LM48822TL datasheet(10/22 Pages) NSC

10 / 22 page

TABLE 1. Device Address

B0 (R/W)

Device

Address

TABLE 2. I2C Control Registers

Address

Name

MODE

CONTROL

SDL

SD_BIAS

CP_ONLY

MUTE_LE

SDR

MUTE_

RIGHT

VOLUME

CONTROL

SHDN

VOL5

VOL4

VOL3

VOL2

VOL1

VOL0

TABLE 3. Mode Control Register

Bit

Name

Value

Description

SDL

Left channel enabled

Left channel disabled

SD_BIAS

Bias enabled

Bias disabled

CP_ONLY

Normal operation

Charge-pump only mode. Amplifiers and Bias disabled.

UNUSED

Set B3 to 0

MUTE_LEFT

Left channel Normal Operation

Left channel Mute

SDR

Right channel enabled

Right channel disabled. Right channel audio inputs summed

with left channel audio inputs and routed to OUTL

MUTE_RIGHT

Right channel Normal Operation

Right channel Mute

GENERAL AMPLIFIER FUNCTION

The LM48822 headphone amplifier feature National’s ground

referenced architecture that eliminates the large DC-blocking

capacitors required at the outputs of traditional headphone

amplifiers. A low-noise inverting charge pump creates a neg-

ative supply (CPV

SS) from the positive supply voltage (VDD).

The headphone amplifiers operate from these bipolar sup-

plies, with the amplifier outputs biased about GND, instead of

a nominal DC voltage (typically V

DD/2), like traditional ampli-

fiers. Because there is no DC component to the headphone

output signals, the large DC-blocking capacitors (typically

220

μF) are not necessary, conserving board space and sys-

tem cost, while improving frequency response.

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 5. The LM48822 charge

pump only mode eliminates this problem. In charge pump only

mode, the amplifiers are disabled, while the charge pump re-

mains 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 incoming signal

to swing between V

DD and VSS 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 opera-

tion.

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