1781.6560.class d amplifier iowa state university
Contents
1. Wa Figure 4 2 Triangle Wave Generator Schematic in Use The resulting output triangle wave has a frequency of 290 KHz and amplitude of 4 Volts with a power supply of 5 Volts 800ns 1 25GS s SA 10k points 2 50 V 1125 Apr 2014 Value Mean Std Dev 17 01 14 Figure 4 3 Triangle Wave Measured in Oscilloscope 4 3 PWM Pulse Width Modulation PWM was used to encode the audio signal in a way that could be very efficiently amplified in a later stage To create the PWM the audio signal from the equalizer was compared to the triangle wave This modulation works by linearly encoding the amplitude of the audio wave to the width of the PWM The straight line shape is why a triangle wave was used but other waves consisting of linear changes in amplitude through time could have been used To ensure high quality a high soeed TLV3501 comparator was used to create a PMW MAY14 02 Page 22 Audio Amplifier and 3 Band Equalizer EQ Out PWM Triangle Wave V GND Figure 4 4 Comparator 4 4 NON OVERLAPPING CLOCK GATING 4 4 1 WHY A non overlapping clock system is needed to prevent the power MOSFETS from turning on at the same time In the original design the two MOSFETS on one side of the bridge were controlled by the same signal just inversed That way when the high side MOSFET would turn on the low side MOSFET would be turning off at the same time The problem though is that the switch doesn t happen instantane2. Class D Audio Amplifier and EQ lowa State University Tl university program TI Innovation Challenge 2014 Project Report Team Leader Spencer Bell ssbel iastate edu or spencersbell gmail com Team Members Josh Schau jmschau iastate edu Kyle Shearer kshearer98 gmail com Mackenzie Tope mktope iastate edu Seth Weiss sgweiss iastate edu Advising Professor Dr Ayman Fayed aafayed iastate edu Texas Instruments N A Mentor if applicable Date 5 4 2014 Qty List all Tl analog IC or 1 Explain where it was used in the project TI processor part 2 What specific features or performance made this number and URL component well suited to the design 13 OPA1662 Low noise 3 3nV VHz Low distortion 0 00006 With very low noise and low distortion the OPA1662 was used for all op amps in the circuit that operated with either audio signals or the triangle wave This included the input buffer equalizer triangle wave generator high gain stage and feedback op amps 3 TLV3501 4 5ns propagation delay Rail to rail outputs With the high speed and rail to rail outputs the tlv3501 comparator was used to create the PWM signal as well as form the Schmitt trigger for the triangle wave generation circuit 4 UCC27211 4A source sink current High amp Low side Drivers The UCC27211 gate driver was critical in driving the power MOSFETS fast enough to increase our effi
3. D 5 N GND SPEAKER DM 1uF D gt N Feedback 10K 1K GND 13 4uH GND Figure 4 14 Feedback Circuit Ideally a very large open loop gain could be implemented as it allows for stronger correction when negative feedback is implemented This strong error correction reduces distortion and some noise caused by other elements in the amplifier The larger the open loop gain of the amplifier the lower the overall distortion is As the output filter was a two pole system it was important to consider the other poles present in the amplifier as well as the open loop gain so that the phase margin was kept above 70 The high gain stage was implemented using an op amp with a gain bandwidth of 22MHz It was decided that the open loop gain should be kept low enough so that the bandwidth of the high gain stage was considerably higher than the cutoff frequency of the output filter This ensures that any phase shift produced by the op amp would not cause the amplifier to be unstable To determine the open loop gain the phase response of the output filter was examined to find the phase of 90 degrees The attenuation caused by the output filter was then examined to be 26dB To create an overall phase margin of 90 the high gain stage was given a gain of 26dB to counteract the attenuation of the output filter MAY14 02 Page 30 Audio Amplifier and 3 Band Equalizer 10K 500 Feedback AAN EQ Out GND Triangl
4. Figure 5 Enclosure Inside View MAY14 02 Page 33 Audio Amplifier and 3 Band Equalizer 6 FUTURE CONSIDERATIONS 6 1 PROFFESIONAL PCB Currently the board on which the amplifier and equalizer are mounted on is a copper board that we had routed at school Due to time constraints we were unable to order a professional PCB with solder resist and a silk screen Using a professionally made PCB would have made our work a lot easier when actually mounting the components We had issues with shorts and bad connections with the current board but a board with proper routing and a good solder resist layer would solve most of the problems we ran into 6 2 SECOND CHANNEL Again due to time constraints only one channel was implemented A second channel would be ideal because most media devices output sound in stereo mode which has one channel for the left speaker and one for the right speaker 6 3 WIRELESS TRANSMISSION Wireless audio transmission is becoming easier and easier to implement each year and with that it also becomes more commonplace It is hard to find an audio system these days that doesn t have some sort of Bluetooth or wireless audio connection In the beginning we had made a list of additions we would like to add to the amplifier if we had enough time Wireless transmission was on the list In reality it wouldn t have been too hard to implement a Bluetooth connection to the amplifier A lot of manufacturers sell Bluetooth modules that
