Class D Audio Power Amplifier In The Pwm Method And Dead Time Control

As the great requirement of high efficiency in audio power amplifier products, class D audio power amplifier has been developed rapidly in recent years. An improved modulation sheme is proposed in this paper, based on the studies and analysis of the currently existing architecture and technology in class D power amplifier system. In the improved modulation scheme, the voltage across the load sits at zero vlots throughout most of the swithing period greatly reducing the whole losses, and as the greatly reducing of the switching current, the class D amplifier is able to be implemented without output filter. The filter-free design simplifies the components outside, and reduces the bulk and the cost of the designing system.The amplifier is designed especially for portable products that need to be thinner, for example the cellular handsets and PDAs.The system model is firstly analysed with calculation and optimization of system parameters. The principle of class D audio amplifier system is analyzed; the main performance target of system is discussed; the need for modulation part to the system is confirmed. For the environment of special noise and RF disturbance in cellular handsets and PDA application, the circuit structure is studied to improve the performance of PSRR, SNR, and THD. The single feedback structure is selected for system; the factors, which influence the performance target of PSRR, THD, efficiency of the system, are analysed; the requirement to every module is settled.Main parts of the circuits in class D audio power amplifier are designed based on the system model parameter. Improved PTAT current source serves for the compensation for the temperature excursion of trans-conductance gm. The fully differential input amplifier and fully differential integrator are used, which reduces the common mode voltage and eliminates the BYPASS capacitor and two input coupling capacitor. The copper OPA structure is used to reduce the noise and offset voltage. Moreover, the key part of quantizer in PWM modulation scheme (the comparator circuit), the driver circuit of the power MOSFETs and the over temperature protection circuit are designed. The effect of dead-time and non-ideal factors on the system is dealt with, then the dead-time is set by optimal value, considering the tradeoff relation of deadtime, backward recover current and Electro Magnetic Interference (EMI).Based on the improved PWM modulation scheme, an applied class D audio power amplifier IC is built, with an internal triwave signal with 250 KHz frequency, a reference current of 10.24 uA, a power supply range in 2.5~5.5 V. With the cooperation of other colleagues, after all sub-blocks are accomplished, the system simulation of the class D power amplifier has been made by the tool of Cadence. Then the work of layout design is done, and the chip tapes out based on the technology of CSMC 0.5 um double poly triple metal (DPTM) CMOS process. The result of test illustrates that PSRR in low frequency is 61 dB, THD is 0.56% (5 V, 1 W, 1 KHz), output power Po is 2.1 W (5 V, THD=10%, 1 KHz), basically up to the expectation.