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Maxim Notes AMPLIFIER COMPARATOR CIRCUITS
AUDIO CIRCUITS
Keywords: ceramic, piezo, speakers, class audio, cellular, cell phones
2008
APPLICATION NOTE 4164
Amplifier Considerations Ceramic Speaker Applications
Abstract: cell phone designs demand small form factor while maintaining audio sound-pressure level. Speakers have typically been component that limits thinness design. developments ceramic, piezoelectric, speakers have opened door sleek designs. capacitive nature these ceramic speakers, special considerations need taken into account when choosing audio amplifier drive them. This article also featured Maxim's Engineering Journal, vol. (PDF, 1.3MB). Today's portable devices need smaller, thinner, more power-efficient electronic components. Cellular phones have become thin that dynamic speaker limiting factor thin manufacturers make their handsets. ceramic, piezoelectric, speaker quickly emerging viable alternative dynamic speaker. These ceramic speakers deliver competitive sound-pressure levels (SPLs) thin compact package, thus potentially replacing traditional voice-coil dynamic speakers. Some differences between dynamic ceramic speakers shown Table Table Advantages Disadvantages Ceramic Dynamic Speakers Ceramic Speakers Dynamic Speakers Advantages High efficiency Very thin form factor Tight manufacturing tolerances Smaller acoustic cavity required Disadvantages Large drive voltage required Restricted low-frequency response Capacitive load Advantages Inexpensive Proven technology Smooth frequency response Disadvantages Wide manufacturing tolerances Inefficient Thick solution size Larger acoustic cavity required
Amplifier circuits that drive ceramic speakers have different output-drive requirements than those that drive traditional dynamic speakers. structure ceramic speaker requires amplifier drive large capacitive load supply increasingly larger currents higher frequencies while maintaining high output voltage.
Ceramic Speaker Attributes
Ceramic speaker manufacturers technology similar that building multilayer ceramic capacitors. This manufacturing technique gives speaker manufacturers tighter control over speaker tolerances compared dynamic speakers. Tight construction tolerances become important when attempting equalize speaker, significant obtaining repeatable sonic characteristics from unit unit. Ceramic speaker impedance, seen driving amplifier, modeled circuit with large capacitance main element (Figure Across most audio frequencies, ceramic speaker mostly capacitive. speaker's capacitive nature dictates that impedance decreases frequency increases. Figure shows similarity ceramic speaker's impedance versus frequency that capacitor. This impedance also point resonance above which speaker most efficient producing sound. impedance around 1kHz indicates speaker's resonant frequency.
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Figure Ceramic speaker impedance large capacitance main element.
Figure impedance versus frequency ceramic speaker very similar that capacitor.
Sound Pressure versus Frequency Amplitude
alternating voltage placed across terminals ceramic speaker causes piezoelectric film inside speaker deform vibrate; amount displacement proportional input signal. vibrating piezoelectric film moves surrounding air, thus producing sound. Increasing voltage across speaker increases piezoelectric element deflection, creating more sound pressure and, therefore, increased volume. Ceramic speaker manufacturers typically rate their speakers with maximum terminal voltage, typically around 15VP-P. This maximum voltage point which ceramic element reaches excursion limits. Applying voltage that greater than rated voltage does result more sound pressure, does increase amount distortion present acoustic output signal. Figure shows graph ceramic speaker's output
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sound-pressure level (SPL) versus frequency when driven with maximum voltage. comparing graphs versus frequency impedance versus frequency, apparent that piezoelectric speaker most efficient producing high SPLs above self-resonant frequency.
Figure Output signal distortion increases when voltage applied that greater than speaker's rated voltage.
Amplifier Requirements when Driving Ceramic Speaker
Ceramic speaker manufacturers specify maximum voltage 14VP-P 15VP-P produce highest levels sound pressure. question quickly becomes generate these voltages from single battery supply. solution switching regulator boost battery voltage Armed with regulated supply, system designer could choose single-supply amplifier that requires bridge-tied load (BTL). Bridge-tying load automatically doubles voltage that speaker perceives. However, supplying amplifier with single supply allows output only theoretically swing 10VP-P. This voltage does allow ceramic speaker output highest SPL. create higher SPLs, power supply must regulated higher voltage. Another approach that employs boost converter regulate battery voltage more issues-namely size components needed. Large peak inductor currents quickly limit small total solution because inductor must physically large that core does saturate. High-current, small-profile inductors available. However, core's saturation current rating these
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inductors high enough handle load current needed drive speaker with high voltage high frequency. High current drive current-limit avoidance needed drive ceramic element. This because ceramic speakers have very impedance high frequencies. amplifier used drive ceramic speaker must have enough current drive available that does into current-limit mode when large amount highfrequency content driven into speaker. Figure shows applications circuit using MAX9788 Class amplifier. Class amplifiers have voltage rails, high low. low-voltage rail used when output signal small. high-voltage rail switched onto output stage when output signal demands higher voltage swing. Because lower power-supply rail, Class amplifier more efficient than Class amplifier when output signal small. Class amplifier still handle peak transients because higher available rail.
