An easy LSB tuning method is proposed for a digitally controlled DC-DC converter to increase the DC gain of the digitally controlled DC-DC converter under conditions of no-limit-cycle and a finite bit number to reduce steady-state error of the digitally controlled DC-DC converter. The LSB of one or more of the coefficients in the denominator of the discrete-time domain transfer function of the digital compensator in the digitally controlled DC-DC converter is so tuned that the sum of all coefficients in the denominator of the discrete-time domain transfer function becomes zero. Therefore, the influence of round-off effect on the coefficients of the digital compensator is mitigated.