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I have a small LED lighting project and I plan to use an AL1791 (1-channel) LED driver.

Due to limited size, I will only use about 4 LEDs. Each LED has 2.2 V Vf.

My power source is 24 VDC.

Can an AL1791 drive just 4 LEDs in series, or does it have to be 10 LEDs in series (24 VDC/2.2 Vf)?

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  • \$\begingroup\$ That is a linear regulator, so any voltage drop across the regulator goes directly to heat. You're limited by how much heat the device can dissipate, so you'll want to sum the LED forward voltages pretty close to 24V. If they're not close to that, get a switching regulator. \$\endgroup\$
    – user1850479
    Oct 18, 2022 at 0:51

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It can work with just 4 LEDs.

The main thing is to calculate the thermals to make sure AL179x doesn't get too hot. See the section "Special System Design Considerations" on page 14 of the datasheet. In particular, these provisos:

  • AL179x has to absorb any additional voltage headroom provided by the voltage input to the emitter strings.
  • PDmax (Maximum Power Dissipation of AL179x) = 55°C/W = 1.364Watts

So, for 4x LEDs and V_LED = 24V, the maximum current at 100% brightness is:

1.364/(24-4*2.2) = 8.9 mA

So you see that a lower V_LED allows for more LED current.

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