible with the NeoPixel.

langwadt:

--- Quote from: aliarifat794 on April 24, 2024, 02:43:25 pm ---If the NeoPixel uses 3.3V compatible CMOS logic, it can generally accept logic signals at 3.3V without issues. This means that the control signals from a microcontroller operating at 3.3V should be compatible with the NeoPixel.

--- End quote ---

datasheet for them usually says Vdd min 3.5V, probably because if the Vf of the LEDs, and Vh_min 0.7*Vdd

so 3.3V violates the datasheet for Vdd, and 3.3V logic isn't enough for 5V Vdd

(Neopixel is an Adafruit brand name, the LEDs are WS2812 or similar)

sebmadgwick:

--- Quote from: langwadt on April 24, 2024, 03:30:07 pm ---Vh_min 0.7*Vdd... and 3.3V logic isn't enough for 5V Vdd
--- End quote ---

It sounds like you've misinterpreted how logic thresholds are specified.  If Vh_min is specified as a function of VDD then VDD refers to the actual voltage on the VDD pins, not the recommended typical value for VDD.  For example, if VDD was 4V then Vh_min would be 0.7*4V.

langwadt:

--- Quote from: sebmadgwick on April 24, 2024, 03:57:59 pm ---
--- Quote from: langwadt on April 24, 2024, 03:30:07 pm ---Vh_min 0.7*Vdd... and 3.3V logic isn't enough for 5V Vdd
--- End quote ---

It sounds like you've misinterpreted how logic thresholds are specified.  If Vh_min is specified as a function of VDD then VDD refers to the actual voltage on the VDD pins, not the recommended typical value for VDD.  For example, if VDD was 4V then Vh_min would be 0.7*4V.

--- End quote ---

I said "for a 5V Vdd", and 5V is the most likely supply voltage

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