lm49350rl National Semiconductor Corporation, lm49350rl Datasheet - Page 42

• Manufacturer Part Number: lm49350rl
• Description: High Performance Audio Codec Sub-system With A Ground-referenced Stereo Headphone Amplifier & An Ultra Low Emi Class D Loudspeaker Amplifier With Dual I2s/pcm Digital Audio Interfaces
• Manufacturer: National Semiconductor Corporation

www.national.com
Headphone Amplifier Function
The LM49350 headphone amplifier features National’s ground referenced architecture that eliminates the large DC-blocking ca-
pacitors required at the outputs of traditional headphone amplifiers. A low-noise inverting charge pump creates a negative supply
(HP_V
amplifier outputs biased about GND, instead of a nominal DC voltage (typically V
no DC component to the headphone output signals, the large DC-blocking capacitors (typically 220μF) are not necessary, con-
serving board space and system cost, while improving frequency response.
Charge Pump Capacitor Selection
Use low ESR ceramic capacitors (less than 100mΩ) for optimum performance.
Charge Pump Flying Capacitor (C6)
The flying capacitor (C6) affects the load regulation and output impedance of the charge pump. A C6 value that is too low results
in a loss of current drive, leading to a loss of amplifier headroom. A higher valued C6 improves load regulation and lowers charge
pump output impedance to an extent. Above 2.2μF, the R
the output impedance. A lower value capacitor can be used in systems with low maximum output power requirements. Please refer
to the demonstration board schematic shown in Figure 23.
Charge Pump Flying Capacitor (C5)
The value and ESR of the hold capacitor (C5) directly affects the ripple on CPV
Decreasing the ESR of C5 reduces both output ripple and charge pump output impedance. A lower value capacitor can be used
in systems with low maximum output power requirements. Please refer to the demonstration board schematic shown in Figure 23.
Auxiliary Output Amplifier
The LM49350’s auxiliary output (AUXOUT) amplifier provides differential drive capability to loads that are connected across its
outputs. This results in output signals at the AUX_OUT+ and AUX_OUT- pins that are 180 degrees out of phase with respect to
each other. This effectively doubles the maximum possible output swing for a specific supply voltage when compared to single-
ended output configurations. The differential output configuration also allows the load to be isolated from ground since both the
AUX_OUT+ and AUX_OUT- pins are biased at the same DC potential. This eliminates the need for any large and expensive DC
blocking capacitors at the AUXOUT amplifier outputs. The load can then be directly connected to the positive and negative outputs
of the AUXOUT amplifier which then isolates it from any ground noise, thereby improving signal to noise ratio (SNR) and power
supply rejection ratio (PSRR).
The AUXOUT amplifier has two modes of operation. The primary mode of operation is high current drive mode (Earpiece Mode)
where the AUXOUT amplifier can be used to differentially drive a mono earpiece speaker. The secondary mode of operation is
low current drive mode where the AUXOUT amplifier operates in a power saving mode (AUX_LINE_OUT Mode) to provide a
differential output that is used as a mono differential line level input to a standalone mono differential input class D amplifier
(LM4675) for stereo loudspeaker applications.
Bits
Bits
0
1
2
3
4
5
0
1
2
3
4
5
SS
) from the positive supply voltage (LS_V
DACR_HPR
AUXR_HPR
DACR_AUX
DACL_HPR
AUXR_AUX
MICR_HPR
AUXL_HPR
DACL_AUX
MICR_AUX
AUXL_AUX
MICL_HPR
MICL_AUX
Field
Field
TABLE 19. RIGHT HEADPHONE_OUTPUT (0x12h)
The right DAC output is added to the right headphone output.
The left DAC output is added to the right headphone output.
The right MIC input is added to the right headphone output. Setting this bit enables the MIC
BIAS output.
The left MIC input is added to the right headphone output. Setting this bit enables the MIC
BIAS output.
The right AUX input is added to the right headphone output.
The left AUX input is added to the right headphone output.
The right DAC output is added to the AUX output.
The left DAC output is added to the AUX output.
The right MIC input is added to the AUX output. Setting this bit enables the MIC BIAS output.
The left MIC input is added to the AUX output. Setting this bit enables the MIC BIAS output.
The right AUX input is added to the AUX output.
The left AUX input is added to the AUX output.
TABLE 20. AUX_OUTPUT (0x13h)
DD
). The headphone amplifiers operate from these bipolar supplies, with the
DS(ON)
of the charge pump switches and the ESR of C6 and C5 dominate
42
SS
Description
. Increasing the value of C5 reduces output ripple.
Description
DD
/2), like traditional amplifiers. Because there is