LM48580
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SNAS491A –FEBRUARY 2010–REVISED MAY 2013
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
the output signal and adjusts the supply accordingly. When the amplifier output is below 3VP-P, the nominal boost
voltage is 6V. As the amplifier output increases above 3VP-P, the boost voltage tracks the amplifier output as
shown in Figure 17. When the amplifier output falls below 3VP-P, the boost converter returns to its nominal output
voltage. Power dissipation is greatly reduced compared to conventional Class AB drivers.
Figure 17. 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 10Ω 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.
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