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 RIN. The -3dB point of the
high-pass filter is found using Equation (1) below.
f = 1 / 2πRINCIN (Hz) (1)
Where the value of RIN 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 VDD 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 LSVDD. 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 220μF) 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 100mΩ) 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 2.2μF, the RDS(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 CPVSS. 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.
21 www.national.com
LM49150