LM4836_15 Datasheet, PDF (26/39 Pages) Texas Instruments – Stereo 2W Audio Power Amplifiers with DC Volume Control, Bass Boost, and Input Mux

LM4836
SNAS045F – JUNE 1999 – REVISED MAY 2013
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BASS BOOST FUNCTION
The LM4836 has a bass-boost feature that enhances the low frequency response in applications using small
speakers. The voltage level applied to the BASS BOOST SELECT pin controls the bass-boost function. Applying
GND activates the bass-boost mode. In bass-boost mode, the LM4836's gain is increased at low frequencies,
with a corner frequency set by the external capacitor, C_BASS. Applying V_DD defeats the bass-boost mode and
selects unity gain. Tying the BASS BOOST SELECT pin to V_DD permanently defeats the bass-boost function.
Enabling bass-boost forces the output amplifiers to operate with an internally set low frequency gain of 2 (gain of
4 in bridged mode). The capacitor C_BASS shown in Figure 3 sets the bass-boost corner frequency. At low
frequencies, the capacitor is a virtual open circuit and the feedback resistance consists of two 10kΩ resistors. At
high frequencies, the capacitor is a virtual short circuit, which shorts one of the two 10kΩ feedback resistors. The
results is bridge amplifier gain that increases at low frequencies. A first-order pole is formed with a corner
frequency at
f_C = 1/(2π10kΩC_BASS)
(9)
At f<<f_C, the differential gain of this bridged amplifier is
2(10kΩ + 10kΩ) /10kΩ = 4
(10)
With C_BASS = 0.1µF, the first-order pole has a corner frequency of 160Hz. It is assumed when using Equation 9
that C_O, C_i, f_IC, and f_OC, are chosen for the desired low frequency response as explained in the PROPER
SELECTION OF EXTERNAL COMPONENTS section. See the TYPICAL PERFORMANCE CHARACTERISTICS
MTE SPECIFIC CHARACTERISTICS section for a graph that includes bass-boost performance with various
values of C_BASS.
DC VOLUME CONTROL
The LM4836 has an internal stereo volume control whose setting is a function of the DC voltage applied to the
DC VOL CONTROL pin. The volume control's voltage input range is 0V to V_DD. The volume range is from 0dB
(DC control voltage = 80%V_DD) to -80dB (DC control voltage = 0V). The volume remains at 0dB for DC control
voltages greater than 80%V_DD. When the MODE input is 0V, the LM4836 operates at unity gain, bypassing the
volume control. A graph showing a typical volume response versus DC control voltage is shown in the TYPICAL
PERFORMANCE CHARACTERISTICS MTE SPECIFIC CHARACTERISTICS section.
Like all volume controls, the LM4836's internal volume control is set while listening to an amplified signal that is
applied to an external speaker. The actual voltage applied to the DC VOL CONTROL pin is a result of the
volume a listener desires. As such, the volume control is designed for use in a feedback system that includes
human ears and preferences. This feedback system operates quite well without the need for accurate gain. The
user simply sets the volume to the desired level as determined by their ear, without regard to the actual DC
voltage that produces the volume. Therefore, the accuracy of the volume control is not critical, as long as the
volume changes monotonically, matches well between stereo channels, and the step size is small enough to
reach a desired volume that is not too loud or too soft. Since gain accuracy is not critical, there will be volume
variation from part-to-part even with the same applied DC control voltage. The gain of a given LM4836 can be
set with a fixed external voltage, but another LM4836 may require a different control voltage to achieve the same
gain. The typical part-to-part variation can be as large as 8dB for the same control voltage.
AUDIO POWER AMPLIFIER DESIGN
Audio Amplifier Design: Driving 1W into an 8Ω Load
The following are the desired operational parameters:
Power Output:
Load Impedance:
Input Level:
Input Impedance:
Bandwidth:
1 W_RMS
8Ω
1 V_RMS
20 kΩ
100 Hz−20 kHz ± 0.25 dB
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