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Cool Operating, High Performance Class-D Audio Amplifiers From TI Increase Battery Life, Cut Costs in Flat Panel Displays

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Answering the demand for audio power amplifiers that reduce heat and decrease board space and costs without compromising sound quality and performance, Texas Instruments Incorporated (TI) introduces the industryıs first Class-D audio power amplifiers designed for flat panel displays via its TPA3000D family. Although originally developed for flat panel displays, the technology is quickly proliferating to other applications such as car audio systems, as demonstrated in a JENSEN car stereo at CES. The TPA3000D family is 85 percent efficient as a result of the 250-kHz pulse width modulated operation and the extremely low output FET on resistance (RDS(on)). The high efficiency reduces heat in the system eliminating the need for large heat sinks and through-hole packaging, cutting board space and component costs for manufacturers. The deviceıs efficient Class-D operation can drive up to 20-W of continuous output power with no external heat sink. "TIıs Class-D technology gives us the best of both worlds: excellent output power and fidelity without a large heat sink," said Ken Owens, director of engineering , Electronics, Recoton. "Weıre able to increase the output power performance from the in-dash car stereo and reduce its size. For consumers, Class-D is synonymous with high performance." Devices in the family include the feature-rich TPA3002D2, a 9-W stereo amplifier with DC volume control and pre-outs for headphone drive and the high power TPA3001D1, a 20-W mono audio power amplifier. All devices in the family operate from a nominal 12-V supply, have less than one percent total harmonic distortion plus noise (THD+N) at half the rated output power level and less than ten percent THD+N at the maximum rated output level. Furthermore, the A-weighted noise floor is only 86 micro-VRMS or -81 dB across the entire audio frequency band. The TPA3003D2 and TPA3004D2 are the other devices in the TPA3000D family. Table 1 lists all four devices with their differentiating features and specifications.   Device Out W Min V Max V S/M Vol Ctr Hdphone Preout Pkg Type Price 1k   TPA3001D1 20 8 18 Mono No No 24-pin TSSOP $2.79   TPA3002D2 9 8.5 14 Stereo Yes Yes 48-pin QFP $3.49   TPA3003D2 3 8.5 14 Stereo No No 48-pin QFP $2.99   TPA3004D2 9 8.5 18 Stereo Yes Yes 48-pin QFP $3.64 TPA3000D features and specifications "We have design-ins at the leading flat panel display manufacturers and we are seeing broad base demand in several other markets including car audio," said Eric Droge, product marketing manager, TI Audio Power Amplifiers. "The TPA3000D family is ramping simultaneously with our new TPA2000D family designed for cell phones and notebook computers. Over the past four years weıve helped the market understand the problems Class-D technology can solve and worked to understand what feature set and price the market needs to be successful. As a result, we are seeing the shift from traditional linear amplifiers to our more efficient Class-D amplifiers." Texas Instruments Incorporated, Semiconductor Group, SC-03008, Literature Response Center, P.O. Box 954, Santa Clarita, CA 91380, Phone: 800-477-8924, ext. 4500 Web Site

