Understanding LED Forward Voltage in a Constant Current Boost Driver

I am wishing to control different clusters of LEDs.

Each cluster can have a different number of LEDs in its string, for example, one cluster might have 8 in series and another only 4.

I was thinking of driving with a MAX16384 (constant-current, boost configuration) using PWM to control the brightness - https://datasheets.maximintegrated.com/en/ds/MAX16834EVKIT.pdf

The MAX16384 can be supplied with 7 - 28V.

Since the forward voltage of a LED is say 2.8V then my 8 cluster would require 22.4V, however, my 4 cluster would require 11.2V.

I assume that each MAX16384 could not be powered from a common supply rail, but each MAX16384 would need a specific supply rail/voltage to match the forward voltage drop of the LED cluster it is driving? I wasn't sure whether this needs to be the case or the fact that the driver is constant current might allow all cluster drivers to be supplied with a common voltage (the highest)

If all cluster drivers do need a specific custom rail is there a neat/common way of doing this?

• From what I can see the chip will boost the voltage so that the required current is achieved. Of course, there are limitations. So I’d say you could use the same input voltage. Aug 13, 2021 at 5:33

Let's look at first what the 2nd paragraph says:

The MAX16834 EV kit operates from a DC supply voltage of 7V to 28V and requires up to 5A. The MAX16834 EV kit circuit is configured to deliver up to 1A of current into series LEDs with a maximum forward voltage of 28V.

The driver works as a boost converter. For proper operation, the input voltage should be lower than or equal to the output voltage. In your case, I understand that the minimum output (load) voltage is around 12V, and the maximum is 28V.

So,

I assume that each MAX16384 could not be powered from a common supply rail, but each MAX16384 would need a specific supply rail/voltage to match the forward voltage drop of the LED cluster it is driving?

Not exactly. Multiple MAX... drivers can be operated from the same supply unless its voltage is higher than the minimum output voltage. If you have a supply voltage of 10-11V at maximum with enough current delivering capability then you can supply your multiple MAX... drivers from that.

• From looking at the MAX16384 datasheet, how do you set the VLED voltage i.e. how does it know what to "boost" it too? Aug 13, 2021 at 7:51
• @tomdertech it's a constant current driver, it regulates the drive current rather than the output voltage. However, there's a limit that the output voltage can go. From the datasheet, it's something like 250VDC. So the chip may not allow the output voltage to go further than that. Aug 13, 2021 at 7:53
• OK, so if I powered all my MAX drivers from say 12V with enough current, it would not matter whether there were 1 or 8 LEDs in the series? It is just counterintuitive to me that I know that the LEDs have a forward voltage of 2.8V and get I am measuring more than 11.2V across them all ... how do the LEDs withstand the voltage? Aug 13, 2021 at 8:06
• @tomdertech it would not matter whether there were 1 or 8 LEDs in the series? As I expressed in my answer, the input voltage should be lower than or equal to the output voltage, because the regulator is a boost converter. So if you apply 12V input then the driver will not work with only one LED at the output (i.e. 2.8V output voltage). Aug 13, 2021 at 8:16
• @tomdertech I know that the LEDs have a forward voltage of 2.8V and get I am measuring more than 11.2V across them all I don't understand. Are you measuring these voltages across only one LED or across a branch of series-connected LEDs? From your question, I understand you have 4- and 8-series-connected modules. The Vf of each LED adds up when connected in series. Aug 13, 2021 at 8:20