5. GATE DRIVE Se AMPLIFIER L T Cr R ae Figure 3 1 Function Level Description 3 2 EQ DESIGN 3 2 1 SIMULATED INDUCTORS Our equalizer design consists of 5 constant quality factor frequently called Constant Q band pass filters controlled by 5 individual potentiometers The design relies heavily on the use of simulated inductors for its frequency shaping characteristics It s no secret that inductors have tremendous frequency shaping capabilities when combined with capacitors and resistors but inductors are bulky expensive and difficult to find in precise values Luckily the characteristics of an inductor can be realized using an op amp two resistors and a capacitor as shown in the circuit below Vin Figure 3 2 Simulated Inductor MAY14 02 Page 11 Audio Amplifier and 3 Band Equalizer The circuit in Figure 3 2 works by basically reversing the operation of a capacitor thus artificially simulating an inductor The following equations relate the RLC band pass filter in Figure 3 3 right to the simulated RLC band pass filter in Figure 3 3 left 1 E Equation 3 1 L R R R C Equation 3 2 fo zaa Ry R2 Ro Cy ze Zmzstfozl Q Q Equation 3 3 C Equation 3 4 R2 270 f 9 R1 Rz fi 1 BW 2 Equation 3 5 C gt Equation 3 6 Q 210 fo R2 Q note the quality factor Q refers to the ratio between the center frequency f and the 3dB bandwidth BW C2 7 R
6. factor of four compared to a half bridge topology 48V 48V T PVM Out 1 PWM Out 2 SPEAKER 10uH 10uH 1uF UF PWM Out 2 L PWM Out 1 V V GND GND y b GND GND Figure 4 10 Power Stage Schematic Showing H Bridge and Output Filter MAY14 02 Page 27 Audio Amplifier and 3 Band Equalizer For a highly efficient system the MOSFETs had to be carefully chosen to have low gate capacitances and low on resistances It was decided to choose the MOSFET with the lowest gate capacitance available to reduce switching losses as much as possible as most of the amplifiers pin efficiencies where due to switching losses The CSD1853KCS was chosen as it possesses a very low gate capacitance at 3nF and a low on resistance 4m9 4 7 OUTPUT FILTER The output filter is used means to reduce EMI and increase the longevity of the speaker This is achieved by filtering the signal produced by the power stage which is a 48V square wave using an LC filter which consists of an inductor and a capacitor An LC filter was used as it theoretically contains no resistive elements meaning an LC filter consumes almost no power Another advantage of an LC filter is that it contains two poles that cause a decay of 40dB after the cut off frequency The values of the output filter where important to calculate as the filter needed to pass the audio spectrum but filter out the 300 KHz square wave LC filters can also have a resonate
7. frequency which creates a spike in the frequency response just before the cutoff frequency The desired characteristics of the output filter where to have a cutoff frequency of 30 KHz and a Q factor of 0 707 This would provide a pass band with no ripple and an attenuation of close to 40dB to the 300KHz square wave SPEAKER 10uH 10uH Ny e E a tuF tuF GND GND Figure 4 11 Output Filter To find the ideal values for the inductors and capacitors the following equations where used R speaker AT f cutoff L 10 6uH 1 20 feutopp ZL C 1 3uF These values could not fully be realized so L was rounded down to 10uH and C was rounded downto 1uF These values gave a Q factor of 0 63 The cutoff frequency was also increased slightly to 43 KHz and attenuation of 34 dB was seen at 300 KHz MAY14 02 Page 28 Audio Amplifier and 3 Band Equalizer When tested the ripple from the output filter had an amplitude of 800mVwhich is an attenuation o 35 6dB Tek a A gt EE SOOMS s a BH 10k points 300mY 1125 Apr 2014 Value Mean Std Dev 17 19 30 4 8 FEEDBACK Negative feedback was implemented to eliminate any dc offset voltage across the speaker as well as to reduce distortion Feedback was taken from across the speaker terminals using an instrumental amplifier topology and fed back into a high gain stage MAY14 02 Page 29 Audio Amplifier and 3 Band Equalizer Y 13 4uH 1K 1K T 1uF we fal ID GND S d G
8. regulators must be used to provide consistent power to the components requiring specific voltages The linear regulator outputs a constant voltage given sufficient input 2 3 2 TRIANGLE WAVE GENERATOR In order to amplify a signal properly with a class D amplifier the signal must first be compared to a triangle wave This triangle wave is created by integrating a square wave which originated from a Schmitt Trigger Following this formation the triangle wave is amplified by a third operational amplifier to obtain the proper shape required for generating a pulse width modulated signal PWM 2 33 PWM PWM is accomplished by comparing a signal with a triangle wave This allows the signal to be amplified very efficiently using a power amplification stage 2 3 4 COMPLEMENTARY NON OVERLAPPING CLOCK GATE The power stage consists of two MOSFETs that if not controlled carefully can both be turned on at the same time This creates a short between the supply voltage rails that wastes a lot of power and produces a very large current called shoot through 2 3 5 DRIVERS Drivers take the PWM signal and drive the power MOSFETS with the same signal Drivers are needed as they can greatly reduce turn on and off times of the MOSFETS and will reduce switching losses and distortion of the audio signal 2 3 6 H BRIDGE The H bridge consists of a series of power MOSFETS that amplify the power of the PWM signal This is a crucial portion because all of the amp
