News and New Products

Class G speaker amplifier includes charge pump

By Paul Rako, Technical Editor -- EDN, 1/29/2007

Maxim has recently introduced two monophonic audio amplifiers to drive speakers in mobile phones and other handheld systems. The MAX9730 drives conventional dynamic speakers; the MAX9788 works with new ceramic speakers. The parts provide 2.2W peak output and operate on 2.7 to 5.5V rails. They have 500-mA internal charge pumps that create negative supplies. This feature allows the bridge-tied output voltage to swing to twice the amplitude. The charge pump requires an external 4.7-μF capacitor.

Class G output stages comprise conventional Class AB outputs with modulated supply voltages to reduce the voltage drop and hence power dissipation across the output transistors. Designers have traditionally implemented Class G output stages using Class D stages to feed power to Class AB stages. The chips either enable or disable the internal charge pump to create two supply-rail levels. The AB output stages in these parts have no efficiency advantage over any other AB output stage except that they allow designers to choose speaker impedances. This feature thus allows the designer to associate the maximum speaker volume with voltage swings fairly close to the battery-voltage supply rails. This technique reduces power loss in the AB stage; however, you cannot employ this approach by itself because, as the battery voltage drops over the discharge cycle, the audio would start to clip as the battery-voltage supply rails drop. At this point, these chips enable the internal charge pump and boost the output-stage supply voltage by lowering the negative rail from 0V to the negative supply of the battery voltage, ensuring efficiency and audio fidelity. PSRR (power-supply-rejection ratio) is almost –80 dB to 10 kHz, which ensures immunity from battery-voltage changes.

The combination of the internal charge pump and the bridge-tied output stage means that the MAX9788 can swing 14V p-p and thereby drive highly capacitive ceramic speakers. These new speakers are far thinner than dynamic speakers and can allow mobile phones to be slimmer. The use of a charge-pump technology means that fewer EMI and RFI issues will occur.

The MAX9730 and MAX9788 come in a 28-pin TQFN (thin-quad-flat-no-lead) packages, which dissipate 1.7W, and 20-bump micro-chip-scale packages, which dissipate 0.8W. The devices cost 73 cents (10,000).