LM48411

SNAS399G – SEPTEMBER 2007 – REVISED MAY 2013

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External Components Description

(Figure 2)

Components

Functional Description

1.

CS

Supply bypass capacitor which provides power supply filtering. Refer to the Power Supply Bypassing section for

information concerning proper placement and selection of the supply bypass capacitor.

2.

CI

Input AC coupling capacitor which blocks the DC voltage at the amplifier's input terminals.

APPLICATION INFORMATION

GENERAL AMPLIFIER FUNCTION

The LM48411 features a filterless modulation scheme. The differential outputs of the device switch at 300kHz

from VDD to GND. When there is no input signal applied, the two outputs (VO1 and VO2) switch with a 50% duty

cycle, with both outputs in phase. Because the outputs of the LM48411 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 LM48411 outputs changes. For increasing output

voltages, the duty cycle of VO1 increases, while the duty cycle of VO2 decreases. For decreasing output voltages,

the converse occurs, the duty cycle of VO2 increases while the duty cycle of VO1 decreases. The difference

between the two pulse widths yields the differential output voltage.

SPREAD SPECTRUM MODULATION

The LM48411 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 LM48411 spreads that energy over a larger bandwidth. The

cycle-to-cycle variation of the switching period does not affect the audio reproduction of efficiency.

ENHANCED EMISSIONS SUPPRESSION SYSTEM (E

2S)

The LM48411 features TI’s patent-pending E2S system that reduces EMI, while maintaining high quality audio

reproduction and efficiency. The E2S system features a synchronizable oscillator with selectable spread

spectrum, and advanced edge rate control (ERC). The LM48411 ERC greatly reduces the high frequency

components of the output square waves by controlling the output rise and fall times, slowing the transitions to

reduce RF emissions, while maximizing THD+N and efficiency performance.

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

LM48411 and in the transducer load. The amount of power dissipation in the LM48411 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 LM48411 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 LM48411 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 LM48411 also offers the possibility of

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