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參數(shù)資料
| 型號： | LM4857GR/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 消費家電 |
| 英文描述： | SPECIALTY CONSUMER CIRCUIT, PBGA49 |
| 文件頁數(shù)： | 25/40頁 |
| 文件大小： | 1948K |
| 代理商： | LM4857GR/NOPB |
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Application Information (Continued)
The maximum power dissipation point given by Equation (9)
must not exceed the power dissipation given by Equation
(10):
P
DMAX’= (TJMAX -TA)/
θ
JA
(10)
The LM4857’s T
JMAX = 150C. In the ITL package, the
LM4857’s
θ
JA is 62C/W. At any given ambient temperature
T
A, use Equation (10) to find the maximum internal power
dissipation supported by the IC packaging. Rearranging
Equation (10) and substituting P
DMAX-TOTAL for PDMAX’ re-
sults in Equation (11). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4857’s maximum junction
temperature.
T
A =TJMAX -PDMAX-TOTAL
θ
JA
(11)
For a typical application with a 5V power supply, stereo 8
loudspeaker load, and the stereo 32
headphone load, the
maximum ambient temperature that allows maximum stereo
power dissipation without exceeding the maximum junction
temperature is approximately 66.4C for the ITL package.
T
JMAX =PDMAX-TOTAL
θ
JA +TA
(12)
Equation (12) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4857’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 (9) is
greater than that of Equation (10), 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
JA is the sum of
θ
JC,
θ
CS, and
θ
SA.(
θ
JC is the junction-to-case thermal impedance,
θ
CS is
the case-to-sink thermal impedance, and
θ
SA 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 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
LM4857’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4857’s
power supply pin and ground as short as possible.
SELECTING EXTERNAL COMPONENTS
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires a high
value input coupling capacitor (C
i in Figure 1). In many
cases, however, the speakers used in portable systems,
whether internal or external, have little ability to reproduce
signals below 50Hz. Applications using speakers with this
limited frequency response reap little improvement; by using
a large input capacitor.
The internal input resistor (R
i) and the input capacitor (Ci)
produce a high pass filter cutoff frequency that is found using
Equation (13).
f
c =1/(2
πR
iCi)
(13)
As an example when using a speaker with a low frequency
limit of 50Hz and R
i = 20k
,C
i, using Equation (13) is
0.19F. The 0.22F C
i shown in Figure 4 allows the LM4857
to drive high efficiency, full range speaker whose response
extends below 40Hz.
Output Capacitor Value Selection
Amplifying the lowest audio frequencies also requires the
use of a high value output coupling capacitor (C
O in Figure
1). A high value output capacitor can be expensive and may
compromise space efficiency in portable design.
The speaker load (R
L) and the output capacitor (CO) form a
high pass filter with a low cutoff frequency determined using
Equation (14).
f
c =1/(2
πR
LCO)
(14)
When using a typical headphone load of R
L =32
with a low
frequency limit of 50Hz, C
O is 99F.
The 100F C
O shown in Figure 4 allows the LM4857 to drive
a headphone whose frequency response extends below
50Hz.
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
B determines how fast
the LM4857 settles to quiescent operation, its value is critical
when minimizing turn-on pops. The slower the LM4857’s
outputs ramp to their quiescent DC voltage (nominally V
DD/
2), the smaller the turn-on pop. Choosing C
B equal to 2.2F
along with a small value of C
i (in the range of 0.1F to
0.39F), 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 10
times the value of C
i. This ensures that output transients are
eliminated when the LM4857 transitions in and out of shut-
down mode. Connecting a 2.2F capacitor, C
B, between the
BYPASS pin and ground improves the internal bias voltage’s
stability and improves the amplifier’s PSRR. The PSRR im-
provements increase as the bypass pin capacitor value in-
creases. However, increasing the value of C
B will increase
wake-up time. The selection of bypass capacitor value, C
B,
LM4857
www.national.com
31
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相關代理商/技術參數(shù) |
參數(shù)描述 |
|---|---|
| LM4857ITL | 制造商:Texas Instruments 功能描述:1.2W STEREO SUBSYST 3D SMD 4857 |
| LM4857ITL/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4857ITLBD | 功能描述:BOARD EVALUATION LM4857ITL RoHS:否 類別:編程器，開發(fā)系統(tǒng) >> 評估板 - 音頻放大器 系列:Boomer® 產(chǎn)品培訓模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標準包裝:1 系列:- |
| LM4857ITLX/NOPB | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
| LM4857SP | 功能描述:音頻放大器 RoHS:否 制造商:STMicroelectronics 產(chǎn)品:General Purpose Audio Amplifiers 輸出類型:Digital 輸出功率: THD + 噪聲: 工作電源電壓:3.3 V 電源電流: 最大功率耗散: 最大工作溫度: 安裝風格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel |
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