9. switching mode power supply and so the efficiency of the class D amplifier is very high The audio signal to be amplified is connected to one of the terminals of a comparator The other terminal has a high frequency triangle wave pulsing at a constant frequency and amplitude the triangle waves frequency must be about 15 times greater than the frequency of the signal to be amplified This creates a digital pulse wave The width of a pulse is directly related to the audio signals amplitude at that specific time Dueto the high frequency of the triangle wave the PWM signal acts as a digital sample of the audio signal Afterward the pulse wave modulated signal is connected to a gate driver to help drive the next stage This stage is a push pull power MOSFET configuration When the inputted digital pulse wave arrives at the gates of the MOSFETs the push pull network amplifies the signal When the digital pulse signal values at high the PUN MOSFET puts VDD as the output When the digital pulse signal is low the PDN MOSFET pushes the signal to Vss This effectively is a digital signal amplifier whose gain is related to MAY14 02 Page 10 Audio Amplifier and 3 Band Equalizer the difference between Vdd and Vss This digital signal is then put through a low pass filter to attenuate the high frequency PWM and pass the audio signal components The resulting signal is sent to the speaker Volt VoD ji TRIANGLE WAVE GND t OSCILLATOR Le out
10. 0 Uk EE 38 A EEN 39 Table of Figures RE LoS TEM DESC Nini 8 FOURE SA TR AUDIOTA tira 10 FIGURE 3 1 FUNCTION LEVEL DESCRIPTION sas ns rv Eege geed 11 FIGURE 32 gt IMULATED INDUCTO aria net ee estaenunu a ueriae gatos ge a pes onc vo aaotentaia ima natca a un nea bauer enuandauanuuaaaweumentes 11 EQUATION Laa 12 ECU TON Oz par 12 EQUATON Si ae 12 EOI BOI Se ia 12 EUA TON dido dis 12 EQUATON EE 12 FIGURE 3 3 SIMULATED RLC LEFT RLC FILTER pen 12 FIGURE 3 4 RUC FILTER LEFT SIMULATED RUG I RIGHT sosa id 13 FIGURE 3 5 FREQUENCY RESPONSE OF RLC VS SIMULATEDRIC 13 FIGURE 3 6 ADJUSTABLE GAIN BAND PASS FILTER eege Eege 14 TOI D EE 14 EQUATION a EE 14 FIGURE S27 ATIENUATING FILER CIRCUIT eet 15 FIGURE 370 AMPLIFYING FILTER CIRCUIT WEE 15 EQUATION E 15 FIGURE 3 9 BAND EQUALIZER CONFIGURATION cai dnialoe EE AEAEE T E heise 15 TABLE 3 1 FIVE BAND EQUALIZER e TEE 16 MAY14 02 Page 6 Audio Amplifier and 3 Band Equalizer TABLE 3 2 FIVE BAND EQUALIZER WALES A dd 16 FIGURES 10FIVE BAND EQUALIZER FULL SCHEMATIC aara aiara ios 16 FIGURE 3 11 GAIN DB VS POTENTIOMETER ROTATION AANEREN ENER 17 FIGURE 3 12 WE TFAPER CHARACTERISTIC re Aar aa 18 FIGURE 3 13 FREQUENCY RESPONSE HIGH BASS LOW MIDS MEDIUM HIGHS coocccccnncccncnoconnnnccnnnnnonnnnnconnnnncnnonarononarononanonnnns 19 FIGURE 3 14 ALL POTENTIOMETERS MAXIMUM add 19 FIGURE 315 ALLPOTENTIOMETERS CH NEG 20 FIGURE e EQUAL GAIN e En IS 20 Sable Ee EE 21 FIGURE 4 1 TYPICAL TRIA
11. 000 5 0000 10 0000 15 0000 1 H 7 10 40 70100 400 700 1k 4k Jk 10k 40k 100k Frequency Hz 16Hz 4K 1KHz 250Hz 60Hz Output MT mu yo ue y y Figure 3 13 Frequency Response High Bass Low Mids Medium Highs 25 Se am 50 N Fi et E Gen 25 75 X 0 100 75 K 2 100 20 1 o 2 10 o 3 2 S d e 5 a DS 10 1 4 7 10 40 70100 400 700 1k Ak 7k 10k Frequency Hz Figure 3 14 All Potentiometers Maximum MAY14 02 Page 19 Audio Amplifier and 3 Band Equalizer 50 N Y 50 so N 4 4 J E Le mE un Magnitude 200 25 0 1 4 710 40 70100 400 7001 k 7k10k q Frequency Hz Figure 3 15 All Potentiometers Minimum Equal Gain Adjustment 250Hz 1KHz 4KHz Pming Time Sunday Apa 77 2014 92223 PM AC Analysis 20 0000 15 0000 10 0000 E EEN Ge 5 BS KA ESA EE PARRA ae 5 0000 Ee Si TES 0 0000 5 0000 00 1k 4k 7k 10k 40k 100 1 4 7 10 40 70 100 400 7 Frequency Hz Figure 3 16 Equal Gain Adjustment MAY14 02 Page 20 Audio Amplifier and 3 Band Equalizer Figure 3 16 paints a good picture of how the equalizer s performance changes with an equal increase in pot rotation At low gains the output is very uniform and as the gains of each band increase the development of the 3dB ripple becomes more and more prominent 4 CLASS D DESIGN 4 1 4 2
12. 2 i j C a a AAA A Figure 3 3 Simulated RLC left RLC Filter right For example if an RLC band pass filter with a center frequency fo of 1KHz and a quality factor Q of 85 is desired R2 can be chosen freely and the values of L and C2 can be calculated using Equations 3 1 and 3 3 With R2 and C2 known the equivalent band pass filter utilizing a simulated inductor can be calculated using Equations 3 2 3 4 and 3 6 Figure 3 4 shows one possible pair of a solution MAY14 02 Page 12 Audio Amplifier and 3 Band Equalizer VoutRLC R20 v2 100 i a 1 Q q ES Figure 3 4 RLC filter left Simulated RLC right Running a frequency sweep from 20Hz to 20KHz on both circuits produces the following plot in Figure 3 5 While it s difficult to see what appears to be a single output is actually one output lying almost perfectly on top of the other Now with the ability to build a precise and inexpensive filter the question becomes how to arrange five band pass filters to make a fully functional and user friendly equalizer 20 10 Magnitude 1 4 710 40 70 400 1k 4k 10k Frequency HZ Figure 3 5 Frequency Response of RLC VS Simulated RLC MAY14 02 Page 13 Audio Amplifier and 3 Band Equalizer 3 2 2 GAIN CONTROL To give the user the ability to adjust the gain of the band pass filter we used the configuration in Figure 3 6 Vout Figure 3 6 Adjustable Gain Band Pass Filter In
13. 4 02 x 100 Output Volta Input Voltage ari LI ttt l O 10 20 30 40 50 60 70 80 90100 Potentiometer Rotation Figure 3 12 W Taper Characteristics 3 2 5 TESTING Testing was carried out both physically and virtually Circuit simulation software proved to be a valuable resource when building and modifying the filter designs Below are some examples of equalizer s output under various band adjustments The solid black line depicts the output of the equalizer in each case In Figure 3 13 the bass pots 60Hz and 250Hz and treble pots 4KHz 16KHz are set high with the mid pot 1KHz set to its minimum value The band overlap keeps each filter from achieving its full gain potential in the output but the frequency response is still reasonable Qualitatively it sounds very good Figure 3 14 and Figure 3 15 depict the output of the equalizer when all potentiometers are at their extremes In the case of Figure 3 14 all pots are at maximum and in Figure 3 15 all pots are at minimum As is the case in both instances with all pots at their extreme values a 3dB ripple occurs caused by an increased Q factor Since Equalizers are for shaping sound and not amplifying all frequencies it s not anticipated that this characteristic would bother the end user If the user wants their sound louder they need to increase the volume on the amplifier not the equalizer Page 18 Audio Amplifier and 3 Band Equalizer 20 0000 15 0000 10 0
