- There is no change in Vf for white colour temperature, that is purely a control of the phosphor layer over blue LED substrate
- Your document from a 3rd party supplier copied from some unknown source has a typo error in the published specs for Vf
Vf typ should read Vmin
LEDs like all diodes, zeners and saturated transistor switches starts from a "knee shaped" threshold Vt (or Vzt in Zeners) is obvious from VI charts.
What causes the voltage to rise above this depends on how small the chip is and some fabrication controls.
- the slope of rising current is steeper for bigger chips ( and also different scales in VI plots)
- size of the chip and thus the package power rating are inversely related to the slope of the VI curve.
- the slope increases with chip size suitable increased power dissipation in the package. above the Vf max is only due to the diode "bulk" resistance
- when saturated all diodes have this effective series R, ESR and ESR=k/Pmax
- although the datasheet does not specify this I/V slope or "typ" VI plot due to wide manufacturing variances
- i.e. quality controls, yield, cost of large epi-wafer and many 2nd sources which may be qualified by a cost saving factory and also historically from supplier process improvements
- The wider spread is on the high side of Vf so a White LED might be
or single bin +/-.1V or less
- or WORSE as in your case with a spread
of 1.5V from 2.7 to 4.2V. which I believe is
But again I believe these are bogus specs. , mislabelled & no test conditions I expect this should be
ESR nom. varies from 1 / Pmax to 0.25 / Pmax for the best higher power LEDs typ.
So a 5mm 65 mW LED = 15 Ohms typ and a 1W LED is 1 Ohm typical while 3W LEDs are <1/3 Ohm typ. while the best power LEDs are 1/4 of that per diode.
For a Vf using a white 5mm LED at 20mA with Vf=Vt(3mA)+ESR*If @25’C
Assuming Vt~2.8 @2mA. (knee threshold voltage, Vt or ~10% of If nominal current.
For Vf = 3.1,3.4
ESR = 15, 30 Ohms
15 Ohms is the expected nominal value of a white 5mm 65mW part.
I would never accept this part with a spec of 4.2V even if the supplier said this was actually at 30 mA.
ESR =(4.2-2.8)/30mA=47 Ohms or 3x the nominal “good quality” value. This is equivalent to +300% tolerance unacceptable by today’s standards. And most likely another error in the spec.
The same inverse power to ESR relationship is true for silicon diodes.
Here are some 1/3W 5050 LEDs
- best sources are few but exist
If you prefer 100 % single bin White,
This is what I used to sell and still have some leftover in 200 pc bags.
30 deg 16,000~20,000 mcd ( too bright to watch at close range )
( also similar in Red, Yellow, Blue, true green , Aqua)