led drivers and voltage overhead
The LED used by SL in their:i4012 pico board 434 MHz P/N 4012-PSC10B434B (zip contains BOM) 
is a  Lite-On 1.8V 10mA LED RED CLEAR 0603 SMD:
LiteOn Datasheet P/N LTST-C190CKT

So the LED should be a 1.8V 10ma Red LED