參數(shù)資料
型號(hào)： LM4894MWC
廠(chǎng)商： NATIONAL SEMICONDUCTOR CORP
元件分類(lèi)： 音頻／視頻放大
英文描述： 1 W, 1 CHANNEL, AUDIO AMPLIFIER, UUC
封裝： WAFER
文件頁(yè)數(shù)： 3/21頁(yè)
文件大?。?/td> 781K
代理商： LM4894MWC
Typical Performance Characteristics
Non-LD Specific Characteristics (Continued)
THD vs
Common-Mode Voltage
THD vs
Common-Mode Voltage
20013782
20013783
Application Information
DIFFERENTIAL AMPLIFIER EXPLANATION
The LM4894 is a fully differential audio amplifier that fea-
tures 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
DD/2. When
setting the differential gain, the amplifier can be considered
to have ’halves’. Each half uses an input and feedback
resistor (R
i1and RF1) to set its respective closed-loop gain
(see Figure 1). With R
i1 =Ri2 and RF1 =RF2, the gain is set
at -R
F/Ri for each half. This results in a differential gain of
A
VD =-RF/Ri
(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 Proper Selection of Exter-
nal Components section for more information. A differential
amplifier works in a manner where the difference between
the two input signals is amplified. In most applications, this
would require input signals that are 180 out of phase with
each other. The LM4894 can be used, however, as a single
ended input amplifier while still retaining its fully differential
benefits. In fact, completely unrelated signals may be placed
on the input pins. The LM4894 simply amplifies the differ-
ence between them. Figures 2 and 3 show single-ended
applications of the LM4894 that still take advantage of the
differential nature of the amplifier and the benefits to PSRR,
common-mode noise reduction, and ’click and pop’ reduc-
tion. Figure 4 shows the LM4894 in a single-ended input
mode with a minimum of external components. This last
application, although saving board space, does not retain
the advantages of the fully differential configuration.
All of these applications, either single-ended or fully differ-
ential, provide what is known as a ’bridged mode’ output
(bridge-tied-load, BTL). This results in output signals at V
o1
and V
o2 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 be-
tween the amplifier output and ground. A bridged amplifier
design has distinct advantages over the single-ended con-
figuration: it provides differential drive to the load, thus dou-
bling 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. In order to choose
an amplifier’s closed-loop gain without causing excess clip-
ping, please refer to the Audio Power Amplifier Design
section.
A bridged configuration, such as the one used in the
LM4894, also creates a second advantage over single-
ended amplifiers. Since the differential outputs, V
o1 and Vo2,
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 Proper
Selection of External Components). 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 con-
figuration, 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 LM4894 include increased power supply rejection
ratio, common-mode noise reduction, and click and pop
reduction.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4894’s exposed-DAP (die attach paddle) package
(LD) provide a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane and, finally, surrounding
air. The result is a low voltage audio power amplifier that
produces 1.4W at
≤ 1% THD with a 4 load. This high power
is achieved through careful consideration of necessary ther-
mal design. Failing to optimize thermal design may compro-
LM4894
www.national.com
11
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