TI Confidential - NDA Restrictions

LM49151

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SNAS482F – MARCH 2009 – REVISED MARCH 2013

Table 10. Attack Time Coefficient

B5

B4

αATK

0

0

2.667

0

1

2

1

0

1.333

1

1

1

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 11) 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%.

Table 11. Release Time Coefficient

B5

B4

αRL

0

0

2

0

1

2.5

1

0

3

1

1

5

PROPER SELECTION OF EXTERNAL COMPONENTS

ALC Timing (CSET) Capacitor Selection

The recommended range value of CSET is between .01μF to 1μF. Lowering the value below .01μF can increase

the attack time but LM49151 ALC ability to regulate its output can be disrupted and approaches the hard limiter

circuit. This in turn increases the THD+N and audio quality will be severely affected.

Charge Pump Capacitor Selection

Use low ESR ceramic capacitors (less than 100m

Ω) for optimum performance.

Charge Pump Flying Capacitor (C1)

The flying capacitor (C1), see Figure 1, affects the load regulation and output impedance of the charge pump. A

C1 value that is too low results in a loss of current drive, leading to a loss of amplifier headroom. A higher valued

C1 improves load regulation and lowers charge pump output impedance to an extent. Above 2.2µF, the

RDS(ON) of the charge pump switches and the ESR of C1 and CPVSS dominate the output impedance. A lower

value capacitor can be used in systems with low maximum output power requirements.

Charge Pump Hold Capacitor (CPVSS)

The value and ESR of the hold capacitor (CPVSS) directly affects the ripple on CPVSS. (see Figure 1) Increasing

the value of CPVSS reduces output ripple. Decreasing the ESR of CPVSS reduces both output ripple and charge

pump output impedance. A lower value capacitor can be used in systems with low maximum output power

requirements.

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