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I'm designing a UVC LEDs strip using WS2811 LED driver ICs. I'm using the Vishay UVC emitting diodes [3] having the forward voltage of 6.8 V and the radiant power typically 2.5 mW at 20 mA. I used one LED per WS2811 chip as the typical output constant current of the WS2811 IC is 18.5 mA. If I use more than one LED per IC then a constant current of 18.5 mA will be divided which I don't want. I used a 5 V regulator to regulate 6.8 V to 5 V to power the WS2811 chip.

  • In the WS2811 datasheet, there are two sample circuits, (i) using 5 V and (ii) 12 V power supply. I'm a little bit confused about how they fed 12 V to the VDD pin of the IC as the maximum voltage of the VDD pin is 6-7 V.

  • Also, they say that R1 is used as the IC internal LDO divider resistance and the value is 2.7K. how they calculate this resistance value.

  • And what would be the R1 value for 6.8 V?

  • if I use a 5 V to power the IC and 6.8 V to power the UVC LED and use one LED per IC would work? Any recommendations would be very helpful. Thank you.

Circuit designs provided in the datasheet

UVC LEDs strip UVC LEDs strip

[3] https://www.farnell.com/datasheets/3098116.pdf

[4] https://cdn-shop.adafruit.com/datasheets/WS2811.pdf

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  • \$\begingroup\$ For me, at least, this question is confusing because you are using an addressable RGB LED PWM driver to drive UVC LEDs for which I can see needing constant current, but that's about it. \$\endgroup\$
    – JYelton
    Commented Nov 2, 2021 at 23:06
  • \$\begingroup\$ @jyelton UVC LED brightness can be controlled by PWM. Question why somebody need it. \$\endgroup\$
    – user263983
    Commented Nov 3, 2021 at 0:31

1 Answer 1

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Ws2811 can control 3 LEDs, your design is excessive. 3 channels, each is 18.5 mA maximum, PWM to control brightness. Totally chip with LEDs may consume around 50mA. Power on Vdd 6-7V, chip has internal LDO. In case of 12V voltage is divided, some internal and external resistance. 6.8V could be connected to Vdd, 100 Ohm resistor for filter, separate power source unnecessary.

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  • \$\begingroup\$ Thank you very much for your comments. Could you please also explain how did you calculate the total current of the circuit? In my opinion, If each LED will draw current of about 18.5 mA then the total current drawn by the LEDs would be around 90 mA, not 50 mA. Is this right? \$\endgroup\$
    – user16449631
    Commented Nov 3, 2021 at 13:00
  • \$\begingroup\$ @user16449631 3 channels, each 18.5 mA. 18.5x3=55.5. How did you get 90? \$\endgroup\$
    – user263983
    Commented Nov 3, 2021 at 13:02
  • \$\begingroup\$ I meant the total current consumption of the circuit, the total number of the LEDs in the circuit are 5, so 5*18.5 = 92.5 mA. \$\endgroup\$
    – user16449631
    Commented Nov 3, 2021 at 15:07
  • \$\begingroup\$ I got another circuit design in which I used 6 LEDs and two ICs (3 LEDs per IC) but when made a prototype of the circuit and tested it, the whole circuit draw about 40 mA current. Do you know why? \$\endgroup\$
    – user16449631
    Commented Nov 3, 2021 at 15:20
  • \$\begingroup\$ @user16449631 the guiscient current of chip appr 1mA. So the max current will be according to your calculation. WS2811 gives the possibility to control current, you can make less. \$\endgroup\$
    – user263983
    Commented Nov 3, 2021 at 15:23

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