LM4675TL datasheet(11/19 Pages) NSC

11 / 19 page

Application Information

GENERAL AMPLIFIER FUNCTION

The LM4675 features a filterless modulation scheme. The

differential outputs of the device switch at 300kHz from V

to GND. When there is no input signal applied, the two

outputs (V

O1 and VO2) switch with a 50% duty cycle, with

both outputs in phase. Because the outputs of the LM4675

are differential, the two signals cancel each other. This re-

sults in no net voltage across the speaker, thus there is no

load current during an idle state, conserving power.

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

the LM4675 outputs changes. For increasing output volt-

ages, the duty cycle of V

O1 increases, while the duty cycle of

O2 decreases. For decreasing output voltages, the con-

verse occurs, the duty cycle of V

O2 increases while the duty

cycle of V

O1 decreases. The difference between the two

pulse widths yields the differential output voltage.

SPREAD SPECTRUM MODULATION

The LM4675 features a fitlerless spread spectrum modula-

tion scheme that eliminates the need for output filters, ferrite

beads or chokes. The switching frequency varies by TBD%

about a 300kHz center frequency, reducing the wideband

spectral contend, improving EMI emissions radiated by the

speaker and associated cables and traces. Where a fixed

frequency class D exhibits large amounts of spectral energy

at multiples of the switching frequency, the spread spectrum

architecture of the LM4675 spreads that energy over a larger

bandwidth. The cycle-to-cycle variation of the switching pe-

riod does not affect the audio reproduction of efficiency.

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

(>22kHz) are not useful. The difference between the power

flowing from the power supply and the audio band power

being transduced is dissipated in the LM4675 and in the

transducer load. The amount of power dissipation in the

LM4675 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 LM4675 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 LM4675 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 LM4675 also

offers the possibility of DC input coupling which eliminates

the two external AC coupling, DC blocking capacitors. The

LM4675 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 LM4675 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 LM4675 and the load results is lower output

power and decreased efficiency. Higher trace resistance

between the supply and the LM4675 has the same effect as

a poorly regulated supply, increased 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

DD in each case. From an EMI stand-

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 LM4675 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 LM4675 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

S) location should be

as close as possible to the LM4675. 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

LM4675. A 4.7µF tantalum capacitor is recommended.

SHUTDOWN FUNCTION

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

LM4675 contains shutdown circuitry that reduces current

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

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