VBOOST

(2V/DIV)

VOUT

(2V/DIV)

2 ms/DIV

LM48560

SNAS513F – AUGUST 2011 – REVISED NOVEMBER 2015

www.ti.com

Feature Description (continued)

Figure 24. Class H Operation

8.3.3 Differential Amplifier Explanation

The LM48560 device 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.

8.3.4 Automatic Level Control (ALC)

The ALC is available in software mode only, and only in boosted mode. In hardware mode ALC is always

disabled. 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 to limit the output voltage to the programmed value. The available gain adjustment

range is typically 8 dB. When the input amplitude is further increased beyond the ALC attenuation range, the

output will again increase. This is illustrated in the Typical Performance Graphs, as seen on the 14 VPP plot 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 CSET. Typically CSET is 0.1 μF, although it can be changed from 0.1 μF

to 4.7

μF to select other ranges of attack and decay time.

8.3.5 Attack Time

Attack time (tATK) is the time it takes for the gain to be reduced by 6 dB once the audio signal exceeds the ALC

threshold. Fast attack times allow the ALC to react quickly and prevent transients such as symbol crashes from

being distorted. However, 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 passages. 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 CSET and the attack time coefficient as given by

Equation 1:

tATK = 20 kΩCSET / αATK

(1)

Where

αATK is the attack time coefficient (Table 1) set by bits B4:B3 in the Voltage Limit Control Register. The

attack time coefficient allows the user to set a nominal attack time. The internal 20 k

Ω resistor is subject to

temperature change, and it has tolerance between –11% to +20%.

Table 1. Attack Time Coefficient

B4

B3

αATK

0

0

2.4

0

1

1.7

1

0

1.3

1

1

0.9

12

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