參數(shù)資料
型號(hào)： LM4810MA/NOPB
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類(lèi)： 音頻／視頻放大
英文描述： 0.105 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO8
封裝： SOIC-8
文件頁(yè)數(shù)： 4/18頁(yè)
文件大?。?/td> 928K
代理商： LM4810MA/NOPB
Application Information (Continued)
creased, the load impedance increased or T
A reduced. For
the typical application of a 5V power supply, with a 32
load,
the maximum ambient temperature possible without violating
the maximum junction temperature is approximately 133.2C
provided that device operation is around the maximum
power dissipation point. Power dissipation is a function of
output power and thus, if typical operation is not around the
maximum power dissipation point, the ambient temperature
may be increased accordingly. Refer to the Typical Perfor-
mance Characteristics curves for power dissipation infor-
mation for lower output powers.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. Applications that employ a 5V regulator typically
use a 10F in parallel with a 0.1F filter capacitors to stabi-
lize the regulator’s output, reduce noise on the supply line,
and improve the supply’s transient response. However, their
presence does not eliminate the need for a local 1.0F
tantalum bypass capacitance connected between the
LM4810’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4810’s
power supply pin and ground as short as possible. Connect-
ing a 4.7F capacitor, C
B, between the BYPASS pin and
ground improves the internal bias voltage’s stability and
improves the amplifier’s PSRR. The PSRR improvements
increase as the bypass pin capacitor value increases. Too
large, however, increases the amplifier’s turn-on time. The
selection of bypass capacitor values, especially C
B, depends
on desired PSRR requirements, click and pop performance
(as explained in the section, Selecting Proper External
Components), system cost, and size constraints.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4810’s performance requires properly se-
lecting external components. Though the LM4810 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4810 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise ra-
tio. These parameters are compromised as the closed-loop
gain increases. However, low gain demands input signals
with greater voltage swings to achieve maximum output
power. Fortunately, many signal sources such as audio
CODECs have outputs of 1V
RMS (2.83VP-P). Please refer to
the Audio Power Amplifier Design section for more infor-
mation on selecting the proper gain.
Input and Output Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input and output coupling capacitors (C
I and CO in Figure 1).
A high value capacitor can be expensive and may compro-
mise space efficiency in portable designs. In many cases,
however, the speakers used in portable systems, whether
internal or external, have little ability to reproduce signals
below 150Hz. Applications using speakers with this limited
frequency response reap little improvement by using high
value input and output capacitors.
Besides affecting system cost and size, C
i has an effect on
the LM4810’s click and pop performance. The magnitude of
the pop is directly proportional to the input capacitor’s size.
Thus, pops can be minimized by selecting an input capacitor
value that is no higher than necessary to meet the desired
3dB frequency. Please refer to the Optimizing Click and
Pop Reduction Performance section for a more detailed
discussion on click and pop performance.
As shown in Figure 1, the input resistor, R
I and the input
capacitor, C
I, produce a 3dB high pass filter cutoff fre-
quency that is found using Equation (3). In addition, the
output load R
L, and the output capacitor CO, produce a -3db
high pass filter cutoff frequency defined by Equation (4).
f
I-3db=1/2
πR
ICI
(3)
f
O-3db=1/2
πR
LCO
(4)
Also, careful consideration must be taken in selecting a
certain type of capacitor to be used in the system. Different
types of capacitors (tantalum, electrolytic, ceramic) have
unique performance characteristics and may affect overall
system performance.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to the value of C
B, the capacitor
connected to the BYPASS pin. Since C
B determines how
fast the LM4810 settles to quiescent operation, its value is
critical when minimizing turn-on pops. The slower the
LM4810’s outputs ramp to their quiescent DC voltage (nomi-
nally 1/2 V
DD), the smaller the turn-on pop. Choosing CB
equal to 4.7F along with a small value of C
i (in the range of
0.1F to 0.47F), produces a click-less and pop-less shut-
down function. As discussed above, choosing C
i no larger
than necessary for the desired bandwith helps minimize
clicks and pops.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4810 contains circuitry that minimizes turn-on and
shutdown transients or “clicks and pop”. For this discussion,
turn-on refers to either applying the power supply voltage or
when the shutdown mode is deactivated. During turn-on, the
LM4810’s internal amplifiers are configured as unity gain
buffers. An internal current source charges up the capacitor
on the BYPASS pin in a controlled, linear manner. The gain
of the internal amplifiers remains unity until the voltage on
the BYPASS pin reaches 1/2 V
DD. As soon as the voltage on
the BYPASS pin is stable, the device becomes fully opera-
tional. During device turn-on, a transient (pop) is created
from a voltage difference between the input and output of the
amplifier as the voltage on the BYPASS pin reaches 1/2 V
DD.
For this discussion, the input of the amplifier refers to the
node between R
I and CI. Ideally, the input and output track
the voltage applied to the BYPASS pin. During turn-on, the
buffer-configured amplifier output charges the input capaci-
tor, C
I, through the input resistor, RI. This input resistor
delays the charging time of C
I thereby causing the voltage
difference between the input and output that results in a
transient (pop). Higher value capacitors need more time to
reach a quiescent DC voltage (usually 1/2 V
DD)
when
charged with a fixed current. Decreasing the value of C
I and
R
I will minimize the turn-on pops at the expense of the
desired -3dB frequency.
Although the BYPASS pin current cannot be modified,
changing the size of C
B alters the device’s turn-on time and
LM4810
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