14. MAY14 02 SPEAKER SPECIFICATIONS For our system we will use two speaker systems one for each channel that each have an impedance of 4 ohms Each speaker set should also be rated for 250 W RMS to handle the full power of the amplifier Each speaker set should contain multiple speakers with their own crossover TRIANGLE WAVE GENERATOR In order to amplify a signal properly with a class D amplifier the signal must first be compared to a triangle wave This triangle wave is created by integrating and amplifying a square wave using four operational amplifiers two in series two in parallel see figure 4 2b The triangle wave generator was designed by combining a Schmitt trigger and a Miller integrator Figure 4 1 details the schematic The frequency of the triangle wave generator was needed to be close to 300 kHz and the resulting equation for finding the component values is as follows f Zi Equation 4 1 out AR R C The amplitude of the triangle wave is determined by the ratio of R1 R2 Di Figure 4 1 Typical Triangle Wave Generator Circuit The values of the components were found as R1 1Kohm R2 6Kohm R 5000hm C 5nf It was found that the op amp used in the Miller integrator had a large impact on the quality of the wave and so an opa1662 precision op amp was used to create a consistently accurate triangle wave Figure 4 2 details the schematic with component values Page 21 Audio Amplifier and 3 Band Equalizer
15. NGLE WAVE GENERATOR CIRCUIT sssccssscsosseccnscceuscncusesesscenssenuseneuseenssensusencusesenssceesstensesensssesesenes 21 FIGURE 4 2 TRIANGLE WAVE GENERATOR SCHEMATIC IN USE csccsscosccscascosccscescesscsecaecascessaecasceecescesseseceecaecsecaecaecescesseees 22 FIGURES ERENNERT ECHTEN 22 FIGURE 4 A COMPARATOR EE 23 FIGURE 4 gt NON OVERLAPPING CLOCK GATING e EE 24 FIGURE 4 6 NON OVERLAPPING CLOCK GATING WAVEFORM csccsccascosccscascescescescesccsecaecasceecaecascceceestessseceecaececeecaecescesseees 25 FIGURE 4 7 NON OVERLAPPING CLOCK GATING DEAD TIME ccscceccsccsecscescaecsecesceecseesseeseeeeceeseeceeceecseceeeseeseseeceseeaeseeceeeeeess 25 FIGURE 4 8 DEAD TIME OF 20NS BETWEEN HIGH SIDE BLUE AND LOW SIDE YELLOW sccccceeccccsececescecceeceseueceseeceeseeecetees 26 FIGURE4 9 GATE DRIVER CONNECTED TO MOSFET E 27 FIGURE 4 10 POWER STAGE SCHEMATIC SHOWING H BRIDGE AND OUTPUT FILTER occoccnccnccnccncnncnnconcnnonnnnonconcnncnncnncnncnnannannnnnnos 27 FIGURE LLOUTPUT GI EE 28 Sleigh 29 FIGURES AS OUTPUT FILTER RIPPER 29 FIGURE A TA FEEDBACK ee 30 FIGURE 4 205 HICA GAINS EE 31 FIGURE telen FIO el E EN FIGURE 5 2 AMPLIFIER CONNECTEDTO EE KEREN enee ere ek EEN dae 32 FIGURE SID EV IEW cri riada ERROR BOOKMARK NOT DEFINED FIGURES A ROUTED PCB E 32 O ae bare hs hails wllte gle pa otag tb pueiict ated atv anu aae clans a oeemieiee sane 36 FIGURE S 2 EE 37 FIGURE E 37 FIGURE EE tnd sta wae memaia sai
16. These are the technical requirements of the project Other goals we had were to implement a 5 band equalizer make sure the equalizer is user friendly and can adequately change the sound of the amplifier have a housing case for the entire system and enter in the TI Innovation Challenge competition Please submit your class report with this one page document Your class report should include the following Max of 30 pages excluding appendix e Table of contents e List of figures and tables e A detailed written description of the project design e Hardware Design e Any Software Architecture used include any software code as part of Appendix e Testing and Results Conclusions e Future Work Recommendations e Acknowledgements and or References e Appendix schematics CAD drawings Critical IC Bill of Materials Entrants may use Digikey Online BOM tool on www Digikey com User Manual etc MAY14 02 Page 2 Audio Amplifier and 3 Band Equalizer SSQW057 MAY14 02 Page 3 Audio Amplifier and 3 Band Equalizer CLASS D AMPLIFIER AND 5 BAND EQUALIZER MAY 14 02 Team Members Spencer Bell Kyle Shearer Josh Schau Seth Weiss Mackenzie Tope Advisor Ayman Fayed MAY14 02 Page 4 Audio Amplifier and 3 Band Equalizer Table of Contents as Sn o e o PS O E COME EPOC EEN 8 E DE O e AR ne Se ee E IIA 8 1 2 RO EE 8 LS OST ATION All ENVIO ONE pileta 8 a ROSE CONC riada oO 8 21 8 18 2 ks BEE 8 22 FUNC O
17. ach band so using Equation 3 7 Equation 3 8 and Equation 3 9 to realize these gains at 0 and 100 potentiometer rotation the ratio of R4 to R2 has to equal about 7 1 and R3 must equal R4 Choosing R3 and R4 to equal 3 3KQ and setting R2 to 4700 for all filters gives us the desired result Gain dB 20 x log A Equation 3 9 For a 5 band equalizer the circuit will be constructed as shown in Figure 3 9 GND Figure 3 9 Band Equalizer Configuration Simply by paralleling the potentiometers of each filter across the inverting and non inverting terminals of the op amp shown at the top right of Figure 3 9 an equalizer with virtually any number of bands can be constructed The difficult part becomes deciding which frequencies to assign to each filter to maximize performance and tonal coherence Page 15 Audio Amplifier and 3 Band Equalizer 3 2 3 SELECTING BAND FREQUENCIES Because humans hear pitch logarithmically building an equalizer with filters that are evenly spaced on the frequency spectrum tends to produce changes in pitch that sound disjointed when amplified or attenuated The solution to this is to space the filters out logarithmically as well However while music is very mathematical it s also very subjective Defining something as sounding good is not universal so experimentation and trial and error tends to be a very important part of sound design While keeping Q constant and testing different combinat
