LM49251TLSLASHNOPB datasheet(28/41 Pages) TI1

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0.2

0.4

0.6

1.0

INPUT VOLTAGE (VPP)

0.8

Voltage Limiter off

4VPP

4.8VPP

5.6VPP

ALC max attenuation

VIN > VDD

10m

100m

INPUT VOLTAGE (VPP)

0.01

0.1

1.0

No Clip

Enabled

No Clip

Disabled

LM49251

SNAS498A – FEBRUARY 2011 – REVISED APRIL 2013

www.ti.com

Figure 58. Voltage Limiter Function

Figure 59. No Clip Function

VDD = 3.3V, RL = 15μH+8Ω+15μH

fIN = 1kHz, LS_GAIN = 0

Blue, Green = Output Power vs Input Voltage

Gray, Yellow = THD+N vs Input Voltage

When No Clip is enabled, class D speaker output reduces when it’s about to enter clipping region and power stay

constant as long as VIN is less than VDD for 0 dB volume gain (see Figure 58). For example, in the case of VDD =

3.3V, there is a 6 dB of headroom for the change in input. Please see the ALC typical performance curves for

additional plots relating to different supply voltages and LS_GAIN settings for specific application parameters.

ATTACK TIME

Attack time (tATK) is the time it takes for the gain to be reduced by 6dB (LS_GAIN=0) 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 2:

tATK = 20kΩCSET / αATK(s)

(2)

Where

αATK is the attack time coefficient (Table 14) set by bits B4:B3 in the Voltage Limit Control Register (see

Table 6). The attack time coefficient allows the user to set a nominal attack time. The internal 20k

Ω resistor is

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

Table 14. Attack Time Coefficient

αATK

2.667

1.333

RELEASE TIME

Release time (tRL) is the time it takes for the gain to return from 6dB (LS_GAIN=0) to its normal level once the

audio signal returns below the ALC threshold. A fast release time allows the ALC to react quickly to transients,

preserving the original dynamics of the audio source. However, similar to a fast attack time, a fast release time

contributes to volume pumping. A slow release time reduces the effect of volume pumping. The release time is

set by a combination of the value of CSET and release time coefficient as given by Equation 3:

tRL = 20MΩCSET / αRL(s)

(3)

where

αRL is the release time coefficient (Table 15) set by bits B4:B3 in the No Clip Control Register. The release

time coefficient allows the user to set a nominal release time. The internal 20M

Ω is subject to temperature

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

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