Texas Instruments LM4953 Evaluation Board LM4953SDBD/NOPB LM4953SDBD/NOPB 데이터 시트
제품 코드
LM4953SDBD/NOPB
SNAS299E – SEPTEMBER 2005 – REVISED FEBRUARY 2006
APPLICATION INFORMATION
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 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.
allows the outputs of the LM4953 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 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.
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.
In addition to eliminating the output coupling capacitors, the ground referenced output nearly doubles the
available dynamic range of the LM4953 when compared to a traditional headphone amplifier operating from the
same supply voltage.
available dynamic range of the LM4953 when compared to a traditional headphone amplifier operating from the
same supply voltage.
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 second 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 20k
The first amplifier’s gain is externally configurable, whereas the second 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 20k
Ω
resistors.
shows that the output of amplifier one
serves as the input to amplifier two. This results in both amplifiers producing signals identical in magnitude, but
out of phase by 180°. Consequently, the differential gain for the Audio Amplifier is
out of phase by 180°. Consequently, the differential gain for the Audio Amplifier is
A
VD
= 2 *(Rf/Ri)
(1)
By driving the load differentially through outputs OUT A and OUT B, an amplifier configuration commonly referred
to as “bridged mode” is established. Bridged mode operation is different from the classic single-ended amplifier
configuration where one side of the load is connected to ground.
to as “bridged mode” is established. Bridged mode operation is different from the classic single-ended amplifier
configuration where one side of the load is connected to ground.
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 conditions. 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 excessive clipping, please refer to the Audio Power Amplifier Design section.
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 conditions. 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 excessive clipping, please refer to the Audio Power Amplifier Design section.
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 output coupling capacitor, the half-supply bias across the load would result in both
increased internal IC power dissipation and also possible loudspeaker damage.
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 output coupling capacitor, the half-supply bias across the load would result in both
increased internal IC power dissipation and also possible loudspeaker damage.
OUTPUT TRANSIENT ('CLICK AND POPS') ELIMINATED
The LM4953 contains advanced circuitry that virtually eliminates 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.
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.
ensure a successful design.
states the maximum power dissipation point for a single-ended amplifier
operating at a given supply voltage and driving a specified output load.
P
DMAX
= (V
DD
)
2
/ (2
π
2
Z
L
)
(2)
Since the LM4953 has two operational amplifiers in one package, the maximum internal power dissipation point
is twice that of the number which results from
is twice that of the number which results from
. Even with large internal power dissipation, the LM4953
does not require heat sinking over a large range of ambient temperatures. The maximum power dissipation point
obtained must not be greater than the power dissipation that results from
obtained must not be greater than the power dissipation that results from
Copyright © 2005–2006, Texas Instruments Incorporated
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