Figure MAX9788 shown typical ceramic speaker application circuit. MAX9788 shown Figure uses on-chip charge pump generate negative rail that inverse VDD. This negative rail only applied output stage when output signal demands higher rail. This device provides more efficient method driving ceramic speaker than traditional methods that Class amplifier with boost converter. Speaker manufacturers always recommend fixed resistance (RL) series with ceramic speaker, shown Figure This resistor acts limit amplifier's current output when signal contains great deal highfrequency content. some applications, fixed resistance needed frequency response audio signal passed speaker bandwidth limited. This would ensure that speaker does look like short circuit amplifier. Contemporary ceramic speakers have capacitance approximately 1µF. impedance Figure speaker 8kHz 16kHz. Future ceramic speakers have larger capacitance that will force amplifiers deliver even more current same signal frequency.
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Efficiency Ceramic Speakers Dynamic Applications
Efficiency traditional dynamic speaker application easy calculate. voice coil windings modeled electrically fixed resistance series with high-value inductance. Calculating power delivered load Ohm's problem using resistance value speaker: Much power delivered speaker dissipated heat speaker coil. their capacitive nature, ceramic speakers generate very much heat when they dissipate power. Ceramic speakers dissipate "blind" power. This very small amount power based ceramic element's dissipation factor. Very little heat generated when blind power dissipated. Calculating blind power straightforward instead, calculated (fCV2) (cos Where: capacitance value speaker drive voltage frequency drive voltage phase angle between current through speaker voltage across speaker dissipation factor speaker; this quite depends signal's frequency ceramic speaker's Because phase angle between voltage current ideal capacitor ceramic speaker mostly capacitive, equal zero causes power dissipation capacitive portion ceramic speaker model. Imperfections ceramic material dielectric cause voltage across speaker behind current through speaker phase angle that does quite equal 90°. This small difference between ideal phase shift actual phase shift dissipation factor (DF). ceramic speaker modeled small, effective series resistance (ESR) series with ideal capacitor. Series resistance should confused with isolation resistor that placed between amplifier speaker. ratio capacitive reactance frequency interest:2, RESR/XC example, ceramic speaker with capacitance 1.6µF being driven 5VRMS, 5kHz signal would have blind power 5000 1.6e-6 0.05) 31.4mW
Real Power Dissipation
Though ceramic speaker itself does dissipate real power heat like dynamic speakers heat generated driving amplifier's output stage external resistor (RL) placed between speaker (Figure larger external resistor, more power dissipation moved amplifier, expense low-frequency response. When driving ceramic speaker with series resistor, that blind power small contribution overall load power. Most power dissipated external resistor, shown required amplifier power delivery versus frequency graph Figure
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Figure Blind power very small portion overall ceramic speaker load power; majority power dissipated external resistor. Better low-frequency response requires smaller external resistor, that causes amplifier's output stage dissipate more power. Amplifier efficiency dictates much power dissipated amplifier's output stage. need dissipate power amplifier drives need more efficient solutions, including Class Class amplifiers. load consists series resistance that leads power dissipation load network, though speaker. Even with 100% efficient amplifier, series resistor will burn power that intended speaker. Figure example, 5kHz, total power delivered load 629mW. amplifier with efficiency dissipates 558mW. amount power that amplifier needs dissipate dictates what size package application use. significant amount power dissipation needed high-frequency sine waves must driven into ceramic speaker.
Conclusion
Increasingly thinner portable devices driving need low-profile ceramic speakers. These speakers different than traditional dynamic speakers, different design considerations apply. ceramic speaker's capacitive nature requires that amplifier have high output voltage large output current that high voltage maintained over frequency. amplifier chosen drive ceramic speaker must able deliver both blind real power complex load. Amplifier efficiency must high enough allow small solution size cost. Such demands require different amplifier topologies than traditional Class amplifier. More efficient solutions like Class Class amplifiers becoming more attractive, with Class amplifiers offering best balance between solution cost, component count, efficiency.
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References "Blind Power Dissipation Piezo Ceramic Load (for Sine Wave)," Sonitron "Piezoelectric Transducers: Distributed Mode Actuators," National Semiconductor, www.techonline.com/ learning/techpaper/193600345 "Application Notes Multilayer Ceramic Capacitors," Kemet Electronics, www.kemet.com/kemet/web/
Application Note 4164: www.maxim-ic.com/an4164 More Information technical support: www.maxim-ic.com/support samples: www.maxim-ic.com/samples Other questions comments: www.maxim-ic.com/contact
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AN4164, 4164, APP4164, Appnote4164, Appnote 4164 Copyright Maxim Integrated Products Additional legal notices: www.maxim-ic.com/legal
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