An energy efficient V_PP generator with fast ramp-up time for mobile DRAM is presented in this paper. Instead of using a fixed pumping clock frequency as in the conventional V_PP generator, the proposed V_PP generator adopts a voltage-controlled oscillator (VCO) and uses variable pumping frequencies to improve the ramp-up time as well as energy efficiency. Numerical results show that the VCO based V_PP generators achieve energy savings of up to 34% with 40% improvement on ramp-up time when compared to the conventional ring oscillator (RO) based design. Our proposed V_PP generator, which uses a three-stage voltage doubler as a charge pump, was implemented and fabricated in CMOS 0.13 μm process. The V_PP generator chip's core occupies 0.6 mm² area and consumes 1162 nJ (average power of 47.8 mW during 24 μs ramp-up time) while generating 3.0 V output voltage with 1.0 nF load capacitor, 2.0 mA current load and 1.2 V supply voltage.