War! Maxim is waging war against National Semiconductor! Up to now the National Semiconductor troops (LM48555 and LM4802B) have controlled the territory (the market!), but now Maxim troops (MAX9730 and MAX9788) are ready to launch an attack. But which is the winner award?

The amplifier leadership in driving ceramic speakers! Ceramic speakers need higher voltage, but less current than traditional moving coil speakers. Moreover, compared to the moving coil speakers, they have thinner profile (0.7 mm Vs. 4 mm) and lighter weight (less than 1 gram). Although even grocer's shops are loosing money due to the world crisis the cell phone area and all its branches (smart phones etc.) are still expanding, so we expect a war with no holds barred. Now let's have a look to the fighters. The LM 48555 is an audio power amplifier while LM4802B integrates a boost converter with an audio power amplifier, both designed to drive ceramic speakers in portable applications. When powered by a 3.2 V and 3.0 V supply the LM48555 and LM4802B produce 15.7 Vp-p and 12 Vp-p respectively, with less than 1% THD+N (Total Harmonic Distortion plus Noise). Both have the boomer audio power amplifiers designed to give high quality output power using a very small number of external components and both include an advanced click and pop circuitry to suppress the noises occurring during the turn-on and turn-off transients.

The limited supply voltage available to the speaker amplifier is the main, or at least one of the main problems in the portable audio applications. Lithium ion batteries produce 3.7 V nominally and are generally used in these audio devices, but speaker amplifiers require more voltage to produce an acceptable sound. For this reason most speaker amplifiers use a particular configuration (bridge-tied -load) that double the supply voltage, but some devices, like ceramic speakers and GPS, require even more output power. A solution to satisfy this power demand is to boost the supply voltage using a separate DC-DC boost converter, but the cost and the complexity of the systems increase a lot in this case. Maxim class G power amplifiers, MAX9370 and MAX9388, have solved the problem by using a charge pump to boost the supply voltage.

MAX9370 Block scheme

The charge pump is fully integrated in the amplifier, requires only two external capacitors instead of a large inductor generally used to achieve the same performances, generates a negative voltage with magnitude equal to the supply and, in this way, doubles the supply voltage. The boost converter and the large inductor elimination reduce significantly costs and sizes and with consumers demanding smaller and smaller devices this solution will have great chances in the future. Battery life is an extremely important parameter in portable devices and above all MAX 9370 and MAX9388 use a 10V amplifier instead of the 5V of the common amplifier, so an high efficiency was requested. Class AB did not provide the requested high efficiency, class D is highly efficient, but not compatible with ceramic speakers and suffers the EMI (Electro Magnetic Interferences) penalties, so Maxim found in class G the technology that was looking for. Class G uses several supply voltages instead of a fixed one. At the amplitude variation of the input signals class G technology selects automatically the suitable supply to reduce the voltage fall across the output transistors. In this way the efficiency is significantly improved generating a longer battery life and the current consumption is reduced.

As you can see both National Semiconductor and Maxim have strong fighters, the war is just at the beginning.

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Read the Italian version: Guerra degli amplificatori! Maxim contro National Semiconductor