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參數(shù)資料
| 型號： | MXF10A |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 1W Monaural, 85mW Stereo Headphone Audio Amplifier |
| 中文描述： | 1W的單聲道，85mW立體聲耳機音頻放大器 |
| 文件頁數(shù)： | 12/15頁 |
| 文件大?。?/td> | 384K |
| 代理商： | MXF10A |
Application Information
(Continued)
easily satisfies the LM4914’s requirement for a dual switch
headphone jack. For applications that require an SPDIF
interface in the stereo headphone jack, use a Foxconn
2F1138-TJ-TR.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
and C
in
Figure 2
). A high
value capacitor can be expensive and may compromise
space efficiency in portable designs. In many cases, how-
ever, the speakers used in portable systems, whether inter-
nal or external, have little ability to reproduce signals below
150Hz. Applications using speakers with this limited fre-
quency response reap little improvement by using large
input capacitor.
The LM4914’s advanced output transient suppression cir-
cuitry has eliminated the need to select the input capacitor’s
value in relation to the BYPASS capacitor’s value as was
necessary in some previous Boomer amplifiers. The value of
C
and C
are now strictly determined by the desired low
frequency response.
As shown in
Figure 2
, the input resistors (R
and R
) and the
input capacitors (C
1
and C
2
) produce a high pass filter cutoff
frequency that is found using Equation (7).
f
C
= 1 / 2
π
R
i
C
i
As an example, when using a speaker with a low frequency
limit of 150Hz, and input resistances (R
2
and R
3
) of 16.8k
,
using Equation (7), input capacitances (C
1
and C
2
) are
0.063μF. The 0.1μF capacitors (C
1
and C
2
) shown in
Figure
2
allow the LM4914 to drive high efficiency, full range
speaker whose response extends below 30Hz.
(7)
Bypass Capacitor Value Selection
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
B
, the capacitor con-
nected to the BYPASS pin. Since C
determines how fast
the LM4914 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4914’s
outputs ramp to their quiescent DC voltage (nominally V
/
2), the smaller the turn-on pop. Choosing C
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 C
i
no larger than neces-
sary for the desired bandwidth helps minimize clicks and
pops. C
B
’s value should be in the range of 5 times to 7 times
the value of C
. This ensures that output transients are
eliminated when power is first applied or the LM4914 re-
sumes operation after shutdown.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4914 contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops") and transients that
could occur when switching between BTL speakers and
single-ended headphones. For this discussion, turn-on re-
fers to either applying the power supply voltage or when the
micro-power shutdown mode is deactivated.
As the V
/2 voltage present at the BYPASS pin ramps to its
final value, the LM4914’s internal amplifiers are configured
as unity gain buffers and are disconnected from the RO/MO+
and LO/MO- pins. An internal current source charges the
capacitor connected between the BYPASS pin and GND 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 V
DD
/2. As soon as the voltage on the
bypass pin is stable, the device becomes fully operational
and the amplifier outputs are reconnected to their respective
output pins. Although the BYPASS pin current cannot be
modified, changing the size of C
B
alters the device’s turn-on
time. There is a linear relationship between the size of C
B
and the turn-on time. Here are some typical turn-on times for
various values of C
B
:
C
B
(μF)
0.01
0.1
0.22
0.47
1.0
2.2
T
ON
(ms)
2
20
42
90
200
420
200634A2
FIGURE 4. Headphone Circuit
L
www.national.com
12
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