LM4949 datasheet(32/40 Pages) NSC

32 / 40 page

Application Information (Continued)

GENERAL AMPLIFIER FUNCTION

Class D Amplifier

The LM4949 features a high-efficiency, filterless, Class D

stereo amplifier. The LM4949 Class D amplifiers feature a

filterless modulation scheme, the differential outputs of each

channel switch at 300khz, from V

DD to GND. When there is

no input signal applied, the two outputs (_LS+ and _LS-)

switch with a 50% duty cycle, with both outputs in phase.

Because the outputs of the LM4949 are differential, the two

signals cancel each other. This results in no net voltage

across the speaker, thus no load current during the idle

state, conserving power.

When an input signal is applied, the duty cycle (pulse width)

changes. For increasing output voltages, the duty cycle of

_LS+ increases, while the duty cycle of _LS- decreases. For

decreasing output voltages, the converse occurs, the duty

cycle of _LS- increases while the duty cycle of _LS+ de-

creases. The difference between the two pulse widths yields

the differential output voltage.

Headphone Amplifier

The LM4949 headphone amplifier features three different

operating modes, output capacitorless (OCL), capacitor-

coupled (CC), and external amplifier mode.

The OCL architecture eliminates the bulky, expensive output

coupling capacitors required by traditional headphone ampli-

fiers. The LM4949 headphone section uses three amplifiers.

Two amplifiers drive the headphones while the third (VOC) is

set to the internally generated bias voltage (typically V

DD/2).

The third amplifier is connected to the return terminal of the

headphone jack. In this configuration, the signal side of the

headphones are biased to V

DD/2, the return is biased to

DD/2, thus there is no net DC voltage across the head-

phone, eliminating the need for an output coupling capacitor.

Removing the output coupling capacitors from the head-

phone signal path reduces component count, reducing sys-

tem cost and board space consumption, as well as improv-

ing low frequency performance.

In OCL mode, the headphone return sleeve is biased to

DD/2. When driving headphones, the voltage on the return

sleeve is not an issue. However, if the headphone output is

used as a line out, the V

DD/2 can conflict with the GND

potential that a line-in would expect on the return sleeve.

When the return of the headphone jack is connected to

GND, the VOC amplifier of the LM4949 detects an output

short circuit condition and is disabled, preventing damage to

the LM4949, and allowing the headphone return to be biased

at GND.

Capacitor Coupled Headphone Mode

In capacitor coupled (CC) mode, the VOC pin is disabled,

and the headphone outputs are coupled to the jack through

series capacitors, allowing the headphone return to be con-

nected to GND (Figure 4). In CC mode, the LM4949 requires

output coupling capacitors to block the DC component of the

amplifier output, preventing DC current from flowing to the

load. The output capacitor and speaker impedance form a

high pass filter with a -3dB roll-off determined by:

-3dB =1/2

πR

LCOUT

Where R

L is the headphone impedance, and COUT is the

output coupling capacitor. Choose C

OUT such that f-3dB is

well below the lowest frequency of interest. Setting f

-3dB too

high results in poor low frequency performance. Select ca-

pacitor dielectric types with low ESR to minimize signal loss

due to capacitor series resistance and maximize power

transfer to the load.

External Headphone Amplifier

The LM4949 features the ability to drive and control a sepa-

rate headphone amplifier for applications that require a True

Ground headphone output (Figure 5). Configure the LM4949

into external headphone amplifier mode by setting bit D2

(OCL_LGC) in register 0.0 to 1 and bit D1 (OCL) to 0. In this

mode the VOC output becomes a logic output used to drive

the shutdown input of the external amplifier. The output level

of VOC is controlled by bits D1 (SDB_HPSEL) and D2

(SDB_MUXSEL) in register 0.1. SDB_MUXSEL determines

the source of the VOC control signal. With SDB_MUXSEL =

0, the VOC signal comes from the internal start-up circuitry

of the LM4949. This allows the external headphone amplifier

to be turned on and off simultaneously with the LM4949.

20200105

FIGURE 4. Capacitor Coupled Headphone Mode

LM4949 датащи(PDF) 32 Page - National Semiconductor (TI)

LM4949 датащи(HTML) 32 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40

датащи скачать

ссылки URL