This article refers to the address: http:// Portable electronic products such as mobile phones are integrating more and more multimedia functions, which has caused great interest in adding stereo functions. The mono structure is used for sound amplification and polyphonic ring tones, and in some cases can be used as a hands-free speaker. Although the functionality is sufficient, it is not a satisfactory high quality music playback solution. therefore, A few years ago, mobile audio amplifiers only needed to power the receiver mode, the average power output was much lower than 100mW, and the audio block was not the main battery power consumer. With the advent of polyphonic ringtones and new multimedia applications, the average audio power of the input 2 speakers is a few hundred milliwatts, and the duty cycle or audio usage time has increased. Figures 4 and 5 show the huge advantages of the Class D solution over the AB class. It can save 600mW of dissipated power generated by 300mW on each output and extend battery life. In addition, the temperature of each Class D audio amplifier only rises by 4 ° C, while the Class AB design rises by 35 ° C. The reduction in the internal power dissipation of the mobile phone also increases the reliability of the overall system. In summary, the development of high-performance Class-D audio amplifiers is an important enabler for integrating advanced multimedia audio functions while maintaining overall power budget. HDMI Splitter,HDMI Extender Co., Ltd. , http://www.nshdmicable.com
Currently, portable devices such as 3G mobile phones or portable multimedia players can easily provide audio 3D effects through digital processing. With this device, stereo headphone listening can be embedded so that left and right signals can be obtained from a DAC or FM radio. Although the headphone power is approximately 10mW on a 32Ω load, the main speaker can be easily used to generate and output up to 1W of power for an 8Ω load. Due to the need to provide a stereo solution instead of a conventional mono system, it is difficult to provide a highly efficient clean audio signal at these power levels.
Class AB solutions offer extremely low THD+N, but the best efficiency is only about 65%. Figure 1 shows a schematic diagram of the solution for this solution.
Knowing the power, load, and output power, the power dissipation of each amplifier can be calculated using the following formula:
For example, if we use the typical values ​​used in mobile phones R l = 8 Ω and Vp = 3.6 V (typical lithium ion battery voltage), the relationship between the dissipated power and efficiency curves and output power is shown in Figure 3 and Figure 4.
When the optimum efficiency of the output power range is less than 65%, if the product is to play music for a long time or play a game with an audio amplifier, the battery life will be greatly shortened. In addition, as shown in Figure 5, the junction temperature under the same conditions will also increase significantly. This curve is based on the junction-to-air thermal resistance at 110 °C/W and is typical for a 9-pin flip-chip package.
In a multimedia application with two 8Ω speakers, the average output power is a few hundred milliwatts. As shown by the curve, the junction temperature is 35°C higher than the ambient temperature.
In addition to the Class AB audio products such as the NCP2890, NCP4894 or NCP4896, ON Semiconductor's new Class D filter-free product line offers superior audio performance while increasing efficiency.
The NCP2820 is the first mono filterless Class D amplifier with less than 0.1% THD+N, with a power supply rejection ratio (PSRR) of -65dB and a 2.65W supply at 4Ω BTL load. Figure 6 is a simplified diagram of the stereo system using two NCP2802s. Due to the filterless connection between the amplifier output and the horn, this Class D solution does not increase external component cost.
Efficiency is the main advantage of Class D amplification. As shown in Figure 3, the NCP2820 has the highest efficiency, up to 90%. From 50mW to 600mW covering 92% of the working range, the NCP2820's efficiency is as high as 80% or more, but in the same output power, the AB class can only linearly increase to 70% by 20%. Figure 2 shows the equivalent circuit of the output section in the same operating state. Switching losses are negligible compared to conduction losses. Rp is the total parasitic resistance of metal interconnects, leadframes, and PCB traces. Ron is a PMOS and NMOS resistor.
Pd = ( 2Ron + Rp ) × Iout 2
Pout = Rl × Iout 2