Powering led strings with higher voltage than led driver input voltage

[mariush]

I think I'm right but I want a confirmation.

With linear led drivers, they usually have a maximum input voltage for their internal circuitry but they basically limit the current going through the string of leds by measuring the voltage drop across a sense resistor, right? So the number of leds in the string and the voltage at the first led shouldn't matter, because the led driver is on the low side just measuring a drop of under 1v on the sense resistor and maybe dropping a few volts behaving like a linear regulator

I'm looking in particular at a driver like AL1793AFE : https://www.diodes.com/assets/Datasheets/AL1791_1792_1793_1794.pdf

It has 3 channels, all are set to a maximum current with a resistor, and the driver IC can be powered with ~ 6.7v to 30v, but it looks to me like that's independent of the voltage used on led strings.

So I should be able to power this driver chip with 9v or 12v (as it's above 6.7v and below 30v), but can I have 48v powering  14 x 3.4v leds = 47.6v leds on one channel? 

The driver's channel should drop only ~0.4v - 0.1v dropped on the resistor.
 

[PCB.Wiz]

Yes, you can use led drops to run from higher supplies, but you need margin for led temperatures and process variations and supply ripple and variations, and driver saturation.
47.6v led drop from 48v does not give much headroom.

[ArdWar]

Put a 21v zener or something across the pass element if you're concerned that the voltage across pass element may rise up too high. Just a small one since it only need to shunt whatever forward current the LEDs may conduct at 2V V_F (very small) when the pass transistor is not conducting (very rare, maybe only at startup or fault condition unless you're actually PWMing it).

[ajb]

So I should be able to power this driver chip with 9v or 12v (as it's above 6.7v and below 30v), but can I have 48v powering  14 x 3.4v leds = 47.6v leds on one channel? 

As PCB.Wiz pointed out, that isn't a lot of headroom.  It seems fine per the datasheet, but more LEDs in series means a larger change in the total Vf with current/temperature, so you'll have to make sure that you can meet minimum headroom requirements at max Vf, but also won't trip the short circuit protection at min Vf.  That's probably not an issue unless you're doing analog dimming, though.