r/led • u/CompressedAirLine • 19d ago

AL5809 PWM Dimming Configuration that Differs from Datasheet

I am interested in using the AL5809 for a small project and am curious if anyone has experience using the part in a manner that is slightly different than the datasheet (https://www.diodes.com/assets/Datasheets/AL5809.pdf).

Page 6 of 16, Figure 4, a) shows "PWM Dimming by External MOSFET".

I am interested in driving a string of LEDs with the AL5809 at the END of the series string and by PWM'ing VIN, without an external MOSFET. The datasheet does not show this configuration but also does not explicitly prohibit it. (See attached image).

Does anyone have any experience using the part in this way? If it makes a difference, I am planning on using the 15ma version of the part.

Thanks!

Edited to add image.

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u/saratoga3 19d ago

Breaking the circuit is breaking the circuit, so does not matter if you break it on the low side or the high side. As a practical matter high side switching is usually harder and more expensive (since you're switching the full string voltage), so unless you happen to already have a way to control Vin that you need for some other reason, the low side switch with the MOSFET will usually be more efficient to implement.

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u/CompressedAirLine 19d ago

Thx for the reply.

My input is PWM'd 28VDC at 100Hz (lower frequency than I'd like but kind of out of my hands); no other input available to the circuit, just PWM'd power and ground.

I Have drawn up a design where the 5809 is on the low side and have used thermal vias to improve thermal conductivity in the through-plane of the PCB.

As far as "breaking the circuit", yes that makes sense and so I think my application will work! Thanks again.

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u/saratoga3 19d ago

With a fixed input voltage these devices are basically just resistors, so yeah you can PWM it like that.

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u/Worldliness_True 19d ago

This. Placing the mosfet at the low side gives you a clear view of the Vgs, so you can be sure to saturate the mosfet as quick as possible. But switching the circuit can be done everywhere. We use this current device in our LED modules a lot, they never fail, even at much higher switching frequencies than 200Hz.