18. aged to post your project as early as possible Your submission will be kept offline until the contest has officially closed Instructions e Project Name must be labelled Tl IC Design Contest North America Project Name and School e Fill out project form template is completely flexible and include the following documents o The Tl report template o Your full class report o Supplemental photos o A video of your partially or fully built out design We d love to see your team engaging with TI products Project abstract a short high level written description of the design and motivation behind project 1 000 words max The project is to design test and physically construct a Class D Audio Amplifier This system must be very efficient and have very low noise because those are the important characteristics of a good audio amplifier There are many different types of audio amplifiers but this project is to build a Class D amplifier Class D amplification is just a specific type of amplifier that uses a special scheme to amplifier a signal Our project includes an equalizer so that the user can change the sound of the amplifier Many audio systems have an equalizer because it allows the user to get a sound that he or she likes and it allows for customization from the user The Class D audio amplifier must be at least 80 power efficient the signal to noise ratio must be at least 96 dBs and the system must have a 3 band equalizer
19. as possible A variety of different dead times were tried but best results were found when using 20ns Dead times longer than 20ns did not improve on the switching losses and THD was within inaudible limits 20 0ns 2 50GS S 10k points 4 36 V 25 Apr 2014 Value Mean Min Std Dev 1117 13 46 No period found Figure 4 8 Dead Time of 20ns Between High Side blue and Low Side yellow The driver chosen was the UCC27211 The UCC27211 is capable of driving a full half bridge with the addition of a bootstrap capacitor It is able to source and sink 4A of current to the gates of the power MOSFETs that can turn them off and on in 7ns MAY14 02 Page 26 Audio Amplifier and 3 Band Equalizer 48v UCC27211A PWM Out Ju HS d M PWM Out 2 mu HO TUF VSS HB L EO VDD GND 10V oo ch V GND Figure 4 9 Gate Driver Connected to MOSFET The power losses per channel where measured to be 9W which is 2 of the maximum rated output power of 500W This loss doubles to 4 when stereo is implemented This loss was primarily due to switching losses 4 6 POWER STAGE To increase the power of the PWM signal a MOSFET H bridge was implemented The H Bridge consists of two pairs of n channel MOSFETs that end up driving both sides of the speaker The advantage of the H bridge is that it allows for the full supply voltage to be deliverable across the speaker which increases the maximum output power by a
20. can be configured and attached to a PCB Most modules that we briefly researched come equipped to be directly connected to speakers and only require power However in order to configure it more serial connections would be needed and a USB port would be needed on the board as well 6 4 DIGITAL PROCCESING The basis of our design was built on using analog components However about hallway through the project when we were researching potentiometers for the equalizer we stumbled upon digital pots We found that they would solve our problem with the linearization of the frequency band gains While we were researching this we also realized that the majority of our components could be replaced by a microprocessor This microprocessor would be able to handle the equalization PWM and the non overlapping clock gating If we were to try and commercialize an amplifier this would be the route we would take because it is way cheaper and less problem prone MAY14 02 Page 34 Audio Amplifier and 3 Band Equalizer 7 APPENDIX I OPERATION MANUAL 1 5 Equalizer control 1 Adjust gain of 60Hz band 2 Adjust gain of 250Hz band 3 Adjust gain of 1kHz band 4 Adjust gain of 4kHz band 5 Adjust gain of 16kHz band 6 Auxiliary input 7 Master volume control 8 9 Power Control 8 Power switch 9 Power cord connector 10 12 Output Control 10 External speakerenable 11 Left channel output 12 Right channel output MAY14 02 Page 35 Aud
21. ciency and improve the audio quality of the amplifier 8 CSD18533 60V Vds High current capacity 28nQ Gate Charge The CSD18533 was used in the power amplifier stage of the amplifier The high voltage and current ratings allowed us to output sufficient power to our speaker The ultra low gate charge was essential for fast switching speeds that where needed to keep the efficiency of the amplifier high and the distortion of the amplifier low 3 LM317HV 60V input voltage Adjustable output voltage The Im317HV was used to create the power rails for the majority of the amplifier circuitry The high input voltage allowed for each rail voltage to be dependent on just one regulator instead of a cascade of regulators The ability to easily adjust the voltage allowed us to use the same part for all of the rail voltages needed in the amplifier 4 SN74LVC1G02 3 6ns propagation delay The SN74LVC1G02 2 input nor gate was used in the non overlapping clock gating circuit The low propagation delay was attractive as it allowed to create short dead times in the PWM signals Spring 2014 Audio Amplifier and 3 Band Equalizer 6 SN74LVC2GU04 3 7ns propagation delay The SN74LVCGU04 dual inverter was used in the non overlapping clocked gating circuit The low propagation delay was attractive as it allowed to create short dead times in the PWM signals Submit your TI Innovation Challenge project to TI s Project repository Your team is encour
