Implantable devices like artificial cardiac pacemakers are generally used to control irregular pulses of heart. For 10-year usage, high accuracy supply voltage is essential with the unavoidable aging of the power system to achieve near-natural beat rhythm. A Self-Calibration DC-DC Buck converter (SC-Buck) is proposed to achieve high output voltage accuracy by eliminating offset caused by mismatch of process, voltage and temperature (PVT). Furthermore, the discontinuity which affects transient performance in conventional offset cancellation is also overcome. Large area occupation, which is required to decrease mismatch in conventional operational amplifier, can be largely decreased according to Monte Carlo analysis. Without trimming process, the proposed SC-Buck technique achieves high accuracy for long-term usage (years) and low cost. The test chip fabricated in UMC 28nm CMOS process proves the output voltage accuracy of 97% since the mismatch of error amplifier can be suppressed from 122% to 1%.