Typical Performance Characteristics (Continued)
Supply Current vs Supply Voltage
ZL = 2µF+30Ω
20142127
By driving the load differentially through outputs OUT A and
Application Information
OUT B, an amplifier configuration commonly referred to as
“bridged mode” is established. Bridged mode operation is
different from the classic single-ended amplifier configura-
tion where one side of the load is connected to ground.
ELIMINATING THE OUTPUT COUPLING CAPACITOR
The LM4953 features a low noise inverting charge pump that
generates an internal negative supply voltage. This allows
the outputs of the LM4953 to be biased about GND instead
of a nominal DC voltage, like traditional headphone amplifi-
ers. 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.
A bridge amplifier design has a few distinct advantages over
the single-ended configuration. It provides differential drive
to the load, thus doubling the output swing for a specified
supply voltage. Four times the output power is possible as
compared to a single-ended amplifier under the same con-
ditions. This increase in attainable output power assumes
that the amplifier is not current limited or clipped. In order to
choose an amplifier’s closed-loop gain without causing ex-
cessive clipping, please refer to the Audio Power Amplifier
Design section.
Eliminating the output coupling capacitors also improves low
frequency response. In traditional headphone amplifiers, the
headphone impedance and the output capacitor form 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 LM4953 does not require the
output coupling capacitors, the low frequency response of
the device is not degraded by external components.
The bridge configuration also creates a second advantage
over single-ended amplifiers. Since the differential outputs,
OUT A and OUT B, are biased at half-supply, no net DC
voltage exists across the load. This eliminates the need for
an output coupling capacitor which is required in a single
supply, single-ended amplifier configuration. Without an out-
put coupling capacitor, the half-supply bias across the load
would result in both increased internal IC power dissipation
and also possible loudspeaker damage.
In addition to eliminating the output coupling capacitors, the
ground referenced output nearly doubles the available dy-
namic range of the LM4953 when compared to a traditional
headphone amplifier operating from the same supply volt-
age.
OUTPUT TRANSIENT (’CLICK AND POPS’)
ELIMINATED
BRIDGE CONFIGURATION EXPLANATION
The Audio Amplifier portion of the LM4953has two internal
amplifiers allowing different amplifier configurations. The first
amplifier’s gain is externally configurable, whereas the sec-
ond amplifier is internally fixed in a unity-gain, inverting
configuration. The closed-loop gain of the first amplifier is set
by selecting the ratio of Rf to Ri while the second amplifier’s
gain is fixed by the two internal 20kΩ resistors. Figure 1
shows that the output of amplifier one serves as the input to
amplifier two. This results in both amplifiers producing sig-
nals identical in magnitude, but out of phase by 180˚. Con-
sequently, the differential gain for the Audio Amplifier is
The LM4953 contains advanced circuitry that virtually elimi-
nates output transients (’clicks and pops’). This circuitry
prevents all traces of transients when the supply voltage is
first applied or when the part resumes operation after coming
out of shutdown mode.
POWER DISSIPATION
Power dissipation is a major concern when using any power
amplifier and must be thoroughly understood to ensure a
successful design. Equation 1 states the maximum power
dissipation point for a single-ended amplifier operating at a
given supply voltage and driving a specified output load.
AVD = 2 *(Rf/Ri)
2
PDMAX = (VDD
)
/ (2π2ZL)
(1)
7
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