LM4842 National Semiconductor, LM4842 Datasheet - Page 22

• Manufacturer Part Number: LM4842
• Description: Stereo 2W Amplifiers with DC Volume Control Transient Free Outputs/ and Cap-less Headphone Drive
• Manufacturer: National Semiconductor

www.national.com
Application Information
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
C
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-
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).
BB
across pins 17 and 18, as shown in Figure 2 (MT/MH
FIGURE 6. The PCDN042 provides additional ESD protection beyond the 6500V shown in the
BA
across pins 25 and 26 and
Absolute Maximum Ratings for the AMP2A output
(Continued)
22
cies, C
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/(2π10kΩ C
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.
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.
BA
and C
200281A4
BB
will be virtual open circuits and have no
BA
) (9) With C
BA
=