automotive class d digital amplifier output stage

1、Audio Engineering SocietyConvention Paper 6861Presented at the 121st Convention2006 October 58San Francisco, CA, USAThis convention paper has been reproduced from the authors advance manuscript, without editing, corrections, or consideration by the Review Board. The AES takes no responsibility for t

2、he contents. Additional papers may be obtained by sending request and remittance to Audio Engineering Society, 60 East 42nd Street, New York, New York 10165-2520, USA; also see . All rights reserved. Reproduction of this paper, or any portion thereof, is not permitted without direct permi

3、ssion from the Journal of the Audio Engineering Society.Automotive Class D Digital Amplifier Output StageBrad Stewart1, Jim Lee2, Daniel Wildhaber3, Ondrej Pauk4, Richard Deken5, and James Babb61 Freescale Semiconductor, Chandler, Arizona, 85224, USA 2 Freescale Semiconducto

4、r, Tempe, Arizona 85284, USA 3 Freescale Semiconductor, Ridgeland, Mississippi, 39157, USA 4 Freescale Semiconductor, Tempe, Arizona 85284, USA 5 Freescale Semiconductor, Ridgeland, Mississippi, 39157, USA 6 Freescale Sem

5、iconductor, Ridgeland, Mississippi, 39157, USA ABSTRACTAutomotive applications using Class D Digital PWM switching amplifiers have long been limited by the approachs perceived higher cost, the potential for radiated electromagnetic interference affecting in-vehicle electronics

6、and the difficulties of designing this type of amplifier using discrete components. An integrated circuit built on a high performance analog mixed signal plus power silicon process coupled with innovative circuit design now allows the deployment of high power Class D amplifiers in vehicles that prev

7、iously have been confined to only Class AB amplifier designs. Class D PWM amplifiers, switching to 400 kHz or more, can achieve an un-weighted dynamic range exceeding 100 dB, linearity in excess of 80 dB, and a PSRR greater than 80 dB with a full scale audio signal.Stewart et al.Automotive Class D A

8、mplifier Power Stagepace of the semiconductor industry requires new design paradigms that meld powerful digital signal processing with semiconductor capabilities which integrates digital and analog sound processing electronics to give the vehicle audio designer more design flexibility. This can be i

9、n the form of vehicle system packaging with the ability to add more channels or decoding another audio format. Ever present is the desire to keep the audio content digital for noise and content protection till the last possible moment. Semiconductor processing advances allow consumers to have their

10、audio CDs, DVDs, or MP3 encoded (and other proprietary) audio streams to be decoded, enjoyed, and kept in the digital1. INTRODUCTIONDigital Class D switching power amplifiers are capable of very high power audio output with substantially less heat dissipation and greatly improved channel packing den

11、sity with excellent audio quality. Despite these obvious benefits and the trend to keep the audio content in the digital domain until the last possible moment, the adoption of the Class D amplifier approach has been primarily seen in consumer applications and almost nonexistent in automotive. Automo

12、tive applications add another significant set of challenges that often outweigh their benefits. This paper explores the challenges to designing and deploying high power multi-channel Class D Amplifier Power Integrated Circuits using an H-Bridge (also known as a Bridged Tied Load) output configuratio

13、n built with on-chip Field Effect Transistors (FETs).domain till just before the speaker. semiconductor integration drives prices leading to lower consumer manufacturing efficiencies.At a macro level, lower component prices and higherThis is in contrast to most in-car audio products where an end-to-

14、end digital processing chain is not common. In fact we find often find the audio content is already in a ones and zeros digital format but is converted back to the analog realm well before the speakers. A good example of this is the in-dash car radio with CD player. The electronics used for the broa

15、dcast tuner or a CD player decodes the audio stream and is often available as a digital signal, especially with the advent of terrestrial digital FM broadcasting and satellite radio. The need for converting from the analog domain to the digital domain will still remain a requirement to support legac

16、y technologies such as AM, FM, and cassette tape sources (along with the inherent loss of audio quality and increased noise.)In brief, the MC33851 is a mixed signal analog power IC that incorporates sophisticated gate drivers driving two independent integrated H-Bridges capable of providing 50 watts

17、 RMS per channel (bridgeable to 100W) power rating at system THD levels of 1%. This is achieved using a Class D amplifier drive approach. The part contains a mix of low voltage digital logic, analog circuits and high power analogpowerblocksimplementedinFreescale fabricationSemiconductors SmartTMOS 8

18、 silicon process.The MC33851 Dual Channel Class DSwitchingAs car makers move toward differentiating their vehicle offerings, the ability to bring high power audio, high channel count with a living room-like listening experience is compelling. This desire coupled with the economics of the semiconduct

19、or industry where digital processing is cheap, with the flexibility of a full digital signal path has gained traction. This evidenced today with consumers able to purchase higher-end passenger vehicles equipped with sound systems that feature multi-channel high power Class D audio amplifiers.Amplifi

20、er is able to deliver its rated audio output power from a single polarity unboosted 12V vehicle batterysource using low impedance speakers.Automotiveproducts that would benefit from a Class D approach include car radios and high power external amplifiers products used with multi-channel audio system

