The LT3009 may be thought of as a lower input voltage version of the LT3008. Summarizing the key differences, the LT3009 has a lower maximum V_IN (20V vs. LT3008’s 45V), lower minimum V_IN (1.6V vs. 2.0V which makes it good for 2xAA or 2xAAA battery powered applications), higher output noise (150µV_RMS vs. 92µV_RMS primarily due to the larger output cap, and an improved Darlington stage in the error amplifier). A smaller output capacitor is also required (1µF vs. 2.2µF) for the LT3009.