The Class D audio amp is a solution targeted at heat- and cost-sensitive applications but itıs design is not as easy as some manufacturers first imagined. Ask TI, the company has been working on a design for several years and initial attempts were mostly successful but there were some performance issues to resolve before it could be considered a winning solution. The TI 3000D family of Class D audio amplifiers could be that winning design. These amplifiers are used for plasma TVs, LCD TVs, cellular car kits, stereo head units and desktop replacement PCs. Before we look at the reasons why the design works lets look at the growth potential in markets that could use the Class D audio amp, such as the flat panel display market. Information from the research company Display Search indicates that the number of flat panel displays was about 25 million in 2002 and will jump to about 70 million by 2005, and these monitors need a small, efficient audio power amplifier. Clearly, this is a good market for the product with the right specs. The Class AB amplifier could be considered a competitive solution to the Class D audio amp but it falls short at power levels above 3 W because of heat issues. There are other solutions available that combine the efficiency and low heat dissipation of a Class C audio amp and the long battery life of Class AB. This combination amp is a good alternative to Class AB audio amps and first generation Class D amplifiers. Compared to the combination audio amp the first generation Class D amp offers similar efficiency but has more noise than the combination amp. The new Class D audio amp from TI is a second generation Class D amp and compares spec for spec to the combined amplifier design. However, the Class D is much smaller than all other audio amps and if you look at any other amp with this range of output power you will see that they need a through-hole package. That requires a more manually intensive manufacturing process. The 3000D family is a surface mount device that can be used in a typical high-volume packaging process. The TI design has also improved on a two-chip Class D solution by integrating everything on one chip and providing better noise performance than what the market demands. To hit the Class D price and performance point that the market needs is not an easy feat and it becomes very difficult with higher power (above 3 W). The car audio product finds another area in the market that is ripe for switching to the Class D audio amplifier. At a recent trade show, TI demonstrated a Class D amp and an off-the shelf design. The retrofitted Class D unit ran about 70 to 80 F cooler than the other unit, and it pulled about half the current. That means you will be able to supply a lot more power in the head unit without increasing the form factor and you will still be able to retrofit units in the dash of your car without making the opening bigger. It also means that audiophiles wonıt have to put an additional power amp in the dash. The Class D audio amp is of interest for customers who use their radio with the engine turned off, which could be while theyıre attending a tailgate party, or washing the car or whatever. Now they donıt have to worry about the power drain on the car battery because the Class D amplifier can provide about twice the battery life than a linear amp. The 3001D1 is expected to be the big seller in the family. It is the full power mono device that provides 20 W into an 8 ohm speaker at 18 V. That reaches the top end of what the LCD monitor manufacturers need, as well as the desk top replacement market (those are the all-in-one units that have the system unit and LCD monitor as one piece with a separate keyboard). The 3000 package is a 24-pin TSSOP, which is quite small for the power it puts out. Other amps with this output power range need a through-hole package. That package type requires a more manually intensive and expensive manufacturing process. The 3001D1 is a surface mount device that is used in the normal high volume packaging process. The differential input of this new audio amp is also important for providing good audio performance. The differential design cancels much of the common mode interference that comes in on the line, with a high common mode rejection ratio. TI did what they could to keep the external component count to a minimum. The four internal gain settings helped reduce the number of external resistors needed. The block diagram indicates the clean design architecture. Earlier Class D designs didnıt have the performance customers needed so they had to add external components to meet the market needs. The data sheet has some interesting information about traditional audio amps versus the 3001D1 Class D amp and it indicates the traditional type requires output filtering that causes lower efficiency. However, if you donıt have an output filter the current is pushed and pulled across the speakers and that action negates any efficiency gain you may have had with the amplifier. The push-pull action burns the current as it moves back and forth. Some manufacturers added an LC filter to act as a storage unit for the current. Instead of pushing and pulling the current across the speaker the current goes to and from the capacitor and inductor acting like a storage tank for the current. That drastically improves the amplifier efficiency. However, this may require a large inductor that adds size and heat. Class D audio amplifiers spike to maximum efficiency about 85% at about 3 W output power. Typically, we listen to audio in the 4 W range, and almost never listen to a notebook, PC, or even home stereo audio at full power. Comparatively, at 4 W the linear amp has about 35% efficiency. So what does efficiency mean to designer? It translates into heat and thatıs important for the LCD monitors and car stereos. These companies want to increase the power or decrease the form factor, or both. They also want to do both while decreasing the amount of heat. Heat is dealt with in the surface mount Class D package with TIıs PowerPAD technology, which dissipates it to the ground plane and out through the PCB. This basically lets you use the ground plane as a heat sink. On their new generation Class D amps TI dropped the positive/negative output signals in phase with each other. That looks to the speaker like there isnıt an input signal, just like the old Class D. When thereıs no input signal on the new Class D amp the positive and negative pulses cancel out and there is zero volts across the speaker. Consequently, no current is pushed/pulled across the speaker and TI could eliminate the large inductor and capacitor in the design. However, thereıs still a second need for a filter (inductor/capacitor) on the output for EMI. TI used a very small ferrite bead inductor (instead of the large inductor used on older version Class D amps) on the higher power Class D amps and thatıs what you see on the 3001D1 block diagram (L1 and L2). The ferrite beads are filtering the 20 MHz and above switching frequencies, the signals that can interfere with exterior components and the ones that the FCC and CE agencies look at for compatibility to the EMI standards. The TPA3002D2 is the feature rich family member. It has stereo operation, also has pre outs for headphone drives, and DC volume controls, as well as all the other features that the 1D1 has, including very low noise floor - that hissing sound when you have the volume very low. The block diagram for the TPA3002D2 is very well thought out. For example, all the inputs are on the left side, the speakers come out from the top and bottom and the outputs for the optional headphone driver are on the right. So the customer can layout the board for different models in exactly the same way and then just add the optional headphone driver for those designs that need it, without any changes to the original design. Thereıs no extra power supply rail for the TPA3002D2. It has all the connections for the headphone option. So the customer can layout the board for different models in exactly the same way and then just add the optional headphone driver for those designs that need it, without any changes to the original design. The TPA3001D1 and TPA3002D2 Class-D audio amplifiers are available now from Texas Instruments and its authorized distributors. The TPA3003D2 and TPA3004D2 Class-D audio amplifiers will be available by late Q1 2003 from Texas Instruments and its authorized distributors. The TPA3001D1 comes in a standard PowerPADı surface mount 24-pin thin scale small outline package (TSSOP). The TPA3002D2, TPA3003D2 and TPA3004D2 come in a standard PowerPAD surface mount 48-pin quad flat pack (QFP). Suggested resale pricing in quantities of 1,000 starts at $2.79. Product Folder

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