/*** check if the MAINREG low power mode is established ***/
while(SUPC->STATUS.bit.ULPVREFRDY==0);

/*** Allow to use low power ref for vreg ***/
SUPC->VREG.bit.VREFSEL = 1;

/*** Set Low Power mode efficiency for the Low Power voltage regulator (LPVREG)
 *** !!!! Apply this when power voltage is above 2.5 V !!!! 
 ***/
SUPC->VREG.bit.LPEFF = 0x1;