LAUNCHXL-CC1350: How to enable 1.8V external power supply in CC1350 launchpad?

Hello there, 

I am running rfPacketTx on the CC1350 launchpad and it works fine if I source 3.3V through the header pins (switching to the external power jumper). How do I make it work if I source 1.8V?

I have tried changing the below code in ccfg.c file: Commented the 'Use the DC/DC' and uncommented the 'Do not use the DC/DC.'  

#ifndef SET_CCFG_MODE_CONF_DCDC_RECHARGE
// #define SET_CCFG_MODE_CONF_DCDC_RECHARGE 0x0 // Use the DC/DC during recharge in powerdown
#define SET_CCFG_MODE_CONF_DCDC_RECHARGE 0x1 // Do not use the DC/DC during recharge in powerdown
#endif

#ifndef SET_CCFG_MODE_CONF_DCDC_ACTIVE
// #define SET_CCFG_MODE_CONF_DCDC_ACTIVE 0x0 // Use the DC/DC during active mode
#define SET_CCFG_MODE_CONF_DCDC_ACTIVE 0x1 // Do not use the DC/DC during active mode
#endif

Doesn't work. 

Thank you, 

Hemanth