22. device very quickly and could cause permanent damage Also note that decoupling capacitors have been placed in strategic locations around the circuit to limit rail noise 3 2 4 W TAPER POTENTIOMETERS One issue inherent with this equalizer design is the nonlinearity of the gain versus potentiometer rotation when using linear potentiometers Because of the way the two gain equations interact with each other when the pots are between extremes a reverse S shaped curve forms as the potentiometers travel from 0 to 100 rotation as shown in Figure 3 10 The result is a series of potentiometers that have almost no perceivable effect on the circuit with the exception of the first and last 5 of rotation This makes it incredibly difficult for the user to subtly adjust tones to suit their liking Gain dB 0 10 20 40 50 60 70 80 90 100 15 20 Pot Rotation Figure 3 11 Gain dB VS Potentiometer Rotation To solve this problem we used W taper potentiometers to counteract the reversed S shaped curve of the equalizer s gain characteristics A graph of gain versus pot rotation is shown below in Figure 3 12 The W taper potentiometer s output characteristics are exactly opposite of the equalizer s natural gain characteristics By replacing the standard linear potentiometers with new W taper pots the resulting gain outputs are effectively linearized Page 17 Audio Amplifier and 3 Band Equalizer MAY1
23. e Wave Figure 4 15 High Gain Stage 5 TESTING AND EVALUATION 5 1 POWER TESTING AND ANALYSIS A single channel draws 300mA at 48V which results in a power usage of 14 5W Which comes to 2 9 of the maximum output power With the addition of the second channel the power losses come to 5 8 Copper losses due to inductor resistances and MOSFET on voltages come to 1 5 This puts the efficiency of the amplifier as a whole at 93 5 2 AUDIO QUALITY TESTING AND EVALUATION The current audio quality test shows THD to be at 1 SNR is hard to distinguish as the oscilloscopes used for testing do not show a noise floor below 60dB and so the true SNR cannot be truly measured _ 400ms H Value Mean Min Low resolution 2 50kS s E 10k points 0 00 Y Max Std Dev Figure 5 1 Noise Floor MAY14 02 Page 31 Audio Amplifier and 3 Band Equalizer 5 3 EQ FUNCTIONALITY TESTING AND EVALUATION Sine waves were inputted at the corner frequencies of the EQ to verify the corner frequencies were in the correct location Qualitative testing was done in the form of a sample audio input and sound verification to confirm that the high middle and low frequencies were adjustable as expected HH Run Hl wl d 7 EK H oe D Ou Figure 5 2 Routed PCB Figure 5 3 Amplifier Connected to Speakers MAY14 02 Page 32 Audio Amplifier and 3 Band Equalizer Figure 4 Enclosure Front View
24. io Amplifier and 3 Band Equalizer 8 APPENDIX II FAILED DESIGNS 8 1 SHOOT THROUGH SOLUTIONS 8 1 1 OFFSET TRIANGLE WAVE Triangle Out GND Triangle Out GND Figure 8 1 This circuit in an attempt to resolve shoot through current created two triangle waves that were offset from one another These triangle waves would then be used to create two non overlapping PWM signals that would have an inherit dead time The circuit added too many extra components and did not accurately produce a short dead time 8 1 2 UNILATERAL DELAY This circuit worked by creating a one way delay on one edge of the PWM signal This created two signals that would not overlap due to each PWM signal being delayed on one side The circuit was not used due to the loading effects from the drivers causing PWM Outi and PWM Out 2 to overlap at the edges This overlapping caused shoot through current therefore invalidating the circuit itself MAY14 02 Page 36 Audio Amplifier and 3 Band Equalizer PVVM Out 1 100 100 MM In EQ Out i Triangle VVave GND 4 PVM Out 2 100 100 WAN 9 V a 1n l Figure 8 2 8 2 EQUALIZER DESIGNS 8 2 1 BAXANDALL This circuit worked by summing together three different filter circuits that were adjustable by a potentiometer It was desirable as it only used one op amp and very few components Audio In 3 6K 100K EQ Out 2 5 GND Figure 8 3 A
25. ions The system will be portable enough for one person to operate and move without significant strain The system should have measures to ensure it does not bring any hazards to the environment in which it is operating 2 DESIGN CONCEPT 2 1 CONCEPT SKETCH Figure 2 1 is a basic sketch of the system The blue blocks represent the system designed and implemented The red blocks are representations of the devices that can be enhanced by using the class D amplifier Audio Amplifier System Class D Equalizer ies Figure 2 1 System Description 2 2 FUNCTIONAL SPECIFICATIONS MAY14 02 Page 8 Audio Amplifier and 3 Band Equalizer The requirements set in section 1 2 are specific goals the system has met The system also accepts different types of media such as iPods CD Players or any type of basic commercial media Given a media input the audio amplifier system drives a speaker with more volume than without using any amplification Speakers have different power ratings and impedance values so the speakers used in this system are ones that the amplifier can drive The following is standard in the audio industry a customer must ensure the speaker and audio amplifier match correctly The equalizer is 5 band and allows changes the amplitudes of specific frequency content outputted to the speaker 2 3 QUALITATIVE DESCRIPTIONS MAY14 02 2 3 1 LINEAR REGULATOR With precise required voltage rails of 2 5V 5V and 10V
26. ions of filters to see how they affected one another we settled on spacing out our filters in a semi logarithmic fashion as shown in Table 3 1 Band1 Band 2 Band3 Band4 Band5 Table 3 1 Five Band Equalizer Spacing With the center frequencies Q factor and gain ratio resistor values known the remaining components can be solved for utilizing equations 3 2 3 4 and 3 6 The component list and full circuit design are shown in Table 3 2 and Figure 3 10 respectively c2 18 80nF 3KO 2KO OOnF 4 8nF R2 1000 VDD wee o Su sbb ut we V RS T T CH Ri AAN AN E a EE H vi 10uF 1000 Ai 241 1 Vpk ae ICH DN Be e ECH VEE 25V c20 c2 cis cs 21 1uF S5 6uF 1 2uF FF os s30nF 8 2F i 13 RS yy Ri m Vv WV los lt 4700 5 1nF e me cs VDD we 180nF sv Snr Kaf 7 la la us ppp gt Ud c a HN ff D Urra e RO 24 A S27kO lt A LR12 5 741 s 130 L 741 Cis 512k0 LA cu cis 47nF e 741 47nF Gi f L D gt i Tv Figure 3 10 Five Band Equalizer Full Schematic MAY14 02 Page 16 Audio Amplifier and 3 Band Equalizer MAY14 02 Note that in the above schematic a buffer amplifier separates the music source from the equalizer so the needed currents are drawn from the buffer stage rather than the music device Hooking a music device directly to the equalizer input would drain the
