參數(shù)資料
型號(hào)： LM4856
廠商： National Semiconductor Corporation
元件分類： 運(yùn)動(dòng)控制電子
英文描述： Integrated Audio Amplifier System
中文描述： 集成音頻放大器系統(tǒng)
文件頁(yè)數(shù)： 18/23頁(yè)
文件大?。?/td> 1321K
代理商： LM4856
Application Information
(Continued)
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4856’s maximum junction tem-
perature.
T
A
= T
JMAX
- P
DMAX-TOTAL
θ
JA
(7)
For a typical application with a 5V power supply and an 8
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 104C for the
IBL package.
T
JMAX
= P
DMAX-TOTAL
θ
JA
+ T
A
(8)
Equation (8) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4856’s 150C, reduce the
maximum junction temperature by reducing the power sup-
ply voltage or increasing the load resistance. Further allow-
ance should be made for increased ambient temperatures.
The above examples assume that a device is a surface
mount part operating around the maximum power dissipation
point. Since internal power dissipation is a function of output
power, higher ambient temperatures are allowed as output
power or duty cycle decreases. If the result of Equation (5) is
greater than that of Equation (6), then decrease the supply
voltage, increase the load impedance, or reduce the ambient
temperature. If these measures are insufficient, a heat sink
can be added to reduce
θ
JA
. The heat sink can be created
using additional copper area around the package, with con-
nections to the ground pin(s), supply pin and amplifier output
pins. External, solder attached SMT heatsinks such as the
Thermalloy 7106D can also improve power dissipation.
When adding a heat sink, the
θ
is the sum of
θ
,
θ
CS
, and
θ
. (
θ
is the junction-to-case thermal impedance,
θ
is
the case-to-sink thermal impedance, and
θ
is the sink-to-
ambient thermal impedance.) Refer to the Typical Perfor-
mance Characteristics curves for power dissipation informa-
tion at lower output power levels.
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 10μF in parallel with a 0.1μF 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 1.0μF
tantalum bypass capacitance connected between the
LM4856’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4856’s
power supply pin and ground as short as possible. Connect-
ing a 1μF 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 turn-on time and can compromise
the amplifier’s click and pop performance. The selection of
bypass capacitor values, especially C
, depends on desired
PSRR requirements, click and pop performance (as ex-
plained in the section, Proper Selection of External Compo-
nents), system cost, and size constraints.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (C
in
Figure 3
). 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.
The internal input resistor (R
i
) and the input capacitor (C
i
)
produce a high pass filter cutoff frequency that is found using
Equation (9).
f
c
= 1 / (2
π
R
i
C
i
)
(9)
As an example when using a speaker with a low frequency
limit of 150Hz, C
, using Equation (9) is 0.063μF. The 0.22μF
C
shown in
Figure 1
allows the LM4856 to drive high effi-
ciency, full range speaker whose response extends below
40Hz.
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 LM4856 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4856’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 C
i
(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
i
. This ensures that output transients are
eliminated when power is first applied or the LM4856 re-
sumes operation after shutdown.
L
www.national.com
18
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