lm48560tlx National Semiconductor Corporation, lm48560tlx Datasheet - Page 15

• Manufacturer Part Number: lm48560tlx
• Description: High Voltage Class H Ceramic Speaker Driver With Automatic Level Control
• Manufacturer: National Semiconductor Corporation

GENERAL AMPLIFIER FUNCTION
The LM48560 is a fully differential, Class H piezo 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 head-
room and improving efficiency compared to a conventional
Class AB driver. The fully differential amplifier takes advan-
tage of the increased headroom and bridge-tied load (BTL)
architecture, delivering significantly more voltage than a sin-
gle-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 ad-
justs the supply accordingly. When the amplifier output is
below TBDV
amplifier output increases above TBDV
tracks the amplifier output as shown in Figure TBD. When the
amplifier output falls below TBDV
turns to its nominal output voltage. Power dissipation is great-
ly reduced compared to conventional Class AB drivers.
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM48560 features a fully differential amplifier. A differ-
ential 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 differen-
tial amplifier reduces sensitivity to ground offset related noise
injection, especially important in noisy systems.
AUTOMATIC LEVEL CONTROL (ALC)
The ALC is available in software mode only, and only in
boosted mode. In hardware mode ALC is always disabled.
P-P
FIGURE 8. Class H Operation
, the nominal boost voltage is TBDV. As the
P-P
, the boost converter re-
P-P
, the boost voltage
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The ALC limits the peak output voltage to the programmed
value. Consequently, it limits the peak boost voltage, as this
is derived from the output voltage. The ALC is continuous, in
that it provides a continuous adjustment of the voltage gain in
order to limit the output voltage to the programmed value. The
available gain adjustment range is typically 8dB. When the
input amplitude is further increased beyond the ALC attenu-
ation range, the output will again increase. This is illustrated
in the Typical Performance Graphs, as seen on the 14V
in the Input voltage vs Output Voltage curve. The attack and
decay of the ALC is programmed by software and works in
conjunction with the external capacitor C
is 1μF, although it can be changed from 0.1μF to 4.7μF to
select other ranges of attack and decay time.
ATTACK TIME
Attack time (t
by 6dB once the audio signal exceeds the ALC threshold. Fast
attack times allow the ALC to react quickly and prevent tran-
sients such as symbol crashes from being distorted. Howev-
er, fast attack times can lead to volume pumping, where the
gain reduction and release becomes noticeable, as the ALC
cycles quickly. Slower attack times cause the ALC to ignore
the fast transients, and instead act upon longer, louder pas-
sages. Selecting an attack time that is too slow can lead to
increased distortion in the case of the No Clip function, and
possible output overload conditions in the case of the Voltage
limiter. The attack time is set by a combination of the value of
C
Where α
in the Voltage Limit Control Register (see ). The attack time
coefficient allows the user to set a nominal attack time. The
internal 20kΩ resistor is subject to temperature change, and
it has tolerance between -11% to +20%.
SET
and the attack time coefficient as given by equation (2):
B5
ATK
0
0
1
1
TABLE 8. Attack Time Coefficient
is the attack time coefficient () set by bits B4:B3
ATK)
t
ATK
is the time it takes for the gain to be reduced
= 20kΩC
B4
0
1
0
1
SET
/ α
ATK
SET
. Typically C
α
2.4
1.7
1.3
0.9
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ATK
PP
plot
SET
(1)