The LED driver circuit i've made here does not output a constant 1A current.

enter image description here

With an input voltage of 12V and 9V total Vf on the three LEDs (CREE LED star), the current is only 0.65A. Am I understanding the datasheet correctly? If so, the AL8860 should output 1A with a 0.1 ohm resistor across the Vin - SET pins (Iout = 0.1/Rs).

When testing on a power supply, I set the voltage limit to 12V and increase the current from 0. The voltage starts at 7V and increases as the current increases until it hits 12V between 1.1 and 1.2A. The power supply will then switch to voltage limiting and the current will drop to 0.65A, staying at 12V. As I then drop the current limit down, it switches back to current limiting at 0.8A, the voltage then changes from 12V to 8-9V.

I'm sorry for the convoluted explanation of my testing but I cannot understand what is happening here. The PCB and components are all correct.

Note: The MOSFET is not currently in the physical design.

  • \$\begingroup\$ It's not immediately obvious where you're actually measuring all these voltages & currents you refer to. Are any of them measured across or through the LEDs? \$\endgroup\$ – brhans Dec 18 '18 at 3:10
  • \$\begingroup\$ Sorry, all voltages are displayed on the power supply excluding the 9V across the LEDs. This 9V is the total forward voltage of the three Cree XP-G2 LEDs in series represented by the "LEDS1" block. \$\endgroup\$ – dos584 Dec 18 '18 at 3:28
  • \$\begingroup\$ And you have actually measured 9V across the LEDs or that's what the datasheet says? \$\endgroup\$ – brhans Dec 18 '18 at 3:31
  • \$\begingroup\$ That's what the datasheet says, i measured around 9.2V. \$\endgroup\$ – dos584 Dec 18 '18 at 4:47

From your first experiment using a benchtop PSU in CC mode, the output voltage reaches nearly 12V if you target is 1A. It means that Vf for your three LEDs is about 12V (4 V per LED), which sounds reasonable. On other words, to get 1 A through the LEDs you need to have a source in excess of 12 V.

However, the AL8860 is a switching regulator, it can't output 12V if Vin = 12V, it must have some room, because it has some limit on duty cycle (75% maximum recommended), and will stop working if Vout approaches Vin. Try to apply 14-16V as your input, everything should be fine then.

From AL8860 datasheet, page 7, to get 1A LED current at 10-11V level, you need at least 14 V of input voltage:

enter image description here

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  • \$\begingroup\$ Thank you for the reply, it makes sense and a lesson learned for reading datasheets. Due to the duty cycle limits, it seems impossible to complete my design with a 12V power supply (final design will run from a 12V alternator). Can you suggest another type of LED driver that can output up to 12V and 0.8A-1A with a 12V input? \$\endgroup\$ – dos584 Dec 18 '18 at 21:34
  • \$\begingroup\$ On second thought, i wasn't measuring it properly. The power supply is showing 12V and 9V is measured from VSET to VSW. The current is showing 0.8A so it looks like 800mA is the max current the AL8860 can output with a 12V input and 9V load. This seems roughly correct looking at the graphs. \$\endgroup\$ – dos584 Dec 18 '18 at 22:48
  • \$\begingroup\$ @dos584, a 2 Ohm resistor, (12V-10V)/1A, might do the job if your power is stable at 12V, and LEDs are well cooled to room temperature, just a thought. \$\endgroup\$ – Ale..chenski Dec 18 '18 at 23:29
  • \$\begingroup\$ It would take a chilled water cooler to bring an XP-G2 down to room temperature running at 1 Amp. A 2Ω resistor with a 12V supply would take them up to 1500 mA or down to 500 mA with a 10V supply. The AL8860 is the wrong buck CC driver for the job. 4V overhead is much too high. An LM3404 may be a good choice. I would be looking at buck drivers aimed at the automotive 12V market. Maybe an Allegro A6213. \$\endgroup\$ – Misunderstood Dec 19 '18 at 11:16

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