Texas Instruments LM4941SD Evaluation Board LM4941SDBD/NOPB LM4941SDBD/NOPB 데이터 시트
제품 코드
LM4941SDBD/NOPB
SNAS347C – JUNE 2006 – REVISED MAY 2013
APPLICATION INFORMATION
OPTIMIZING RF IMMUNITY
The internal circuitry of the LM4941 suppresses the amount of RF signal that is coupled into the chip. However,
certain external factors, such as output trace length, output trace orientation, distance between the chip and the
antenna, antenna strength, speaker type, and type of RF signal, may affect the RF immunity of the LM4941. In
general, the RF immunity of the LM4941 is application specific. Nevertheless, optimal RF immunity can be
achieved by using short output traces and increasing the distance between the LM4941 and the antenna.
certain external factors, such as output trace length, output trace orientation, distance between the chip and the
antenna, antenna strength, speaker type, and type of RF signal, may affect the RF immunity of the LM4941. In
general, the RF immunity of the LM4941 is application specific. Nevertheless, optimal RF immunity can be
achieved by using short output traces and increasing the distance between the LM4941 and the antenna.
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4941 is a fully differential audio amplifier that features differential input and output stages. Internally this is
accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the
output voltages so that the average value remains V
accomplished by two circuits: a differential amplifier and a common mode feedback amplifier that adjusts the
output voltages so that the average value remains V
DD
/ 2. When setting the differential gain, the amplifier can be
considered to have "halves". Each half uses an input and feedback resistor (R
i
and R
F
) to set its respective
closed-loop gain (see
). With R
i1
= R
i2
and R
F1
= R
F2
, the gain is set at -R
F
/ R
i
for each half. This results
in a differential gain of
A
VD
= -R
F
/R
i
(1)
It is extremely important to match the input resistors to each other, as well as the feedback resistors to each
other for best amplifier performance. See the
other for best amplifier performance. See the
section for more
information. A differential amplifier works in a manner where the difference between the two input signals is
amplified. In most applications, input signals will be 180° out of phase with each other. The LM4941 can be used,
however, as a single-ended input amplifier while still retaining its fully differential benefits because it simply
amplifies the difference between the inputs.
amplified. In most applications, input signals will be 180° out of phase with each other. The LM4941 can be used,
however, as a single-ended input amplifier while still retaining its fully differential benefits because it simply
amplifies the difference between the inputs.
All of these applications provide what is known as a "bridged mode" output (bridge-tied-load, BTL). This results in
output signals that are 180° out of phase with respect to each other. Bridged mode operation is different from the
single-ended amplifier configuration that connects the load between the amplifier output and ground. A bridged
amplifier design has distinct advantages over the single-ended configuration: it provides differential drive to the
load, thus doubling maximum possible output swing for a specific supply voltage. Four times the output power is
possible compared with a single-ended amplifier under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited or clipped. Choose an amplifier's closed-loop gain without
causing excess clipping.
output signals that are 180° out of phase with respect to each other. Bridged mode operation is different from the
single-ended amplifier configuration that connects the load between the amplifier output and ground. A bridged
amplifier design has distinct advantages over the single-ended configuration: it provides differential drive to the
load, thus doubling maximum possible output swing for a specific supply voltage. Four times the output power is
possible compared with a single-ended amplifier under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited or clipped. Choose an amplifier's closed-loop gain without
causing excess clipping.
A bridged configuration, such as the one used in the LM4941, also creates a second advantage over single-
ended amplifiers. Since the differential outputs are biased at half-supply, no net DC voltage exists across the
load. This assumes that the input resistor pair and the feedback resistor pair are properly matched (see
ended amplifiers. Since the differential outputs are biased at half-supply, no net DC voltage exists across the
load. This assumes that the input resistor pair and the feedback resistor pair are properly matched (see
). BTL configuration eliminates the output coupling capacitor required in single-
supply, single-ended amplifier configurations. If an output coupling capacitor is not used in a single-ended output
configuration, the half-supply bias across the load would result in both increased internal IC power dissipation as
well as permanent loudspeaker damage. Further advantages of bridged mode operation specific to fully
differential amplifiers like the LM4941 include increased power supply rejection ratio, common-mode noise
reduction, and click and pop reduction.
configuration, the half-supply bias across the load would result in both increased internal IC power dissipation as
well as permanent loudspeaker damage. Further advantages of bridged mode operation specific to fully
differential amplifiers like the LM4941 include increased power supply rejection ratio, common-mode noise
reduction, and click and pop reduction.
POWER DISSIPATION
Power dissipation is a major concern when designing a successful amplifier, whether the amplifier is bridged or
single-ended.
single-ended.
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
R
L
) Single-Ended
(2)
However, a direct consequence of the increased power delivered to the load by a bridge amplifier is an increase
in internal power dissipation versus a single-ended amplifier operating at the same conditions.
in internal power dissipation versus a single-ended amplifier operating at the same conditions.
P
DMAX
= 4 * (V
DD
)
2
/ (2
π
2
R
L
) Bridge Mode
(3)
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