For electronic personal dosimeters (EPDs) based on a spectroscopy system, it is necessary to accurately measure the dose in real-time from the gamma energy spectra. The method of spectrum-to-dose conversion is being used instead of the method of count-to-dose conversion. The G(E) function, a typical method of spectrum-to-dose conversion has been applied to various instruments due to its good dose measurement performance and the advantage of real-time measurements. In this manuscript, we present a method to increase the accuracy of G(E) function for the EPD consisting of a 3 x 3 mm(2) PIN diode coupled with a 3 x 3 x 3 mm(3) CsI(Tl) scintillator. The new G(E) function was calculated using the adaptive moment estimation (ADAM) method based on Monte Carlo simulation. The proposed G(E) function is verified by comparison with the least-square method (LSM), which is the conventional method for calculating the G(E) function and with the gradient-descent method (GDM), which is the basis for the ADAM. The relative difference was acquired to compare the converted dose value using each G(E) function by using Am-241, Co-57, Na-22, Cs-137, Mn-54 and Co-60 radioisotopes. In addition, the energy response to Cs-137 of each G(E) function was obtained. The relative difference of G(E) function according to LSM, GDM, and ADAM was in the range of +/- 28.54, +/- 12.59, and +/- 9.9%, respectively, and the energy response to Cs-137 was 0.71 to 1, 0.87 to 1.02, and 0.9 to 1.03, respectively.