Application Information

GENERAL AMPLIFIER FUNCTION

The output signals generated by the LM4670 consist of two,

BTL connected, output signals that pulse momentarily from

near ground potential to V

independently with the exception that they both may never

pulse simultaneously as this would result in zero volts across

the BTL load. The minimum width of each pulse is approxi-

mately 350ns. However, pulses on the same output can

occur sequentially, in which case they are concatenated and

appear as a single wider pulse to achieve an effective 100%

duty cycle. This results in maximum audio output power for a

given supply voltage and load impedance. The LM4670 can

achieve much higher efficiencies than class AB amplifiers

while maintaining acceptable THD performance.

The short (350ns) drive pulses emitted at the LM4670 out-

puts means that good efficiency can be obtained with mini-

mal load inductance. The typical transducer load on an audio

amplifier is quite reactive (inductive). For this reason, the

load can act as it’s own filter, so to speak. This "filter-less"

switching amplifier/transducer load combination is much

more attractive economically due to savings in board space

and external component cost by eliminating the need for a

filter.

POWER DISSIPATION AND EFFICIENCY

In general terms, efficiency is considered to be the ratio of

useful work output divided by the total energy required to

produce it with the difference being the power dissipated,

typically, in the IC. The key here is “useful” work. For audio

systems, the energy delivered in the audible bands is con-

sidered useful including the distortion products of the input

signal.

Sub-sonic

(DC)

and

super-sonic

components

flowing from the power supply and the audio band power

being transduced is dissipated in the LM4670 and in the

transducer load. The amount of power dissipation in the

LM4670 is very low. This is because the ON resistance of the

switches used to form the output waveforms is typically less

than 0.25

Ω. This leaves only the transducer load as a po-

tential "sink" for the small excess of input power over audio

band output power. The LM4670 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 LM4670 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 LM4670 also

offers the possibility of DC input coupling which eliminates

the two external AC coupling, DC blocking capacitors. The

LM4670 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 LM4670 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 LM4670 and the load results is lower output

power and decreased efficiency. Higher trace resistance

between the supply and the LM4670 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 rising and falling edges are necessarily short in relation

to the minimum pulse width (350ns), having approximately

16ns rise and fall times, typical, depending on parasitic

output capacitance. 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

case. From an EMI standpoint, this is an aggressive wave-

form that can radiate or conduct to other components in the

system and cause interference. 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 tech-

niques are all useful in preventing unwanted interference.

As the distance from the LM4670 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 LM4670 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 LM4670. 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

LM4670. A 1µF tantalum capacitor is recommended.

SHUTDOWN FUNCTION

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

LM4670 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

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