This reference design uses the onsemi MC34063 and a low VCE(sat) BJT to develop the drive voltage for a string of LEDs. The external discrete BJT is used in conjunction with the internal pass transistor to provide optimum efficiency. It has an input voltage range of 7.5V to 14.5V and an output of 120mA at 10V. Application note AN954, available on the onsemi website, covers the selection of the different components.