LM49150 datasheet(21/28 Pages) NSC

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SINGLE-ENDED INPUT CONFIGURATION

The left and right stereo inputs of the LM49150 are configured

for single-ended sources (see Figure 1).

INPUT CAPACITOR SELECTION

Input capacitors may be required for some applications, or

when the audio source is single-ended. Input capacitors block

the DC component of the audio signal, eliminating any conflict

between the DC component of the audio source and the bias

voltage of the LM49150. The input capacitors create a high-

pass filter with the input resistors R

IN. The -3dB point of the

high-pass filter is found using Equation (1) below.

f = 1 / 2

πR

INCIN

(Hz)

(1)

Where the value of R

IN is given in the Electrical Characteris-

tics Table.

High-pass filtering the audio signal helps protect the speak-

ers. When the LM49150 is using a single-ended source,

power supply noise on the ground is seen as an input signal.

Setting the high-pass filter point above the power supply noise

frequencies, 217Hz in a GSM phone, for example, filters out

the noise such that it is not amplified and heard on the output.

Capacitors with a tolerance of 10% or better are recommend-

ed for impedance matching and improved CMRR and PSRR.

INPUT MIXER/MULTIPLEXER

The LM49150 includes a comprehensive mixer multiplexer

controlled through the I2C interface. The mixer/multiplexer al-

lows any input combination to appear on any output of

LM49150. Multiple input paths can be selected simultane-

ously. Under these conditions, the selected inputs are mixed

together and output on the selected channel. Table 5 shows

how the input signals are mixed together for each possible

input selection.

CLASS D AMPLIFIER

The LM49150 features a high-efficiency, filterless, class D

amplifier, which features a filterless modulation scheme.

When there is no input signal applied, the output switches

between V

DD and GND at a 50% duty cycle. Since the outputs

of the LM49150 class D amplifier are differential and in phase,

the result is zero net voltage across the speaker and no load

current during the ideal state, thus conserving power. The

switching frequency of each output is 300kHz.

When an input signal is applied, the duty cycle(pulse width)

changes. For increasing output voltages, the duty cycle of one

output increases while the duty cycle of the output decreases.

For decreasing output voltages, the converse occurs. The

difference between the two pulse widths yields the differential

output voltage across the load.

SPREAD SPECTRUM

The LM49150 features a filterless spread spectrum modula-

tion scheme. The switching frequency varies by +/-30% about

a 300kHz center frequency, reducing the wideband spectral

content, reducing EMI emissions radiated by the speaker and

associated cables and traces. When a fixed frequency class

D exhibits large amounts of spectral energy at multiples of

switching frequency, the spread spectrum architecture of the

LM49150 spreads that energy over a larger bandwidth. The

cycle-to-cycle variation of the switching period does not affect

the audio reproduction, efficiency, or PSRR. To enable

spread spectrum, set the spread spectrum bit, B5 = 1 of the

SHUTDOWN CONTROL register (see Table 3).

ENHANCED EMMISIONS SUPPRESSION (E2S)

The LM49150 features National’s patented E2S system that

reduces EMI, while maintaining high quality audio reproduc-

tion and efficiency. The LM49150 features Edge Rate Control

(ERC) that 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

optimizing THD+N and efficiency performance.

LDO GENERAL INFORMATION

The LM49150 has different supplies for each portion of the

device, allowing for the optimum combination of headroom,

power dissipation and noise immunity. The speaker amplifiers

are powered from LSV

DD. The ground reference headphone

amplifiers are powered from the internal LDO. The separate

power supplies allow the loudspeaker amplifier to operate

from a higher voltage for maximum headroom, while the

headphone amplifiers operate from a lower voltage, improv-

ing power dissipation.

GROUND REFERENCED HEADPHONE AMPLIFIER

The LM49150 features a low noise inverting charge pump that

generates an internal negative supply voltage. This allows the

headphone outputs to be biased about GND instead of a

nominal DC voltage, like traditional headphone amplifiers.

Because there is no DC component, the large DC blocking

capacitors (typically 220

μF) are not necessary. The coupling

capacitors are replaced by two small ceramic charge pump

capacitors, saving board space and cost. Eliminating the out-

put coupling capacitors also improves low frequency re-

sponse. In traditional headphone amplifiers, the headphone

impedance and the output capacitor from a high-pass filter

that not only blocks the DC component of the output, but also

attenuates low frequencies, impacting the bass response.

Because the LM49150 does not require the output coupling

capacitors, the low frequency response of the device is not

degraded by external components. In addition to eliminating

the output coupling capacitors, the ground referenced output

nearly doubles the available dynamic range of the LM49250

headphone amplifiers when compared to a traditional head-

phone amplifier operating from the same supply voltage.

CHARGE PUMP CAPACITOR SELECTION

Use low ESR ceramic capacitors (less than 100m

Ω) for opti-

mum performance.

CHARGE PUMP FLYING CAPACITOR (C1)

The flying capacitor (C1), see Figure 1, affects the load reg-

ulation and output impedance of the charge pump. A C1 value

that is too low results in a loss of current drive, leading to a

loss of amplifier headroom. A higher valued C1 improves load

regulation and lowers charge pump output impedance to an

extent. Above 2.2

μF, the R

DS(ON) of the charge pump switches

and the ESR of C1 and Cs5 dominate the output impedance.

A lower value capacitor can be used in systems with low max-

imum output power requirements.

CHARGE PUMP HOLD CAPACITOR (Cs5)

The value and ESR of the hold capacitor (Cs5) directly affects

the ripple on CPV

SS. Increasing the value of Cs5 reduces

output ripple. Decreasing the ESR of Cs5 reduces both output

ripple and charge pump output impedance. A lower value ca-

pacitor can be used in systems with low maximum output

power requirements.

LM49150 Datasheet(PDF) 21 Page - National Semiconductor (TI)

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