27. lea SS e e EEN 8 Za OMAN EATING DESCHPUON S ee 9 DBD NEO REO O CON EE 9 EE Wave Ge EE 9 7 ike P EE 9 2 3 4 Complementary Non overlapping Clock Gate ccccceccsecceecceeccecsceccecceesseeseueseeseesseeseeeeseeeeseeees 9 2 VES eeh 9 e EE 9 SE SS ed 9 2A Tee o ST ON Ane ne eee 10 PI ANO TOK EE 10 3 System Level Descritores 10 SL Els E e el dE 10 3 EDESA tai 11 SL INMI CO ege TEE 11 EE 14 323 DCICGHNO Band re OUEN COS race 16 324 W Taper Potentiometers eegene 17 Pa We e EE 18 De CA PES i 21 4AL Speaker Specifica e CN 21 D2 Triangle Wave ENEE e WEE 21 4 4 Non overlapping Clock EC 23 A PP e o E O E A 23 E e 24 AS E 24 do DIV rata tit 25 Ao POWER tie 27 A erte TE Ai D rta acti 28 Ds o o AA ie o e E EAS 29 Ss TESIAZARO EVA 1 pepe ne een nae oe os oS eons nr rf or ee ESAE 31 MAY14 02 Page 5 Audio Amplifier and 3 Band Equalizer 5 1 Power Testing and Analysis sonic dic A ici 31 5 2 Audio Quality Testing and Evaluation 31 53 EQ Functionality Testing and Evaluation vera 32 FUE LORO E 34 Gn Polena POB EE 34 SNE e e A Er e toa 34 63 Wireless IN INS ISSO Nr io tte 34 oA DIG ial PrOcCeS INE nl 34 7 Appendix I Operation Manual ssscsssssssessnsssnsessssssssnsessnsessnsesssnsessnsessnnessensessnssesnsessnns 35 5 Appendix ik Failed Des AS pin 36 S L Shoot Througn Solutio EE 36 Sl O R OO WAVE ins EEE AE S EAEE N EA KENE EE AEN 36 A OTA eege 36 ME e TEE EDE EE 37 A A e no ee en ee E 37 eze ER 0 a0
28. lification is done at this stage 2 3 7 OUTPUT FILTER Page 9 Audio Amplifier and 3 Band Equalizer An LC filter is used immediately after the power stage to recover the original audio signal It also acts as a filter for the high powered square wave present with the PWM An LC filter is used as it has no resistive elements and so is very efficient 2 4 STANDARDS 2 4 1 AUDIO JACK The only relevant standard for our application is the use of a 3 5mm audio jack as an input to the system The jack most commonly used in devices today is a TRRS Tip Ring Ring and Sleeve jack This jack has four connections Starting from the tip there is the left channel connection right channel ground and microphone connection There is a discrepancy with the order of the ground and microphone connections on older devices but most new devices follow ground and then microphone Left Channel Right Channel Ground Microphone Figure 2 2 TRS Audio Jack Since our amplifier will not have a microphone on it we will be using a TRS Tip Ring and Sleeve jack that only has 3 connections left channel right channel ground 3 SYSTEM LEVEL DESCRIPTION 3 1 CLASS D AMPLIFIER The class D amplifier is an amplifier design based on the principal of using an audio signal to control a PWM The PWM is amplified and the audio signal is then recovered using a low pass filter The switching power MOSFET s add a similarity between the class D amplifier and a
29. ng it to output high and propagate a high on the final output This design will always put a delay on the rising edge of each output giving just enough time for the one MOSFET to turn off before the other one turns on 4 43 RESULTS Figure 4 6 shows two waveforms one for the high side gate driver and one for the low side gate driver These waveforms came from measuring the output of the circuit built from Figure 4 5 It is easy to see that the signals are never high at the same time Figure 4 7 shows a closer look at the dead time between switches The dead time is the amount of time that both MOSFETs are off Page 24 Audio Amplifier and 3 Band Equalizer Tek A A e MEE 400ns 2 50GS s F3 2 ff H 10k points 4 34 V value Mean Min Max Std Dev 1125 Apr 2014 17 08 30 22 400ns 3 04 Y 233 3305 720mY A21 867ns AS EA IN p i K PORIE 2 50GS S aa 10k points 4 36 V 1125 Apr 2014 Value Mean Min Std Dev 17 13 46 No period found The turn on voltage for our MOSFETS is 3V So in order to measure the dead time we had to measure the time when the falling edge for one side dropped below 3V and the rising edge for the other rose above 3V Figure 4 7 shows that time to be 20 ns 4 5 DRIVERS Drivers are a components that can turn MOSFETs on and off at high speeds by providing high current pulses Suitable drivers are crucial because the performance of class d amplifier is directly related to the MAY14 02 Page 25 Audi
30. nic device can change its output voltage Slew rate is usually measured in volts per microsecond V us MAY14 02 Page 39
31. o Amplifier and 3 Band Equalizer speed at which the power MOSFETs can be switched on and off This is due to a tradeoff between switching losses and audio quality Switching losses occur when a transition in the half bride is made by turning one MOSFET on and another off During this transition both power MOSFETs can be turned on for a short time This creates a short from 48V to ground that causes a large current spike known as shoot through A pause known as dead time is added between the switching times of the high and low side MOSFETs to tackle shoot through Switching losses also occur when turning the high side power MOSFET on and the low side power MOSFET off When the high side MOSFET switches on the drain of the low side power MOSFET increases from O volts to 48 volts Gate to drain capacitances in the low side MOSFET pass through some charge during the transition which in turn increases the voltage on the gate of the low side MOSFET If too much charge is passed through to the gate of the low side MOSFET the MOSFET can be turned on causing a large amount of shoot through Including a dead time between the switching of the low and high side power MOSFETs leaves a period when neither of the MOSFETs are conducting During this time the inductor in the output filter is not provided with the true PWM signal This leads to distortion of the audio signal A high fidelity amplifier will need very short dead times so as to cause as little distortion
