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Consumers desire that new generation of devices will be developed with smaller physical size and lower cost than existing devices. This paper presents the design of an integrated class D audio power amplifier with step-down (Buck) DC to DC converter. The high efficiency pulse-width modulation (PWM) buck converter is used to supply 1.5 V for the class D amplifier. The operating voltage range can be from 3V to 5V which is used for mobile devices. The class D amplifier uses the three level pulse width modulation (PWM) to eliminate the external output low-pass filter. The circuit is designed with TSMC 0.35um 2P4M 3.3V/5V mixed-signal CMOS process. The best total power efficiency of this integrated circuit achieves 84%, and the total harmonic distortion (THD) is less than 0.26% at 10 KHz with 8Ω load.