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LM4866LQX データシートの表示(PDF) - National ->Texas Instruments

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LM4866LQX 2.2W Stereo Audio Amplifier National-Semiconductor
National ->Texas Instruments National-Semiconductor
LM4866LQX Datasheet PDF : 21 Pages
First Prev 11 12 13 14 15 16 17 18 19 20
Application Information (Continued)
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4866’s performance requires properly se-
lecting external components. Though the LM4866 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues.
The LM4866 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 CO-
DECs have outputs of 1VRMS (2.83VP-P). Please refer to the
Audio Power Amplifier Design section for more informa-
tion on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (Ci in Figure 1). A high value capaci-
tor can be expensive and may compromise 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 large input capacitor.
Besides effecting system cost and size, Ci has an affect on
the LM4866’s click and pop performance. When the supply
voltage is first applied, a transient (pop) is created as the
charge on the input capacitor changes from zero to a quies-
cent state. The magnitude of the pop is directly proportional
to the input capacitor’s size. Higher value capacitors need
more time to reach a quiescent DC voltage (usually VDD/2)
when charged with a fixed current. The amplifier’s output
charges the input capacitor through the feedback resistor,
Rf. Thus, pops can be minimized by selecting an input
capacitor value that is no higher than necessary to meet the
desired -3dB frequency.
A shown in Figure 1, the input resistor (RI) and the input
capacitor, CI produce a −3dB high pass filter cutoff frequency
that is found using Equation (7).
(7)
As an example when using a speaker with a low frequency
limit of 150Hz, CI, using Equation (4), is 0.063µF. The 1.0µF
CI shown in Figure 1 allows the LM4866 to drive high effi-
ciency, full range speaker whose response extends below
30Hz.
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of CB, the capacitor con-
nected to the BYPASS pin. Since CB determines how fast
the LM4866 settles to quiescent operation, its value is critical
when minimizing turn−on pops. The slower the LM4866’s
outputs ramp to their quiescent DC voltage (nominally 1/2
VDD), the smaller the turn−on pop. Choosing CB equal to
1.0µF along with a small value of Ci (in the range of 0.1µF to
0.39µF), produces a click-less and pop-less shutdown func-
tion. As discussed above, choosing Ci no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops.
OPTIMIZING CLICK AND POP REDUCTION PERFOR-
MANCE
The LM4866 contains circuitry to minimize turn-on and shut-
down 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. While the power
supply is ramping to its final value, the LM4866’s internal
amplifiers are configured as unity gain buffers. An internal
current source changes the voltage of the BYPASS pin in a
controlled, linear manner. Ideally, the input and outputs track
the voltage applied to the BYPASS pin. The gain of the
internal amplifiers remains unity until the voltage on the
bypass pin reaches 1/2 VDD. As soon as the voltage on the
BYPASS pin is stable, the device becomes fully operational.
Although the bypass pin current cannot be modified, chang-
ing the size of CB alters the device’s turn-on time and the
magnitude of ’clicks and pops’. Increasing the value of CB
reduces the magnitude of turn-on pops. However, this pre-
sents a tradeoff: as the size of CB increases, the turn-on time
increases. There is a linear relationship between the size of
CB and the turn-on time. Here are some typical turn-on times
for various values of CB:
CB
0.01µF
0.1µF
0.22µF
0.47µF
1.0µF
TON
20 ms
200 ms
440 ms
940 ms
2 Sec
In order eliminate ’clicks and pops’, all capacitors must be
discharged before turn-on. Rapidly switching VDD may not
allow the capacitors to fully discharge, which may cause
’clicks and pops’.
NO LOAD STABILITY
The LM4866 may exhibit low level oscillation when the load
resistance is greater than 10k. This oscillation only occurs
as the output signal swings near the supply voltages. Pre-
vent this oscillation by connecting a 5kbetween the output
pins and ground.
13
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