LM48580

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SNAS491 – FEBRUARY 2010

Application Information

GENERAL AMPLIFIER FUNCTION

The LM48580 is a fully differential, Class H ceramic element driver for ceramic speakers and haptic actuators.

The integrated, high efficiency boost converter dynamically adjusts the amplifier’s supply voltage based on the

output signal, increasing headroom and improving efficiency compared to a conventional Class AB driver. The

fully differential amplifier takes advantage of the increased headroom and bridge-tied load (BTL) architecture,

delivering significantly more voltage than a single-ended amplifier.

CLASS H OPERATION

Class H is a modification of another amplifier class (typically Class B or Class AB) to increase efficiency and

reduce power dissipation. To decrease power dissipation, Class H uses a tracking power supply that monitors

voltage. Power dissipation is greatly reduced compared to conventional Class AB drivers.

Figure 7. Class H Operation

PROPERTIES OF PIEZOELECTRIC ELEMENTS

Piezoelectric elements such as ceramic speakers or piezoelectric haptic actuators are capacitive in nature. Due

to their capacitive nature, piezoelectric elements appear as low impedance loads at high frequencies (typically

above 5kHz). A resistor in series with the piezoelectric element is required to ensure the amplifier does not see a

short at high frequencies.

The value of the series resistor depends on the capacitance of the element, the frequency content of the output

signal, and the desired frequency response. Higher valued resistors minimize power dissipation at high

frequencies, but also impacts the frequency response. This configuration is ideal for use with haptic actuators,

where the majority of the signal content is typically below 2kHz. Conversely, lower valued resistors maximize

frequency response, while increasing power dissipation at high frequency. This configuration is ideal for ceramic

speaker applications, where high frequency audio content needs to be reproduced. Resistor values are typically

between 10

Ω and 20Ω.

DIFFERENTIAL AMPLIFIER EXPLANATION

The LM48580 features a fully differential amplifier. A differential amplifier amplifies the difference between the

two input signals. A major benefit of the fully differential amplifier is the improved common mode rejection ratio

(CMRR) over single ended input amplifiers. The increased CMRR of the differential amplifier reduces sensitivity

to ground offset related noise injection, especially important in noisy systems.

THERMAL SHUTDOWN

The LM48580 features thermal shutdown that protects the device during thermal overload conditions. When the

junction temperature exceeds +160°C, the device is disabled. The LM48580 remains disabled until the die

temperature falls below the +160°C and SHDN is toggled.

Copyright © 2010, Texas Instruments Incorporated

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