X0306 4 2019-02-11 0001280452 MONOLITHIC POWER SYSTEMS INC MPWR 0001312593 Tseng Saria 4040 LAKE WASHINGTON BLVD. NE, SUITE 201 KIRKLAND WA 98033 1 VP & General Counsel Common Stock 2019-02-11 4 A 0 74331 30 A 372618 D Common Stock 2019-02-12 4 S 0 21694 130.6571 D 350924 D On February 7, 2017, the reporting person was granted a target of 28,589 shares of restricted stock units, with the right to earn up to 300% of the target shares, subject to satisfaction of pre-determined, performance-based vesting criteria. On February 11, 2019, the Company's Compensation Committee of the Board of Directors determined and approved the achievement of the performance criteria for the restricted stock units, which resulted in 45,742 additional shares being earned by the reporting person. Of the restricted stock units that were awarded, 37,167 will vest immediately and 37,164 will vest quarterly in the next two years. The reported sales were to cover taxes upon the vesting of restricted stock units, as required by the Company's equity incentive plans. /s/ Saria Tseng 2019-02-13