LM48822 Datasheet, PDF (11/22 Pages) National Semiconductor (TI) – Ground-Referenced, Ultra High PSRR, Ultra Low Noise, 35mW/Channel Stereo Headphone Amplifier with CommonMode Sense, and I2C Volume Control

LM48822, LM48822TLEVAL
www.ti.com
SNAS456C – MAY 2008 – REVISED MAY 2013
GENERAL AMPLIFIER EXPLANATION
The LM48822 features a differential input stage, which offers improved noise rejection compared to a single-
ended input amplifier. Because a differential input amplifier amplifies the difference between the two input
signals, any component common to both signals is cancelled. An additional benefit of the differential input
structure is the possible elimination of the DC input blocking capacitors. Since the DC component is common to
both inputs, and thus cancelled by the amplifier, the LM48822 can be used without input coupling capacitors
when configured with a differential input signal.
CHARGE PUMP ONLY MODE
In applications where the headphone jack is used as both an output and input port, signals such as a microphone
input can appear on the headphone amplifier output. Traditional charge pump headphone amplifiers can clamp
or distort the signals that appear on their output. Without the charge pump active, generating the negative
voltage supply, the internal protection diodes of the amplifier clamp the incoming signal, distorting the negative
half cycle, see Figure 24. The LM48822 charge pump only mode eliminates this problem. In charge pump only
mode, the amplifiers are disabled, while the charge pump remains active. The disabled amplifier outputs present
a high impedance (1MΩ) load to the incoming signal. The charge pump maintains the negative rail, allowing the
incoming signal to swing between V_DD and V_SS without any interference from the device.
Set bit B4 (CP_ONLY) of the MODE CONTROL register to 1 for charge pump only mode. Setting CP_ONLY = 1
disables both the left and right channels, regardless of the status of the shutdown control bits. Set CP_ONLY = 0
for normal operation.
V_DD
OUT_
HI-Z
V_SS
CHARGE PUMP
C1P
C1N
CPV_SS
C2
C1
INCOMING AUDIO
SIGNAL
Figure 24. Back-Driving the LM48822 Outputs
COMMON MODE SENSE
The LM48822 features a ground (common mode) sensing feature. In noisy applications, or where the headphone
jack is used as a line out to other devices, noise pick up and ground imbalance can degrade audio quality. The
LM48822 COM input senses and corrects any noise at the headphone return, or any ground imbalance between
the headphone return and device ground, improving audio reproduction. Connect COM directly to the headphone
return terminal of the headphone jack Figure 25. No additional external components are required. Connect COM
to GND if the common-mode sense feature is not in use.
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