LM4674 datasheet(13/18 Pages) NSC

13 / 18 page

Application Information

GENERAL AMPLIFIER FUNCTION

The LM4674 stereo Class D audio power amplifier features a

filterless modulation scheme that reduces external compo-

nent count, conserving board space and reducing system

cost. The outputs of the device transition from V

DD to GND

with a 300kHz switching frequency. With no signal applied,

the outputs (OUT_A and OUT_B) switch with a 50% duty

cycle, in phase, causing the two outputs to cancel. This

cancellation results in no net voltage across the speaker,

thus there is no current to the load in the idle state.

With the input signal applied, the duty cycle (pulse width) of

the LM4674 outputs changes. For increasing output voltage,

the duty cycle of OUT_A increases, while the duty cycle of

OUT_B decreases. For decreasing output voltages, the con-

verse occurs. The difference between the two pulse widths

yields the differential output voltage.

DIFFERENTIAL AMPLIFIER EXPLANATION

As logic supplies continue to shrink, system designers are

increasingly turning to differential analog signal handling to

preserve signal to noise ratios with restricted voltage signs.

The LM4674 features two fully differential amplifiers. A dif-

ferential amplifier amplifies the difference between the two

input signals. Traditional audio power amplifiers have typi-

cally offered only single-ended inputs resulting in a 6dB

reduction of SNR relative to differential inputs. The LM4674

also offers the possibility of DC input coupling which elimi-

nates the input coupling capacitors. A major benefit of the

fully differential amplifier is the improved common mode

rejection ratio (CMRR) over single ended input amplifiers.

The increased CMRR of the differential amplifier reduces

sensitivity to ground offset related noise injection, especially

important in noisy systems.

POWER DISSIPATION AND EFFICIENCY

The major benefit of a Class D amplifier is increased effi-

ciency versus a Class AB. The efficiency of the LM4674 is

attributed to the region of operation of the transistors in the

output stage. The Class D output stage acts as current

steering switches, consuming negligible amounts of power

compared to their Class AB counterparts. Most of the power

loss associated with the output stage is due to the IR loss of

the MOSFET on-resistance, along with switching losses due

to gate charge.

SHUTDOWN FUNCTION

The LM4674 features independent left and right channel

shutdown controls, allowing each channel to be disabled

independently. /SDR controls the right channel, while /SDL

controls the left channel. Driving either low disables the

corresponding channel, reducing supply current to 0.03µA.

It is best to switch between ground and V

DD for minimum

current consumption while in shutdown. The LM4674 may be

disabled with shutdown voltages in between GND and V

DD,

the idle current will be greater than the typical 0.03µA value.

Increased THD+N may also be observed when a voltage of

less than V

DD is applied to /SD_ for logic levels between

GND and V

DD Bypass /SD_ with a 0.1µF capacitor.

The LM4674 shutdown inputs have internal pulldown resis-

tors. The purpose of these resistors is to eliminate any

unwanted state changes when /SD_ is floating. To minimize

shutdown current, /SD_ should be driven to GND or left

floating. If /SD_ is not driven to GND or floating, an increase

in shutdown supply current will be noticed.

SINGLE-ENDED AUDIO AMPLIFIER CONFIGURATION

The LM4674 is compatible with single-ended sources. When

configured for single-ended inputs, input capacitors must be

used to block and DC component at the input of the device.

Figure 3 shows the typical single-ended applications circuit.

AUDIO AMPLIFIER POWER SUPPLY

BYPASSING/FILTERING

Proper power supply bypassing is critical for low noise per-

formance and high PSRR. Place the supply bypass capaci-

tor as close to the device as possible. Typical applications

employ a voltage regulator with 10µF and 0.1µF bypass

capacitors that increase supply stability. These capacitors do

not eliminate the need for bypassing of the LM4674 supply

pins. A 1µF capacitor is recommended.

AUDIO AMPLIFIER INPUT CAPACITOR SELECTION

Input capacitors may be required for some applications, or

when the audio source is single-ended. Input capacitors

block the DC component of the audio signal, eliminating any

conflict between the DC component of the audio source and

the bias voltage of the LM4674. The input capacitors create

a high-pass filter with the input resistors RI. The -3dB point of

the high pass filter is found using Equation 1 below.

f=1/2

πR

INCIN

(1)

The values for RI can be found in the EC table for each gain

setting.

The input capacitors can also be used to remove low fre-

quency content from the audio signal. Small speakers can-

not reproduce, and may even be damaged by low frequen-

cies. High pass filtering the audio signal helps protect the

speakers. When the LM4674 is using a single-ended source,

power supply noise on the ground is seen as an input signal.

Setting the high-pass filter point above the power supply

noise frequencies, 217Hz in a GSM phone, for example,

filters out the noise such that it is not amplified and heard on

the output. Capacitors with a tolerance of 10% or better are

recommended for impedance matching and improved

CMRR and PSRR.

AUDIO AMPLIFIER GAIN SETTING

The LM4674 features four internally configured gain settings.

The device gain is selected through the two logic inputs, G0

and G1. The gain settings are as shown in the following

table.

GAIN

V/V

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