16 C IN C OUT V CC Technical View Class D Amplifer Design By: Hugo Letourneau, Manager, System Design Center, Future Electronics True audiophile electronic designers have always dreamed odesigning the perect amplifer perectly reproducing the recorded soundstage. They may have started to dream about it when they learned in their frst electronic classes, that the class-A topology gives exceptional results in terms olinearity. There is sometimes a passionate student who will try to reinvent the wheel despite the warning ohis teachers and commit himselto design a 150W/channel Class A amplifer to impress the whole class with powerul and perect sound. And every time the design reaches the fnal stage, it is a renewed un to see that the amp is mostly a powerul heater or the cinema room, the casing othe amp being a big hot heat sink or the output stage transistors! This kind opassionate student will usually pur- sue another lowest power design, potentially using class B or AB, or ithe student is patient enough to do research on the various topologies, class D. For the neophyte to amplier design: in class Asystems the output stage transistors are working on 360 degrees othe signal where in class B, 180 degrees only. For class AB, the transistors will work on 180 to 270 degrees approximately, all depending on the quiescent current othe output stage. Class D ampliers are oten reerred to as being “digital” ampliers because the output stage is working with only xed high and low volt- age values, generating a square wave to eed the speakers through lters. The main advantage oClass D topology is its eciency that can get in the high nineties opercents due to its digital nature. Figure 1 shows typical circuits or each output stage topology. Figure 1: Typical basic circuits or Class A, B or A/B and ull bridge Class D ampliers respectively Class D audio ampliers have been around since more than 25 years, but only gained popularit y over the past 10-1 5 years or so. They were mainly used in low requency application and high energy de- mands osubwooers due to their high eciency, but very rarely or medium or high requencies, due to the high distortion caused by a lack operormance othe technology that was just not made or this kind oprecision switching activity at that time. Many parameters have to be taken into consider- ation beore getting to a decently audible class D amplier. Each element othe signal chain must be adequately controlled, to achieve a good audio signal through the whole audible requency range. Figure 2 shows a simple typical signal chain blockdiagram or digital ampliers. Every single detail will have to be adjusted to reach a certain balance that satises the ear. Figure 2: Class D signal path block diagram Analog PWM Conversion Stage The PWM signal can be generated by an analog or a digital circuit, just like the audio source can be analog or digital. PWM is easily achieved in analog by comparing a triangular waveorm to the audio signal. PCM can be converted into PWM using a DSP processor. In any case, the jitter and stability oany oscillator used or the PWM is oprimar y im- portance, as a ew pico seconds RMS ojitter will make the dream ogetting 100dB+ oSNR out oreach. In digital PWM systems, quantization er ror is added on top due to the nite amount oPWM steps. Several digital noise shaping techniques have been developed over time, such as pulse density and delta sigma modulation, which theoretically allows the noise power to be pushed higher in requencies outside the requency band ointerest where the lter will attenuate it better. Ia comparator will be used, a totem pole output with high slew rate is preerred, such as the LMV7239 which has a 45ns propagation delay, 1.2ns rise/all times. The quality othe lay- out will be critical to avoid any ringing, and the power distribution and decoupling have to be taken care ometiculously to avoid urther jitter in the output waveorm. Excessive capacitance has also to be avoided along the output signal path leading to the MOSFET driver. Output MOSFET Stage and Driver While all the stages along the signal path are important, the 2 parts that are the most intimately tied to the signal output quality is the MOSFET Audio in Audio to PWM conversion Output stage driver Low pass filter Half/Full Bridge Speaker V cc V ee Rload cc Click here to buy these products LMV7239 Series 1.800.675.1619 • www.FutureElectronics.com
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.