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High quality audio amp for the home with minimal design effort

National Semiconductor Corporation today announced three new stereo audio amplifiers in its Overture line of products for home stereo systems. National's newest audio amplifiers feature quiet fade-in/fade-out mute mode circuitry that gradually brings the sound up or down. These “easy on the ears” transitions enhance the listening experience in compact stereos, high-definition televisions (HDTVs), 5.1 surround sound systems and other consumer electronic equipment.

“National Semiconductor's high-quality, high-power Overture amplifier line complements our unique portfolio of audio amplifiers,” said Gary Adrig, marketing director for National Semiconductor's audio products group. “This new series of Overture amplifiers provides the highest output power available in a single IC package. These new amplifiers provide customers with robust circuitry and built-in safeguards that produce the high-fidelity audio quality that consumers are demanding.”

National's LM4780 is a stereo audio amplifier capable of delivering 60W per channel of continuous average output power. The LM4781 is a three channel audio amplifier capable of delivering 35W per channel of continuous average output power into an 8 Ohm load, and the LM4782 is a three channel audio channel capable of delivering 25W per channel of continuous average output power into an 8 Ohm load, all with less than 0.5% total harmonic distortion plus noise (THD+N) from 20Hz ” 20kHz.

The LM4780/81/82 audio amplifiers are fully protected by National's self-peak instantaneous temperature (degrees Kelvin) (SPiKe) protection circuitry. SPiKe circuitry provides a dynamically optimized safe operating area. SPiKe protection completely safeguards the LM4780/81/82 outputs against over-voltage, under-voltage, overloads, shorts to the supply or ground, thermal runaway and instantaneous temperature peaks. The advanced protection features of the LM4780/81/82 audio amplifiers are unique in the field of discrete and hybrid amplifiers. The LM4780/81/82 audio amplifiers can easily be configured for bridge or parallel operation for higher power and bi-amp solutions. All three audio amplifiers feature independent, smooth fade-in/out mute. In addition, the LM4782 has a power-conserving stand-by mode. The mute and stand-by modes can be controlled by external logic signals.

With a leading market share in audio amplifiers for cellular handsets, National delivers audio solutions to nearly every handset maker in the world. In addition to providing chips for cellular handsets, National manufactures Class AB and Class D audio amplifiers for consumer stereo and home theater systems, and headphone and speaker drivers in notebook computers and portable audio devices.

For more information on National's audio products, visit www.national.com

These audio amps were no great leap for National since they are additions to the now mature Overture family of amplifiers. The Overture is a Class AB audio amp designed for the higher power product. It's built in a high-voltage bi-polar process, called HV700. It is very mature and optimized for audio. One of the most appealing values is the spike protection. This process includes structures that make it possible to do spike protection. The way the structure is set up is not a typical over current or voltage scheme you might see in other devices. The process does give the ability to create the spike protection structures.

The family is considered a premier class of audio amp. The Overture family has never been positioned as a low-cost family but it is fairly priced. These audio amps have extremely low noise running at 2 μV noise floor, the distortion is very low in the 0.001 to 0.003dB levels at normal to high power levels. As you move up the THD scale the levels go up just like they would in any amplifier. They tend to slew around 18 to 19V/μs.

These amps will be very popular in 5.1 channel DVD receivers, stereo and A/V receivers because they are esentially, bullet proof. They can be abused and put into the spike state and not sound pretty but you won't be able to blow the amp because National uses a technique they call SPiKe. The idea behind SPiKe, which stands for self-peak instantaneous temperature in Kelvin, is unique because it has a series of temperature sensing transistors that are used near the power transistors. It's not just a single thermal sensing device, it senses the whole range of transistors on the output stages. It determines when a transistor goes into a state that's caused by excessive voltage or current and is getting into a dangerous state where heat is going to be a problem. However, only the faulty transistor is isolated and its current is rolled back to bring it into a safe operating region. That can happen on one, two or all transistors, or any combination of transistors on the output stages, depending on how hard they are being driven in an application. The idea of this design is to monitor and limit current flow dynamically. It then limits those output sensors that are in or near their thermal limits.

This new series of Overture is made in packages like the TO-220, which is a 27-lead package. The previous series had a 5, 9, 11, 15 pins, which meant you only had a limited number of inputs and outputs in the system and it also meant that there is a limited amount of heat that it can dissipate. The TO220 is a large package and that allows you to get higher power levels than could be achieved before; and it has three channels. The three channels or triple, is a very handy option because many new audio systems have five and six channels, which means there are five channels and the sub woofer, and the sub woofer has 50% to 100% the power of the free-standing channels. In that case, you have the equivalent of seven channels of power ” five for the main channels and 2 for the sub. There are many two and some single channel amplifiers on the market and that means you have to use three duals and a single and sometimes you have to use four duals with one unused channel to get the seven channels needed. The three channel series from National Semi allows you to use a triple and double for the main channel and then you can bridge a single-ended system with a high-powered mono for a sub woofer.

National says they have seen an audio market even as low as 15W, to about 30W re-emerge as a high-volume market segment. That's where these new Overture audio amps will continue to be used. The new package has given the company a new power level and triple combinations, which is very nice for 7.1, which is emerging very rapidly. That's where you have two speakers on each side, and that's the equivalent of 9 channels of power, so the three triples would will work very nicely in that topology.

Typical Circuit
Click for full size circuit

The LM4781 and 82 are very similar except for the output power. they are rated differently because the voltages they are meant to run at and how they are set up to run internally. The output power is different and is governed primarily by the supply voltages and the loads they can drive. The 4782 is not able to carry quite as much voltage”it's only rated to 25V. The only other significant difference is that the 4782 also has a standby feature in addition to a mute.

So, what external components do you need surrounding these audio amps? National has done a nice job designing these audio amps and optimized the circuits so you'll need an RC input filter, some gain-set resistors, and supply-bypass caps. It should total about three to four capacitors and three to four resistors: pretty minimal addition to your materials list. Compared to Class D architectures it's really small because you would expect to use about 40 components for every two channels in Class D designs.

The LM4780 is available now in 27-lead TO-220 packaging and is priced at $3.25. The LM4781 is available now in TO-220 packaging and is priced at $3.25. The LM4782 is available now in TO-220 packaging and is priced at $2.75. All prices are for 1,000-unit quantities.

LM4780 Product Folder

LM4781 Product Folder

LM4782 Product Folder

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