LM4682 datasheet(14/21 Pages) NSC

14 / 21 page

General Features (Continued)

Thermal Protection

The LM4682 has thermal shutdown circuitry that monitors

the die temperature. Once the LM4682 die temperature

reaches 170˚C, the LM4682 disables the output switching

waveform and remains disabled until the die temperature

falls below 140˚C (typ).

Over-Modulation Protection

The LM4682’s over-modulation protection is a result of the

preamplifier’s (AMP1 and AMP2, Figure 1) inability to pro-

duce signal magnitudes that equal the power supply volt-

ages. Since the preamplifier’s output magnitude will always

be less than the supply voltage, the duty cycle of the ampli-

fier’s switching output will never reach zero. Peak modula-

tion is limited to a nominal 95%.

DC Volume Control

The LM4682 has an internal stereo volume control whose

setting is a function of the DC voltage applied to the volume

control pin VOLCTL.

The LM4682 volume control consists of 31 steps, which are

individually selected by a variable DC voltage level on the

VOLCTL pin. The gain range of Class D amplifiers are from

–48dB to 30dB. The gain range of headphone amplifiers are

from –65dB to 13dB. Each gain step corresponds to specific

input voltage of both Class D amplifiers and headphone

amplifiers are shown in Table 2.

To minimize the effect of noise on the volume control

VOLCTL pin, which can affect the selected gain level, hys-

teresis has been implemented. The amount of hysteresis

corresponds to half of the step width. For highest accuracy,

the voltage shown in the “recommended voltage” column of

the table is used to select a desired gain. The recommended

voltage is exactly halfway between the two closest transi-

tions to the next highest or next lowest gain levels.

LM4682 Datasheet(PDF) 14 Page - National Semiconductor (TI)