The MAX9788 from Maxim Integrated Products is a mono, Class G amplifier specifically designed to drive the high capacitive load of piezoelectric loudspeakers. The device integrates an inverting charge-pump power supply, which provides up to 16 VPK-PK from a single Li-ion battery. This voltage effectively doubles the available supply without using any inductors. Two ceramic capacitors are the only external components required to generate the inverted supply.
Prior to the introduction of the MAX9788, piezoelectric speaker amplifiers typically used inductor-based boost converters and Class AB amplifiers to generate the necessary output voltage. The thin ceramic loudspeakers require a significant amount of voltage drive to achieve respectable sound pressure levels. While a typical speaker used in handheld devices can only withstand 7 VPK-PK, a piezoelectric speaker requires more than twice that voltage, up to 15 VPK-PK to generate the same sound pressure level.
The devices use Class G amplifiers that operate just like Class AB amplifiers, except that Class G uses multiple supply voltages instead of one fixed voltage. As the input signal varies in amplitude, the Class G approach automatically selects the appropriate supply to minimize the voltage drop across the output transistors.
Typically Class G amplifiers operate from two positive supplies and ground. The high supply is used for large output levels and the lower supply used for small output levels. However, the MAX9788 uses Class G technology uniquely, taking advantage of the inverted supply from the charge pump instead of the high and low positive supply.
When the amplifier is generating a small output signal, the MAX9788 amplifier uses the battery voltage and ground as its supply. In this mode the device operates like a typical 5-V, Class AB amplifier. When the output signal exceeds the capability of the supply, the amplifier switches to using the battery voltage and the inverted charge pump output. The amplifier can then output signals sufficient for reaching a ceramic speaker's maximum sound pressure level.
Available in a 2-mm x 2.5-mm UCSP package, samples of the device are available now. Prices start at $0.65 for 1000-piece orders.