LM4675, LM4675SDBD, LM4675TLBD

SNAS353C – AUGUST 2006 – REVISED MAY 2013

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APPLICATION INFORMATION

GENERAL AMPLIFIER FUNCTION

The LM4675 features a filterless modulation scheme. The differential outputs of the device switch at 300kHz from

with both outputs in phase. Because the outputs of the LM4675 are differential, the two signals cancel each

other. This results 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

between the two pulse widths yields the differential output voltage.

SPREAD SPECTRUM MODULATION

The LM4675 features a fitlerless spread spectrum modulation scheme that eliminates the need for output filters,

ferrite beads or chokes. The switching frequency varies by ±30% 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 period 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 considered 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 potential "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 copper 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. Traditional 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

between 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 characteristic 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 corresponding 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.

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Product Folder Links: LM4675 LM4675SDBD LM4675TLBD