band output power. The LM4671 dissipates only a fraction of

the excess power requiring no additional PCB area or cop-

per plane to act as a heat sink.

DIFFERENTIAL AMPLIFIER EXPLANATION

As logic supply voltages continue to shrink, designers are

increasingly turning to differential analog signal handling to

preserve signal to noise ratios with restricted voltage swing.

The LM4671 is a fully differential amplifier that features

differential input and output stages. A differential amplifier

amplifies the difference between the two input signals. Tra-

ditional audio power amplifiers have typically offered only

single-ended inputs resulting in a 6dB reduction in signal to

noise ratio relative to differential inputs. The LM4671 also

offers the possibility of DC input coupling which eliminates

the two external AC coupling, DC blocking capacitors. The

LM4671 can be used, however, as a single ended input

amplifier while still retaining it’s fully differential benefits. In

fact, completely unrelated signals may be placed on the

input pins. The LM4671 simply amplifies the difference be-

tween the signals. A major benefit of a differential amplifier is

the improved common mode rejection ratio (CMRR) over

single input amplifiers. The common-mode rejection charac-

teristic of the differential amplifier reduces sensitivity to

ground offset related noise injection, especially important in

high noise applications.

PCB LAYOUT CONSIDERATIONS

As output power increases, interconnect resistance (PCB

traces and wires) between the amplifier, load and power

supply create a voltage drop. The voltage loss on the traces

between the LM4671 and the load results is lower output

power and decreased efficiency. Higher trace resistance

between the supply and the LM4671 has the same effect as

a poorly regulated supply, increase ripple on the supply line

also reducing the peak output power. The effects of residual

trace resistance increases as output current increases due

to higher output power, decreased load impedance or both.

To maintain the highest output voltage swing and corre-

sponding peak output power, the PCB traces that connect

the output pins to the load and the supply pins to the power

supply should be as wide as possible to minimize trace

resistance.

The use of power and ground planes will give the best

THD+N performance. While reducing trace resistance, the

use of power planes also creates parasite capacitors that

help to filter the power supply line.

The inductive nature of the transducer load can also result in

overshoot on one or both edges, clamped by the parasitic

diodes to GND and V

point, this is an aggressive waveform that can radiate or

conduct to other components in the system and cause inter-

ference. It is essential to keep the power and output traces

short and well shielded if possible. Use of ground planes,

beads, and micro-strip layout techniques are all useful in

preventing unwanted interference.

As the distance from the LM4671 and the speaker increase,

the amount of EMI radiation will increase since the output

wires or traces acting as antenna become more efficient with

length. What is acceptable EMI is highly application specific.

Ferrite chip inductors placed close to the LM4671 may be

needed to reduce EMI radiation. The value of the ferrite chip

is very application specific.

POWER SUPPLY BYPASSING

As with any power amplifier, proper supply bypassing is

critical for low noise performance and high power supply

rejection ratio (PSRR). The capacitor (C

as close as possible to the LM4671. Typical applications

employ a voltage regulator with a 10µF and a 0.1µF bypass

capacitors that increase supply stability. These capacitors do

not eliminate the need for bypassing on the supply pin of the

LM4671. A 1µF tantalum capacitor is recommended.

SHUTDOWN FUNCTION

In order to reduce power consumption while not in use, the

LM4671 contains shutdown circuitry that reduces current

draw to less than 0.01µA. The trigger point for shutdown is

shown as a typical value in the Electrical Characteristics

Tables and in the Shutdown Hysteresis Voltage graphs

found in the Typical Performance Characteristics section.

It is best to switch between ground and supply for minimum

current usage while in the shutdown state. While the

LM4671 may be disabled with shutdown voltages in between

ground and supply, the idle current will be greater than the

typical 0.01µA value. Increased THD may also be observed

with voltages less than V

PLAY mode.

The LM4671 has an internal resistor connected between

GND and Shutdown pins. The purpose of this resistor is to

eliminate any unwanted state changes when the Shutdown

pin is floating. The LM4671 will enter the shutdown state

when the Shutdown pin is left floating or if not floating, when

the shutdown voltage has crossed the threshold. To mini-

mize the supply current while in the shutdown state, the

Shutdown pin should be driven to GND or left floating. If the

Shutdown pin is not driven to GND, the amount of additional

resistor current due to the internal shutdown resistor can be

found by Equation (1) below.

(V

Ω

(1)

With only a 0.5V difference, an additional 8.3µA of current

will be drawn while in the shutdown state.

PROPER SELECTION OF EXTERNAL COMPONENTS

The gain of the LM4671 is set by the external resistors, Ri in

Figure 1, The Gain is given by Equation (2) below. Best

THD+N performance is achieved with a gain of 2V/V (6dB).

A

Ω /R

(2)

It is recommended that resistors with 1% tolerance or better

be used to set the gain of the LM4671. The Ri resistors

should be placed close to the input pins of the LM4671.

Keeping the input traces close to each other and of the same

length in a high noise environment will aid in noise rejection

due to the good CMRR of the LM4671. Noise coupled onto

input traces which are physically close to each other will be

common mode and easily rejected by the LM4671.

Input capacitors may be needed for some applications or

when the source is single-ended (see Figures 3, 5). Input

capacitors are needed to block any DC voltage at the source

so that the DC voltage seen between the input terminals of

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