I had a discussion recently with a former business associate who was telling me about a recently released NXP speaker driver. He said it was a fairly sophisticated device intended to drive the speaker in a cellphone, tablet computer, or MP3 player. My initial reaction was, “Yeah, big deal, another 1W or 2W class-D amplifier. So what else is new?” He explained that this device is somewhat smarter than just a typical 50¢ 1W amplifier we were used to seeing.
He further explained that the problem that usually occurs with the small speakers is that 1) they are subjected to environmental temperature extremes; and 2) they are often driven too hard, which both overheats the voice-coil wire and stresses the mechanical attachment points of the speaker cone. So there is deterioration, both electrically and mechanically. So now I had a better understanding of the problem, but still no clear view of the solution. But that's where this new device enters the picture.
The new device from NXP is the TFA9887. It is a class-D amplifier, but it is also a lot more. For starters, it has a DC-DC converter so that its bus voltage is raised high enough to allow it to deliver 2.65W into a 4Ω speaker (external supply of 3.6V). It has a DSP on board that controls audio levels so that it can deliver the high power to a speaker that's rated for only 0.5W.
By monitoring the voice-coil resistance the IC can detect if the speaker is starting to get too hot. This avoids problems caused by excessive displacement of the speaker diaphragm. Not only does this improve speaker life, but it also improves the sound quality — for free air, closed box, or vented box applications. The device also detects load shorts and shorts to supply and ground.
The DSP also helps avoid clipping, which is an amplifier problem, not a speaker problem. But this is still a good thing — by carefully adjusting levels on the fly, sound quality is improved. It's clever enough to adjust levels as battery voltage falls and even prolong battery life towards end-of-charge. The DSP and its software are compatible with existing acoustic echo cancellers. There are no hidden license fees related to the DSP's software.
The device exhibits low susceptibility to RF interference — a nice feature in a device that operates right next to a small RF transmitter. This is due in part to the digital interface being somewhat impervious to input clock jitter.
The device can be controlled via its I2C interface, and audio is passed into the device via its I2S interface. The IC is housed in a tiny 29-bump package that's around 2mm x 3mm.
What I like about this device is that the guys that designed it thought about a problem that was well beyond the scope of just the IC. In this case, I had never thought about speaker characteristics and the way the speaker deteriorates with time. NXP did and provided a nice solution to the problem.