32. ously There is a brief moment when the signals are switching that the MOSFETS will both be on at the same time When this happens there is a connection between the rail voltages with very little resistance High voltages and low resistance mean high current High current means heat and power loss A delay is introduced to prevent this and will give the signals time to switch MAY14 02 Page 23 Audio Amplifier and 3 Band Equalizer MAY14 02 4 4 2 HOW PWM Out 1 10K AAA PWM IN PWM Out 2 Figure 4 5 Non Overlapping Clock Gating Circuit Figure 4 5 shows the non overlapping clock circuit The circuit has two NOR gates connected to the output of the opposing side To understand how this works it is easiest to walk through the signal changes step by step At a steady state when the PWM is high the bottom NOR gate will have two high signals at the input and therefore a low output at the gate and at the final output as well The top NOR gate will have two low signals at the input and therefore a high output at the gate and the final output Now when the PWM switches to low the signal at the top NOR gate will have a high and a low input therefore changing its output to low but at that instant the bottom gate will also have a high and low input so it will still output low Once the signal change in the top propagates through the inverters the output at the top changes to low Only then will the NOR gate receive two low signals allowi
33. s nterans Spaaaia A Sawua tue mete anna A eeu an tele 38 MAY14 02 Page 7 Audio Amplifier and 3 Band Equalizer 1 INTRODUCTION 1 1 AUDIO SYSTEMS In order to play music from an audio device through speakers usually there is need of a separate power supply that will drive the speakers because media devices don t have that kind of power This is reason for amplifiers Audio amplifiers take the output from a media device such as an iPod and produce the same signal but at a higher power level capable of driving large speakers This way very little power is used from the media device Most audio systems also implement an equalizer that will take signal from the media device and level out a set of frequencies to a desired gain This allows for users to control the volume levels of the low middle or high end frequencies 1 2 REQUIREMENTS The system is designed to take an audio input from a media device such as an iPhone Android tablet etc The explicit Class D amplifier in the system must have a power efficiency of 80 The system also outputs a waveform that has a signal to noise ratio greater than or equal to 96dB The system also has a 5 band equalizer to adjust frequency bands to the desired amplitudes 1 3 OPERATIONAL ENVIRONMENT The system must be operable in normal commercial environments The audio amplifier is designed to be used near humans so no special consideration need to be made about temperature or other operating condit
34. sharper frequency response was required that this circuit did not provide The circuit also shifted the cutoff frequencies as each potentiometer was adjusted which is an axiomatic problem MAY14 02 Page 37 Audio Amplifier and 3 Band Equalizer 8 2 2 CASCADED FILTERS The cascaded equalizer design had the proper frequency responses when tentative adjustments were made to each potentiometer Despite this frequency bands behaved sporadically when approaching maximums and minimums Audio signals passing through also exhibited excess noise producing undesirable output Audio In a MAY14 02 Figure 8 4 Page 38 Audio Amplifier and 3 Band Equalizer 9 GLOSSARY 1 Crossover The sole purpose of a crossover 1s to split one signal into different frequency bands A crossover 1s usually used for a set of speakers where certain speakers such as a bass speaker can only handle a select range of frequencies 2 PWM Pulse Width Modulation PWM is the process of altering the pulses that make up a square wave Specifically the widths of the pulses relative to the square wave s period is changed in PWM 3 OP AMP An OP AMP 1s a device that compares the difference between two voltages and amplifies the difference by a very large amount OP AMPS are often used in conjunction with negative feedback which allows them to behave as amplifiers with very accurate and linear gains 4 Slew rate The fasted speed at which an electro
35. this circuit when the potentiometer is rotated to its most counter clockwise position 0 R4 drops out of the gain equation and the circuit effectively becomes an attenuator for that particular frequency band where the gain equation is given by Equation 3 7 below The equivalent circuit that results is shown in Figure 3 7 When the potentiometer is rotated to its most clockwise position 100 R3 drops out of the equation and the circuit becomes an amplifier where the gain is given according to Equation 3 8 The equivalent circuit is show in Figure 3 8 With the potentiometer at 50 the amplification cancels out the attenuation and a unity gain buffer is created A 1 When the potentiometer lies between the 0 50 and 100 marks a combination of attenuation and amplification takes place resulting in partial attenuation or amplification R2 A R gt R3 Equation 3 7 Equation 3 8 MAY14 02 Page 14 Audio Amplifier and 3 Band Equalizer R3 A A A A A Vout VVVNVNV Ir R3 Vin AAA e J V V V A e SS Vout MAY14 02 Ce R4 A A A A A R4 C2 VVVVV ANNAN C1 c1 Figure 3 7 Attenuating Filer Circuit Figure 3 8 Amplifying Filter Circuit From looking at Equation 3 7 and Equation 3 8 it becomes clear that the gain boundaries of the filter are dictated by the ratio of R3 and R4 to R2 We want to give the user the ability to adjust the output to anywhere between 17dB and 17dB decibels for e
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