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I'm working on a small High power LED driver to drive 3 RGBW-LEDs in series. This is the schematic* of the driver: enter image description here VCC = +12V, PWM channel are connected to a microcontroller (5V level). This is the datasheet of the driver-IC: https://pdf1.alldatasheet.com/datasheet-pdf/view/1119996/DIODES/AL8860WT-7.html

This is from the datasheet of the LEDs: enter image description here

My Problem is: If use the 10k pulldown resistors (R2, R4, R6, R8), The red channel of the LEDs are always on. If I don't use the pulldowns, all channels are constantly on. What am I doing wrong? Is it even possible to drive 4 LED drivers in parallel? Another Shop is using a schottky diode between the input Power and VCC: Link to schematic

*The schematic is based on sparkfuns Picobuck:

I already tried:

  • solder/desolder the oulldown reistors
  • use current limiting resistors for the PWN inputs
  • simple digital operations instead of PWM

Are there other, maybe better ways to drive these LEDs? Thank you in advice.

kokospalme
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  • Link to LED driver IC datasheet please. – winny Jul 14 '23 at 14:58
  • Post has been edit :) – kokospalme Jul 14 '23 at 15:22
  • If the 10k resistor makes any difference then it sounds like you don't have the digital inputs connected to your microcontroller correctly. – user1850479 Jul 14 '23 at 15:39
  • Are all of these grounds connected together, and to the microcontroller ground? – rdtsc Jul 14 '23 at 15:45
  • I can not find in the datasheet how much current is pushed internally to the CTRL pin. If say 500 uA, 10 kohm will produce too high voltage and it won’t shut down. Try 1 kohm. – winny Jul 14 '23 at 16:05
  • the 10k resistors make a (stange) difference: With them only red is on, without them ALL colors are on. I don't know why but I'll try 1k resistors... The digital Outputs of the microcontroller are set correct. – kokospalme Jul 14 '23 at 18:59

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Now I get a clear signal at about 2V at the PWM pins and works with a 1.5kΩ Pulldown and a 2.2kΩ resistor between the Microcontroller and the PWM inputs. So I guess it works like a voltage-divider if I pull the pin on the microcontroller high?

Vpwm = (5V*1.5kΩ) / ( 1,5kΩ + 2.2kΩ) = 2.02V.

toolic
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kokospalme
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