What happens if there's no resistor before the ISET pin of a MAX7219 LED driver chip?
Those pushbuttons have a LED with resistor, as those are meant to be driven at 12V.
Driving it with the MAX7219 cannot drive the pushbutton LEDs very bright, because it is meant to be used with 5V supply.
As per the question itself, the datasheet does not say what is the minimum resistance, but it does not matter. No matter how much you want current out of it, it still can't drive LEDs that have built-in resistors for 12V use. Usually the LEDs are rated at 12V 20mA.
This is also indicated by the curve &quot;SEGMENT DRIVER OUTPUT CURRENTvs. OUTPUT VOLTAGE&quot; on the datasheet page 4.
For all three resistor values, at the output voltage of 5V, current has dropped to about 5mA anyway.