21、s.2. AUTOMOTIVE CLASS D THE FUTURE IS NOWTodays car sound systems use audio amplifiers that are mostly linear Class AB designs. In contrast, Class D switching amplifiers are found in all manner of high volume products from a host of consumer electronics companies. The quickening pace in the automoti

22、ve infotainment market coupled with the already quick3. THE STEPS TO A SYSTEM SOLUTIONDesigning audio products based on Class D amplifiers have certain constraints that are not seen in the design of Class AB linear amplifiers. Examples include:AES 121st Convention, San Francisco, CA, USA, 2006 Octob

23、er 58Page 2 of 9Stewart et al.Automotive Class D Amplifier Power StageInteraction and isolation of any digital logic supplies from the analog power suppliesThe transient response of the power supplies to wide extremes in operating currentsThe chip set required to implement an end-to- end Class D dig

24、ital solutionLow voltage operation requirement for portable products with headphones or ear budsThe Switching Amplifiers Printed Circuit Board (PCB) layout affects the audio quality The direct impact of switching frequency and Output filtering on audio qualityThispaperdescribesthe system levelsilico

25、npartitioningandtheuse ofamixedsignalsemiconductor processto implement an intelligentswitching amplifier design using high current capable H-Bridges. During the product definition phase, there was close collaboration with leading suppliers of audio systems to understand and overcome these obstacles.

26、 Figure 1 shows a high level block diagram of the MC33851 Dual Channel Class D Switching Amplifier IC. Figure 2 shows a 6 channel Class D Switching Amplifier implemented with the FSA95601 6 channel Modulator and 3 MC33851s.In this paper we will examine several of the key impediments that formerly li

27、mited the use of Class D switching amplifiers use for automotive applications.Automotive electronics adds another set of unique challenges to a switching amplifier sound system designEMI/ RFI interference to Infotainment and other in-car electronic equipment such as AM and FM radio receivers.Little

28、or no airflow in a mounting location(s) (trunk, kick panel, under seat etc.)Higher ambient and in-module temperatures Thecostofsemiconductorcomponents qualified to AEC-Q100 guidelinesExtended life cycle component support costs (10 year plus)Figure 1AES 121st Convention, San Francisco, CA, USA, 2006

29、October 58Page 3 of 9Stewart et al.Automotive Class D Amplifier Power StageFigure 2The FSA95601PB digital amplifier controller, in tandem with the MC33851 Dual Channel Switching Amplifier power stage IC implements a holistic approach to suppressing pops and clicks. These4. A TOTAL VEHICLE SYSTEM APP

30、ROACHannoying and sometime dangerous transients can easily create a negative impression of the cars sound system that otherwise has laudable specifications and sonic performance.A switching amplifier used in automotive applications will suffer audio quality issues and reliability concerns if it rece

31、ives power directly from the vehicles battery- charging system without consideration of how the battery-charging system operates and how this power source can be disturbed during normal every day driving.For discussion, lets assume a basic high power external Class D system that uses the aforementio

32、ned PCM to PWM Modulator to drive a MC33851. Both of theMC33851sH-BridgeoutputsasquarewaveIn a normal vehicles use, the sound system has power applied during the ignition “Key On” or engine start sequence and conversely the power is removed at the “Key Off” or engine off state. Consideration of how

33、the vehicles power system behaves during these moments and understanding how the typical car sound system is operated drove two key decisions embodied in the design and development of the MC33851 Dual Channel Class D Switching Amplifier IC and the companion FSA95601PB PCM to PWM Modulator IC.differe

34、ntially and provide a low-impedance source to the Low Pass Filter (LPF) and speaker load. The LPF is present to remove the switching frequency components.At ignition, the amplifier module is powered on with its electronics including the FSA95601 digital amplifier controller and the Class D Switching

35、 Amplifier coming out of a forced reset state. The digital amplifier controller immediately signals the MC33851 to connect a DC current source to slowly charge the output LC filters and speaker load so that both sides of the H- Bridge outputs can get close to half the power supply voltage. In the ne

36、xt step of the power-up procedure, the digital amplifier controller starts the PWM signals while the MC33851 uses a weak FET output stage to drive the output LC filters and the speaker load. This is necessary in order to allow the internal feedback system of the digital amplifier controller to use a

37、 DC servo loop to further reduce the differential offset on the speaker terminals. At the final stage of the power up phase, the strong FET power transistors are enabled in theThe key areas addressed were “Pop and Click” suppression and power supply noise suppression or power supply rejection ratio

38、(PSRR). We will not dwell on whether the MC33851 Class D amplifier approach is better than other approaches but tackle the challenges of deploying its topology. While this paper deals primarily with the MC33851 Class D switching amplifier some discussion of the FSA95601PB Modulators role in the appl

39、ication is warranted.AES 121st Convention, San Francisco, CA, USA, 2006 October 58Page 4 of 9Stewart et al.Automotive Class D Amplifier Power StageMC33851, achieving full functionality.The result istypically less than a 5 mV transient across the speaker coil. Without a means to “soft start” the H-Br

