I’m working on a project where I want to drive a CREE XM-L RGBW power led from a WEMOS D1. The circuit is powered from a 4v supply, and I use the 4x CAT4101 with 1.74 kΩ on rset to deliver 350mA to my led.
Now my question: as the CAT4101 drops 0.5v, all leds get 3.5v while typical Forward voltage should be 2.25v for red, 3.3v for green and 3.1 for blue and white. I plan to add a resistor between each led and CAT4101 (R: 4.7 Ohm, G: 2.2 Ohm, B&W: 7 Ohm) to get the right voltage, however google tells me “resistor + power led = bad idea” and all other CAT4101 circuit I found basically ignore the voltage difference between the colors.
Should I go with the resistors or find another solution?
hpvanriemsdijk:
as the CAT4101 drops 0.5v, all leds get 3.5v
No, then you don't understand what the CAT4101 does
The CAT4101 acts as a variable resistor itself. One that changes it resistance so the current at the output is what you set (via Rset). The voltage will be at whatever voltage the load (aka the LED) will need to keep that current aka the forward voltage.
What VDOUT tells you is the max voltage at the output possible. Aka, that can't go higher then Vin - VDOUT, even if at that moment the current is lower then you set it. For example, when there is no load.
Aha, that’s an helpful insight, totally misunderstood what was going on. Wil try to do some recalculations on what resistors are needed on Rset. Thanks!
They don't specify a formula to calculate Rset but in figure 11 you can see it's a weird non-linear graph. So easiest is to look at table 6. For example, you need 1690Ω for 300mA, no matter the LED connected. Linear interpolation gives you need around 1k5 for 350mA.
septillion:
They don't specify a formula to calculate Rset but in figure 11 you can see it's a weird non-linear graph. So easiest is to look at table 6. For example, you need 1690Ω for 300mA, no matter the LED connected. Linear interpolation gives you need around 2k3 for 350mA.
And NO resistors in series with the LEDs
Its linear in conductance, not resistance. LED current is approx Rset conductance x 500, or
put more simply LED current (in amps) = 500/Rset.
Did some more reading and it seems easier that expected. I can just connect a 1k5 resistor to rset and 4v to the led, the CAT4101 will sink the access voltage, regardless the forward voltage of the led dies color. Magical, thanks for the help!
Yep, only keep an eye on the heat dissipation. This will be the most for the red led. With 350mA the heat dissipation of the CAT4101 will be (4V - 2,25V) x 350mA = 612,5mW. Because of the low input voltage not major but do check if that's in spec.
