CHARGE PUMP

C1P

C1N

C1

C2

HI-Z

INCOMING AUDIO

SIGNAL

OUT_

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 (1M

Ω) load to the incoming signal. The charge pump maintains the negative rail, allowing the

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.

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.

Copyright © 2008–2013, Texas Instruments Incorporated

Submit Documentation Feedback

11

Product Folder Links: LM48822 LM48822TLEVAL