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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.

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  • \$\begingroup\$ Link to LED driver IC datasheet please. \$\endgroup\$
    – winny
    Jul 14 at 14:58
  • \$\begingroup\$ Post has been edit :) \$\endgroup\$
    – kokospalme
    Jul 14 at 15:22
  • \$\begingroup\$ If the 10k resistor makes any difference then it sounds like you don't have the digital inputs connected to your microcontroller correctly. \$\endgroup\$
    – user1850479
    Jul 14 at 15:39
  • \$\begingroup\$ Are all of these grounds connected together, and to the microcontroller ground? \$\endgroup\$
    – rdtsc
    Jul 14 at 15:45
  • \$\begingroup\$ 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. \$\endgroup\$
    – winny
    Jul 14 at 16:05

1 Answer 1

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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.

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