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I'm working on an extremely compact circuit, and I'm trying to figure out the best way to current limit three LEDs. The issue is that they are RGB LEDs, so there are actually 9 of them. They dont need to be driven individually, so only three control inputs are needed, but I'm having trouble figuring out a compact way to tie them together.

I can't just put them in parallel (because of possible minute Vf differences), and I don't have enough voltage to put them in series. The voltage drive varies between 3-6 volts, and I want a current of around 10mA in each of the LEDs, +/- 10%. I'm controlling them with a 1.8V mcu, so I was just going to use a mosfet for switching, but I need something to limit the current more precisely than just having a resistor. Any thoughts?

It's very important that it be compact, because every mm2 counts here. I know I could use a purpose made chip, but I haven't found any that are small enough and have enough inputs. I certainly wouldn't want to be using 9 PSSI2021SAYs (although three would be ok, but how do you tie the LEDs together?).

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  • \$\begingroup\$ 1. R G and B LEDs won't have even close to the same Vf. 2. If a (0.1%, for example) resistor is not accurate enough for you, what is your actual spec for accuracy of the current? 3. 0201 resistors are quite small. \$\endgroup\$
    – The Photon
    Oct 12, 2017 at 16:59
  • \$\begingroup\$ Why do you need more precise control than a resistor? Is your voltage source unstable enough to flicker visibly? \$\endgroup\$
    – AaronD
    Oct 12, 2017 at 17:00
  • \$\begingroup\$ Apparently the OP is not even talking about current regulation but about current limiting. Making even less sense... \$\endgroup\$
    – Eugene Sh.
    Oct 12, 2017 at 17:02
  • \$\begingroup\$ The voltage source will fluctuate between 3 and 6 volts, over long periods of time (not flickering). Using a resistor would therefore cause a current variation of more than 10%. When I said the same Vf, I mean putting the Reds in parallel with Reds, Blues with Blues etc. \$\endgroup\$
    – BeB00
    Oct 12, 2017 at 17:10
  • \$\begingroup\$ Then regulate the voltage! Put a zener diode in parallel. \$\endgroup\$
    – Eugene Sh.
    Oct 12, 2017 at 17:11

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The smallest package is the 6 Lead On-Semi SC−70 2x2mm, or 2x2mm TSOT−23 with their CAT4003B.

2.4-5.5Vin (20mA at power on)

The CAT4003B has 3 drivers each with a single enable/dim pin.

CAT4003B 3 drivers 2x2mm=4mm², 3 of these would take up 12mm²

On-Semi CAT4003B LED Driver

On-Semi CAT4003B Datasheet

On-Semi's CAT4004A an eight pin 2x2mm has four channels and a Rset pin to set the power on current. LEDs can be adjusted between 2mA to 40mA with 1.3v logic level signal.

LED Current for all 4 channels set by External Resistor and dimming with a single Enable/dim pin.

On-Semi CAT4004A

On-Semi CAT4004A Datasheet

You could drive the 3 colors in series with the Microchip CL320, 3 drivers 3 dim pins 4.9x3.9mm=19mm² 4-15v Three currents 20mA, 25mA, and 30mA.

Microchip CL320

Microchip CL320/325/330 Datasheet

On-Semi CAT4003B Datasheet

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