40、idge, any turn on power circuit settling could create a large differential voltage and current across the speaker coil, resulting in an audible artifactPower supply noise rejection is another key requirement when designing with a Class D approach. Class D amplifier modules operating from a car batte

41、ry without an internal power supply to isolate the high current switching circuits from the nominal 12 volt charging system face having power line noise added to the amplified audio. Noise can come from the vehicle charging system or appear from operating high current loads like seat positioning mot

42、ors or the pump and solenoids of the Antilock Braking System. Class D switching amplifiers not isolated from this noise will see it pass through to the speakers. This becomes a problem when the MC33851s H-Bridges output stages are powered directly by the vehicle battery. This presented a serious sys

43、tem challenge.Figure 3With this approach we remove the need for an internal power supply along with the complications and costs. Amplifier designs using the MC33851 may have a power supply to boost the voltage supply to 24 volts for application using higher impedance speakers. Class D solutions offe

44、r a savings in external power supply requirements over a linear approach due to its high efficiency.Open loop Class D amplifiers have essentially no power supply noise rejection, and any noise present in the power supply will result in high intermodulation distortion with the audio signal. The same

45、digital feedback mechanism described above is also effective at substantially increasing the power supply noise rejectionThe FSA95601 digital amplifier controller includes a digital feedback circuit that samples the scaled output ofeach H-Bridge.After digitization by the digitaland reducing intermod

46、ulation distortion.Figure 4amplifier controller and applying real time pulse width correction on a per channel basis, the objectionableshows the Power Supply Rejection Ratio (PSRR) of aclosed loop versus open loop Class D system.The upper trace is without feedback, and the lower trace isnoise artifa

47、cts become inaudible.This requires nointeraction with the Amplifier Module DSP or any other electronics. More details on this approach are part of another paper. A high level view of a singled-ended block diagram is shown in Figure 3.with feedback. 0 dB.The input signal is a 1 kHz sine wave at+ 0- 1

48、 0- 2 0- 3 0- 4 0- 5 0- 6 0d Br- 7 0- 8 0A- 9 0- 1 0 0- 1 1 0- 1 2 0- 1 3 0- 1 4 0- 1 5 0- 1 6 02 k4 k6 k8 k1 0 k H z1 2 k1 4 k1 6 k1 8 k2 0 kFigure 4AES 121st Convention, San Francisco, CA, USA, 2006 October 58Page 5 of 9Stewart et al.Automotive Class D Amplifier Power Stage5. THERMAL MODELING AND

49、PACKAGING SELECTIONto 90% efficiency. This suggests 8.57 Watts of heat must be removed from the IC package chosen for the part. This condition was analyzed with the input and help from industry leaders in amplifier module design. Using their recommendations and simulating a typical listening scenari

50、o using a pink noise spectrum, it was determined that the heat dissipation levels and die temperature rise were in a range allowing a 54 lead Small Outline Integrated Circuit (54ld SOIC) package to be used. This package has an exposed thermal pad on the top surface allowing a heat sink to be attache

51、d. This allows multiple MC33851s to be placed side by side using a common heat spreader assembly. A quick example of one thermal modeling scenario is shown in Figure 5. The peak temperature reached by the package indicates that the thermal budget (140C) is met at the current powering and operating c

52、onditions. The heat sink is maintained isothermal at 100CA class D switching amplifier can achieve electrical conversion efficiencies approaching 90% while a linear class AB design often languishes in the 25% to near50% levels.When a Class AB amplifier is notamplifying it generates waste heat.Increa

53、singly, audio designers are given the task of increasing the per channel audio power and/or tasked to add more amplifier channels. By doing one, the other, or both, the designer must still remove some waste heat. This is often exacerbated by the amplifiers mounting location or in the case of an in-d

54、ash application fitting it into a 1 to 2 DIN high radio head-unit. Both mounting locations offer little to no air flow and the electronics must operate at an elevated 85 C ambient temperature. Also, one must contend with the amplifiers internal temperature rise above ambient.The Thermal Simulation P

55、owering Conditions:“Pink Noise” Conditioning“Full Power Test” = 10min1.071W per FET forOn the component side, several candidate packages were analyzed for thermal capabilities to determine which were suitable for the MC33851 design. The effort balanced off the material cost, the lead count, assembly

56、 cost, and package size coupled with the worst case operating environment of a car. The scenarios analyzed included assuming a 50 watt RMS per channel continuous output power (an unrealistic situation except for lab testing) times the two channels operating with up“10dB Power continuousTest”=0.5Wper

57、FETFigure 5AES 121st Convention, San Francisco, CA, USA, 2006 October 58Page 6 of 9Stewart et al.Automotive Class D Amplifier Power Stage6. AMPLIFIER MODULE AND IC SELF PROTECTIONinternal fault. The engine ECU is designed to provide a basic level of engine control that allows limited engine operatio

58、n under a variety of failure conditions. With this capability the vehicle can be driven short distances to get off the road or for assistance. This capability owes a lot from robustly designed automotive quality semiconductors that self protect for over-voltage, over- current or over temperature conditions.In consumer applications, a product failureis oftenrectified by offering a replacement produc