SOURCE: Maxim Integrated Products

March 05, 2007 12:07 ET

Thin Is in... Maxim's Class G Technology Enables Piezoelectric Drive With Minimum Support Components

SUNNYVALE, CA -- (MARKET WIRE) -- March 5, 2007 -- Maxim Integrated Products (NASDAQ: MXIM) introduces the MAX9788, the industry’s first mono, Class G amplifier specifically designed to drive the high capacitive load of piezoelectric speakers.

Synopsis for Investors:

* MAX9788 uses novel technology to provide an efficient, compact solution
for powering the industry’s newest, thin ceramic (piezoelectric) speakers.

* This amplifier is ideal for next-generation, ultra-slim electronic

* The MAX9788 overcomes traditional amplifier voltage limitations that
prevented the use of tiny piezoelectric speakers in most devices.

* Using a proprietary design, the MAX9788 integrates an inverting charge-
pump that effectively doubles the available voltage supply without
inductors. Space is saved and sufficient voltage is generated to drive the
piezoelectric speaker.
One of the main attractions for using ceramic piezoelectric speakers is their thin form factor. But to maintain the advantage of the ceramic speaker, the amplifier must also provide a compact solution. Since battery life is, moreover, one of the most important design criteria in mobile phone design, amplifier efficiency is also essential for an optimal audio solution. Responding to these design challenges, the MAX9788 uses novel technology to provide an efficient, compact solution for powering ceramic speakers.

The MAX9788 integrates an inverting charge-pump power supply, which provides up to 16Vp-p voltage drive from a single lithium ion (Li+) battery (See Figure 1). This voltage effectively doubles the available supply without using any inductors. Available in a small, 2mm x 2.5mm UCSP™ package, the amplifier is ideal for next-generation, ultra-slim electronic devices.

The Design Challenges of Ceramic Piezoelectric Speakers

Piezoelectric speakers are transducers that rely on the piezoelectric effect instead of a moving wire coil to generate sound. Compared to the 4mm or greater thickness of traditional moving coil speakers, standard ceramic piezoelectric speakers are only 0.7mm thick and often weigh less than one gram. The small form factor associated with piezoelectric speakers is quite attractive to the portable electronics market where size and weight are significant factors.

The industry has not seen widespread use of piezoelectric speakers, despite their obvious benefit in size. The lack of a good amplifier was a major obstacle preventing use of the piezoelectric speakers.

These thin ceramic speakers require a significant amount of voltage drive to achieve respectable sound pressure levels. While a typical speaker used in handheld devices can only withstand 7Vp-p, a piezoelectric speaker requires more than twice that voltage, up to 15Vp-p to generate the same sound pressure level. This higher voltage requirement poses a problem for speaker amplifiers typically used in handheld applications, because these amplifiers are not designed to generate such high voltages. Piezoelectric speakers are, furthermore, electrically similar to a ceramic capacitor; a moving coil speaker is electrically similar to an inductor. The capacitive nature of a piezoelectric speaker results in higher current draw as frequency increases, while a typical speaker requires less current as frequency increases. This fundamental difference poses great challenges for the amplifier.

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. While functional, the inductor for that solution requires significant board space, which leads to both a costly and a space-consuming solution. The Class AB amplifier is, moreover, inefficient and wastes battery power.

The New Charge-Pump Approach

The MAX9788 uses charge-pump technology to allow up to 16Vp-p voltage drive from a single Li+ battery with minimal external support components. Li+ batteries typically generate 4.2V maximum and operate down to 3.0V as the battery discharges. Using an inverting charge pump, similar to the Company's DirectDrive™ technology, the MAX9788 doubles the available supply without using any inductors. Two small and inexpensive ceramic capacitors are the only external components required to generate the inverted supply.

With the creation of the second negative supply, the MAX9788's amplifier can operate from a dual supply of up to +/-5.5V, enabling the amplifier to output nearly 20Vp-p. In typical applications the amplifier can operate from either a regulated 5V supply or directly from the Li+ battery, in either case generating sufficient voltage to drive a piezoelectric speaker.

Maximizing Frequency Response

Not only can the MAX9788 generate the voltage required for ceramic speakers, but it also delivers the necessary current. A ceramic speaker has the same impedance characteristics as a capacitor. Consequently, the higher the frequency, the lower the speaker's impedance becomes. To ensure that the charge pump does not collapse at high frequencies when the most current is required, the MAX9788 uses a significantly larger charge pump than typical DirectDrive amplifiers. This charge pump can deliver up to 500mA of continuous current, which allows the amplifier to maintain flat frequency response at high frequencies and maximize sound quality.

Class G Technology Uniquely Suits this Application

Since the MAX9788 utilizes a 10V amplifier instead of the typical 5V amplifiers found in handheld equipment, maintaining high efficiency is essential to maximizing battery life. While Class D technology is highly efficient and often used in handheld devices, it is not compatible with ceramic speakers due to the purely capacitive load. The design challenge was clear: something else was required for the MAX9788. Class G, a relatively unknown amplifier technology for low-power applications, emerged as the perfect fit.

Class G amplifiers 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. Efficiency improves significantly. Typically Class G amplifiers operate from two positive supplies and ground (See Figure 2). The high supply is used for large output levels, and the lower supply used for small output levels.

The MAX9788 also 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 5V, 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.

In summary, MAX9788's novel approach uses an inverting charge pump and Class G technology to provide an efficient, compact solution for powering ceramic piezoelectric speakers. With samples available now, the MAX9788 is an excellent solution for next-generation, ultra-slim electronic devices.

The MAX9788 operates over the -40 degrees Celsius to +85 degrees Celsius temperature range. Prices start at $0.80 (1000-up, FOB USA). For more information please visit:

Maxim Integrated Products is a leading international supplier of quality analog and mixed-signal products for applications that require real-world signal processing. Maxim leverages its proprietary wafer fab technologies and world-class engineering teams to develop innovative solutions for rapidly growing markets. This business model has consistently resulted in double-digit sales growth. Net revenues were $1.859B for the fiscal year that ended on June 24, 2006.

UCSP is a trademark of Maxim Integrated Products, Inc.

Figure 1.

Figure 2.

DATA SHEET: A Data Sheet for this product is available on the web.

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Contact Information

  • Editors' Contact:
    Drew Ehrlich
    Public Relations Coordinator
    408-737-7600, X4252

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