LM4842 datasheet(22/31 Pages) NSC

22 / 31 page

Application Information (Continued)

to ground to enable shutdown. In a system with a micropro-

cessor or a microcontroller, use a digital output to apply the

control voltage to the SHUTDOWN pin.

BASS BOOST

The ’Bass Boost Mode’ is enabled whenever the LM4842 is

NOT in headphone mode (please refer to the ’Cap-Less

Headphone’ description in the previous section for head-

phone on/off conditions). In BTL output mode, the Bass

Boost feature is enabled and the low frequency gain of the A

and B Channel amplifiers can be set by installing the exter-

nal bass boost capacitors, C

BA across pins 25 and 26 and

BB across pins 17 and 18, as shown in Figure 2 (MT/MH

package pin numbers). If the Bass Boost feature is not

desired, then the capacitors can be left out of the circuit.

When the capacitors are installed and the bass boost is

enabled, the output amplifiers will be internally set at a gain

of 4 in bridged mode, as shown in Figure 2. At low frequen-

cies, C

BA and CBB will be virtual open circuits and have no

effect on the overall gain of the circuit. However at higher

frequencies, the capacitors will be virtual short circuits and

this will cause the gain of the bridge amplifiers to be reduced

from a gain of 4 to a gain of 2. An example of actual values

for this circuit are as follows: a first order pole is formed with

a corner frequency at: fc = 1/(2

π10kΩ C

BA) (9) With CBA =

0.1µF, which results in a corner frequency of 160Hz. This

bass boost feature can be useful in systems where speakers

are housed in small enclosures and require a little more

acoustic output at lower frequencies to make the sound

more natural and balanced.

ESD Protection

As stated in the Absolute Maximum Ratings, pin 28 on the

MT and MH packages have a maximum ESD susceptibility

rating of 6500V. For higher ESD voltages, the addition of an

PCDN042 dual transil (from California Micro Devices), as

shown in Figure 6, will provide additional protection.

DC VOLUME CONTROL

The LM4842 has an internal stereo volume control whose

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

CONTROL pin.

The LM4842 volume control consists of 31 steps that are

individually selected by a variable DC voltage level on the

volume control pin. The range of the steps, controlled by the

DC voltage, are from 0dB - 78dB. Each gain step corre-

sponds to a specific input voltage range, as shown in table 2.

To minimize the effect of noise on the volume control pin,

which can affect the selected gain level, hysteresis has been

implemented. The amount of hysteresis corresponds to half

of the step width, as shown in Volume Control Characteriza-

tion Graph (DS200133-40).

For highest accuracy, the voltage shown in the ’recom-

mended voltage’ column of the table is used to select a

desired gain. This recommended voltage is exactly halfway

between the two nearest transitions to the next highest or

next lowest gain levels.

The gain levels are 1dB/step from 0dB to -6dB, 2dB/step

from -6dB to -36dB, 3dB/step from -36dB to -47dB, 4dB/step

from -47db to -51dB, 5dB/step from -51dB to -66dB, and

12dB to the last step at -78dB.

200281A4

FIGURE 6. The PCDN042 provides additional ESD protection beyond the 6500V shown in the

Absolute Maximum Ratings for the AMP2A output

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