ALED7709, Multiple Devices?

[patricke]

I am reworking a discrete driver design for a custom LED emitter device. While the driver can be changed, the custom emitter board currently cannot. It's designed for common V+ with multiple sinks, 10 groups of 21 LEDs in series. Our drive current would range from ~70mA to ~150mA.

We really like the ALED7709 and wanted to see if there was a way to cascade units to work with this custom LED circuit (three ALED7709s, one of which has two channels disabled). Obviously this is tricky concerning the voltage feedback circuit. As I expected, if I completely bypass VOUT and power the high side of the LEDs directly, control and dimming work but drive is lower than wanted. All lights will be dimmed/controlled in unison, so I was curious how bad the ALEDs would fight each other if I ran VOUT from one ALED, and tied all three feedback pins together.

[Peter BENSCH]

Welcome @patricke, to the community!

I am worried that the ALED7709 with 21 serial LEDs per channel cannot supply sufficient current: the ALED7709 is only designed for a VOUT of max 42V, from which the minimum headroom voltage of max. 1.3V must also be subtracted, which leaves 40.7V for the LED chain in the worst case.

With 21 LEDs, only ~1.94V/LED would then be possible, which is typically no longer sufficient for today's high-performance LEDs with a forward voltage of >3V/LED.

I am afraid that you will not be able to use the ALED7709 if the length of the LED chains cannot be changed.

Regards
/Peter

[patricke]

We are using a custom COB design, with emitters that have a max Vf of 1.8, typically 1.4. Currently we power them at 38V with no problem.

[Peter BENSCH]

You hadn't mentioned the forward voltages before and I was assuming white or similar high-power LEDs. With infrared, on the other hand, you do have such small forward voltages.

This should therefore work with 38V of a chain, but not with connected VFB of several devices. In the data sheet, section 7.7.4, you will find a description of the optimisation of the output voltage and thus the power dissipation, which the device can only regulate with itself.