LM4674A

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SNAS366A – SEPTEMBER 2006 – REVISED MAY 2013

APPLICATION INFORMATION

GENERAL AMPLIFIER FUNCTION

The LM4674A stereo Class D audio power amplifier features a filterless modulation scheme that reduces

external component count, conserving board space and reducing system cost. The outputs of the device

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 LM4674A 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 converse 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 LM4674A features two fully

differential amplifiers. A differential amplifier amplifies the difference between the two input signals. Traditional

audio power amplifiers have typically offered only single-ended inputs resulting in a 6dB reduction of SNR

relative to differential inputs. The LM4674A also offers the possibility of DC input coupling which eliminates 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 efficiency versus a Class AB. The efficiency of the

LM4674A 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 LM4674A 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.

The LM4674A shutdown inputs have internal pulldown resistors. 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 LM4674A 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 25 shows the typical

single-ended applications circuit.

AUDIO AMPLIFIER POWER SUPPLY BYPASSING/FILTERING

Proper power supply bypassing is critical for low noise performance and high PSRR. Place the supply bypass

capacitor 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

LM4674A supply pins. A 1µF capacitor is recommended.

Copyright © 2006–2013, Texas Instruments Incorporated

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