LM49250 Datasheet, PDF (22/36 Pages) National Semiconductor (TI) – Enhanced Emissions Suppression Stereo Class D Audio Sub-System with Ground Referenced Headphone Amplifier and Mono Earpiece

LM49250
SNAS406C – JANUARY 2008 – REVISED MAY 2013
www.ti.com
not only blocks the DC component of the output, but also attenuates low frequencies, impacting the bass
response. Because the LM49250 does not require the output coupling capacitors, the low frequency response of
the device is not degraded by external components. In addition to eliminating the output coupling capacitors, the
ground referenced output nearly doubles the available dynamic range of the LM49250 headphone amplifiers
when compared to a traditional headphone amplifier operating from the same supply voltage.
Charge Pump Capacitor Selection
For optimal performance, low (<100mΩ) ESR (equivalent series resistance) ceramic capacitors with X7R
dielectric are recommended. Low ESR capacitors keep the charge pump output impedance to a minimum,
extending the headroom on the negative supply. Higher ESR capacitors result in a reduction of output power
from the audio amplifiers. Charge pump load regulation and output impedance are affected by the value of the
flying capacitor (C_C). A larger valued C_C (up to 3.3μF) improves load regulation and minimizes charge pump
output resistance. The switch-on resistance dominates the output impedance for capacitor values above 2.2μF.
The output ripple is affected by the value and ESR of the output capacitor (C_SS). Larger capacitors reduce output
ripple on the negative power supply. Lower ESR capacitors minimize the output ripple and reduce the output
impedance of the charge pump.
The LM49250 charge pump design is optimized for 2.2μF, low ESR ceramic capacitors for both C_C and C_SS (See
Figure 1).
Input Mixer / Multiplexer
The LM49250 includes a comprehensive mixer/multiplexer controlled through the I²C interface. The
mixer/multiplexer allows any input combination to appear on any output of the LM49250. Control bits
LSR_SSEL and LSL_SSEL (loudspeakers), and HPR_SSEL and HPL_SSEL (headphones) select the individual
stereo input channels. For example, LSR_SSEL = 1 outputs the right channel stereo input on the right channel
loudspeaker, while LSL_SSEL = 1 outputs the left channel stereo input on the left channel loudspeaker. Control
bits LSR_MSEL and LSL_MSEL (loudspeaker), and HPR_MSEL and HPR_LSEL (headphones) direct the mono
input to the selected output. Control bits LS_XSEL (loudspeaker) and HP_XSEL (headphone) selects both stereo
input channels and directs the signals to the opposite outputs. For example, LS_XSEL = 1 outputs the right
channel stereo input on the left channel loudspeaker, while the left channel stereo input is output on the right
channel loudspeaker. Setting __XSEL selects both stereo inputs simultaneously, unlike the __SSEL controls
which select the stereo input channels individually.
Multiple input paths can be selected simultaneously. Under these conditions, the selected inputs are mixed
together and output on the selected channel. Table 5 and Table 6 show how the input signals are mixed together
for each possible input selection combination.
LS MODE
0
1
2
3
4
5
6
7
LS_XSEL
0
0
0
0
1
1
1
1
Table 5. Loudspeaker Multiplexer Control
LSR_SSEL/
LSL_SSEL
0
0
1
1
0
0
1
1
LSR_MSEL/
LSL_MSEL
0
1
0
1
0
1
0
1
LEFT CHANNEL OUTPUT
Mute
M
L'
M + L'
R'
M + R'
L' + R'
M + L' + R'
RIGHT CHANNEL OUTPUT
Mute
M
R'
M + R'
L'
M + L'
L' + R'
M + L' + R'
HPMODE
0
1
2
HP_XSEL
0
0
0
Table 6. Headphone Multiplexer Control
HPR_SSEL/
HPL_SSEL
0
0
1
HPR_MSEL/
HPL_MSEL
0
1
0
LEFT CHANNEL OUTPUT
Mute
M
L'
RIGHT CHANNEL OUTPUT
Mute
M
R'
22
Submit Documentation Feedback
Product Folder Links: LM49250
Copyright © 2008–2013, Texas Instruments Incorporated