This circuit uses the SYNCOUT pin of the LM20154 to drive the SYNC input of the LM20134 so that the two converters will run 180 degrees out of phase. The LM20154 was chosen for this application because it features a SYNCOUT pin that provides a 180 degree phase shift. The SYNCOUT pin is pulled up to the ExtVcc pin by resistor RPULLUP. This is a convenient pull-up voltage to use since the voltage at ExtVcc is 2.7V, well within the operating range of SYNC pin of the LM20134. This resistor could also have been pulled up to the input voltage provided it did not exceed 3.6V.