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參數(shù)資料
| 型號(hào)： | LM4834MS |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻控制 |
| 英文描述： | 1.75W Audio Power Amplifier with DC Volume Control and Microphone Preamp |
| 中文描述： | 2 CHANNEL(S), VOLUME CONTROL CIRCUIT, PDSO28 |
| 封裝： | SSOP-28 |
| 文件頁數(shù)： | 10/12頁 |
| 文件大小： | 443K |
| 代理商： | LM4834MS |
Application Information
(Continued)
is from 2.9dB (pin22 = 4V) to -75dB (pin22 = 0V). Any DC
voltage greater than 4V will result in a gain of 2.9dB. When
the mode pin is given a logic low, the Line/HP amplifier will
be fixed at a gain of 2.9dB regardless of the voltage of pin
22. Refer to the
Typical Performance Characteristics
for
detailed information of the attenuation characteristics of the
DC Volume Control pin.
MICROPHONE PREAMPLIFIER
The microphone preamplifier is intended to amplify low-level
signals. The mic input can be directly connected to a micro-
phone network or to low level signal inputs. The mic amplifier
has enough output capability to drive a 1k
load. A power
supply buffer is included for microphones which require ex-
ternal biasing.
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 bridged amplifier operating at a given
supply voltage and driving a specified load.
P
DMAX
= 4(V
DD
)
2
/(2
π
2
R
L
)
Along with the bridged amplifier, the LM4834 also incorpo-
rates two single-ended amplifiers. Equation 2 states the
maximum power dissipation point for a single-ended ampli-
fier operating at a given supply voltage and driving a speci-
fied load.
P
DMAX
= (V
DD
)
2
/(2
π
2
R
L
)
Even with the power dissipation of the bridged amplifier
andthe two single-ended amplifiers, the LM4834 does not re-
quire heatsinking. The power dissipation from the three am-
plifiers, must not be greater than the package power dissipa-
tion that results from Equation 3:
P
DMAX
= (T
JMAX
T
A
)/
θ
JA
For the LM4834 SSOP package,
θ
= 95C/W and T
=
150C. Depending on the ambient temperature, T
, of the
system surroundings, Equation 3 can be used to find the
maximum internal power dissipation supported by the IC
packaging. If the result of Equation 1 and 2 is greater than
that of Equation 3, then either the supply voltage must be de-
creased, the load impedance increased, or the ambient tem-
perature reduced. For the typical application of a 5V power
supply, with an 8
bridged load and 32
single ended loads,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 82C
provided that device operation is around the maximum
power dissipation points. Power dissipation is a function of
output power and thus, if typical operation is not around the
maximum power dissipation point, the ambient temperature
can be increased. Refer to the
Typical Performance Char-
acteristics
curvesfor power dissipation information for differ-
ent output powers.
(1)
(2)
(3)
GROUNDING
In order to achieve the best possible performance, there are
certain grounding techniques to be followed. All input refer-
ence grounds should be tied with their respective source
grounds and brought back to the power supply ground sepa-
rately from the output load ground returns. Bringing the
ground returns for the output loads back to the supply sepa-
rately will keep large signal currents from interfering with the
stable AC input ground references.
LAYOUT
As stated in the Grounding section, placement of ground re-
turn lines is imperative in maintaining the highest level of
system performance. It is not only important to route the cor-
rect ground return lines together, but also to be aware of
where the ground return lines are routed with respect to each
other. The output load ground returns should be physically
located as far as possible from low signal level lines and their
ground return lines. Critical signal lines are those relating to
the microphone amplifier section, since these lines generally
work at very low signal levels.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is criti-
cal for low noise performance and high power supply rejec-
tion. The capacitor location on both the bypass and power
supply pins should be as close to the device as possible. The
effect of a larger half supply bypass capacitor is improved
PSRR due to increased half-supply stability. Typical applica-
tions employ a 5 volt regulator with 10 μF and a 0.1 μF by-
pass capacitors which aid in supply stability, but do not elimi-
nate the need for bypassing the supply nodes of the
LM4834. The selection of bypass capacitors, especially C
B
,
is thus dependant upon desired PSRR requirements, click
and pop performance as explained in the section,
Proper
Selection of External Components,
system cost, and size
constraints. It is also recommended to decouple each of the
V
DD
pins with a 0.1μF capacitor to ground.
PROPER SELECTION OF EXTERNAL COMPONENTS
Proper selection of external components in applications us-
ing integrated power amplifiers is critical to optimize device
and system performance. While the LM4834 is tolerant of
external component combinations, consideration to compo-
nent values must be used to maximize overall system qual-
ity.
The LM4834’s bridged amplifier should be used in low gain
configurations to minimize THD+N values, and maximize the
signal to noise ratio. Low gain configurations require large in-
put signals to obtain a given output power. Input signals
equal to or greater than 1Vrms are available from sources
such as audio codecs.
Besides gain, one of the major considerations is the closed-
loop bandwidth of the amplifier. To a large extent, the band-
width is dictated by the choice of external components
shown in Figure 1 Both the input coupling capacitor, C
, and
the output coupling capacitor form first order high pass filters
which limit low frequency response given in Equations 4 and
5.
f
IC
= 1/(2
π
R
i
C
i
)
f
OC
= 1/(2
π
R
L
C
O
)
These values should be chosen based on required fre-
quency response.
(4)
(5)
Selection of Input and Output Capacitor Size
Large input and output capacitors are both expensive and
space hungry for portable designs. Clearly, a certain sized
capacitor is needed to couple in low frequencies without se-
vere attenuation. In many cases the speakers used in por-
table systems, whether internal or external, have little ability
to reproduce signals below 100 Hz–150 Hz. In this case, us-
inga large input or output capacitor may not increase system
performance.
In addition to system cost and size, click and pop perfor-
mance is effected by the size of the input coupling capacitor,
www.national.com
10
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