How does changing the load voltage affect the output current of a boost driver?

Flashlight Modding and DIY Parts
1

I’m wondering if boost drivers are limited to a set current (and adjust voltage), or are limited to a set voltage (and adjust current), or some combination of the two? For example, if there’s a 12V driver designed to run a GT-FC40 at 2.5V and you instead connect it to a slightly higher voltage will it still provide the 2.5A or will it output a lower current? Can the new output be calculated easily or is it a bit of an unknown?

I ask this because I’m thinking of building a Convoy S12 with 3 UV emitters in series and a 12V driver and I’m trying to work out the output

2

Unless it’s a badly designed boost driver, the output is constant current, so Iout remains the same as Vin changes, unless there is a limit (usually input current limit).
For example the Convoy 22mm FC40 driver has an average input current limit around 10A (it’s a feature of the MP3431 boost IC used, the limit is set by a resistor), as Vin decreases Iin increases to provide the same output power, until it reaches 10A, then Iout decreases.

~~See the measurements there : https://budgetlightforum.com/t/-/68528~~

The SC700 driver is the same, here it’s the boost IC max input limit that is met.

The Acebeam E70 driver does something similar but the limit is implemented in the firmware instead.

3

Oh my bad I misunderstood the question, it was rather about the voltage of the LED (s) connected to it.
It depends but usually there is an output voltage range, I don’t remember exactly but the 22mm FC40 driver can’t go very high, for example it can only drive an XHP35 at something like 1.2A because it hit the top of the Vout range, this can be solved by swapping one resistor of the FB voltage divider.
Another example is the noctigon boost driver which can’t go low Vout enough for red LED, so the minimum output will be higher than it should, After I mentionned this to Hank he now swap a resistor for red leds.

4

No no, these are both very helpful answers thank you! It is interesting to hear that input current is normally the limiting factor. I have built a couple of very basic 7135 drivers but I think it’s time I dipped into switching modes.

5

What are the leds you’re going to use ?

6

Initially I think I’ll build it with the SST-10-UV emitters. If that works well and I want more output I’ll probably upgrade it to SBT-10X emitters (but they are quite expensive at £18 each from the Convoy store). The low Vout range and 10A input limit may be slightly limiting here, but I’m keen to see what I can get out of it.