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參數(shù)資料
| 型號： | LM4981 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | CHOKE, COM/ASYM MODE 2X0.7MH 4A |
| 中文描述： | 地為參考，80mW立體聲耳機(jī)放大器，具有數(shù)字音量控制 |
| 文件頁數(shù)： | 13/18頁 |
| 文件大?。?/td> | 1193K |
| 代理商： | LM4981 |
Application Information
(Continued)
down pin to the same state as the Shutdown Mode pin. For
simplicity’s sake, this is called "shutdown same", as the
LM4981 enters shutdown mode whenever the two pins are
in the same logic state. The trigger point for either shutdown
high or shutdown low is shown as a typical value in the
Supply Current vs Shutdown Voltage graphs in the
Typical
Performance Characteristics
section. It is best to switch
between ground and supply for maximum performance.
While the device may be disabled with shutdown voltages in
between ground and supply, the idle current may be greater
than the typical value of 0.1μA. In either case, the shutdown
pin should be tied to a definite voltage to avoid unwanted
state changes.
In many applications, a microcontroller or microprocessor
output is used to control the shutdown circuitry, which pro-
vides a quick, smooth transition to shutdown. Another solu-
tion is to use a single-throw switch in conjunction with an
external pull-up resistor (or pull-down, depending on shut-
down high or low application). This scheme guarantees that
the shutdown pin will not float, thus preventing unwanted
state changes.
AUTOMATIC STANDBY MODE
The LM4981 features Automatic Standby Mode circuitry
(patent pending). In the absence of an input signal, after
approximately 12 seconds, the LM4981 goes into low cur-
rent standby mode. The LM4981 recovers into full power
operating mode immediately after a signal, which is greater
than the input threshold voltage, is applied to either the left or
right input pins. The input threshold voltage is not a static
value, as the supply voltage increases, the input threshold
voltage decreases. This feature reduces power supply cur-
rent consumption in battery operated applications. Please
see also the graph entitled Representation of Automatic
Standby Mode Behavior in the Typical Performance Charac-
teristics section.
To ensure correct operation of Automatic Standby Mode,
proper layout techniques should be implemented. Separat-
ing PGND and SGND can help reduce noise entering the
LM4981 in noisy environments. Auto Standby mode works
best when output impedance of the audio source driving
LM4981 is equal or less than 50 Ohms. While Automatic
Standby Mode reduces power consumption very effectively
during silent periods, maximum power saving is achieved by
putting the device into shutdown when it is not in use.
OUTPUT TRANSIENT (’CLICK AND POPS’)
ELIMINATED
The LM4981 contains advanced circuitry that virtually elimi-
nates output transients (’clicks and pops’). This circuitry
prevents all traces of transients when the supply voltage is
first applied or when the part resumes operation after coming
out of shutdown mode.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATION
The LM4981’s exposed-dap (die attach paddle) package
(LD) provides a low thermal resistance between the die and
the PCB to which the part is mounted and soldered. This
allows rapid heat transfer from the die to the surrounding
PCB copper traces, ground plane, and surrounding air.
The LD package should have its DAP soldered to a copper
pad on the PCB. The DAP’s PCB copper pad may be con-
nected to a large plane of continuous unbroken copper. This
plane forms a thermal mass, heat sink, and radiation area
However, since the LM4981 is designed for headphone ap-
plications, connecting a copper plane to the DAP’s PCB
copper pad is not required. The DAP on the LM4981 should
be connected to GND to ensure correct functionality.
SELECTING PROPER EXTERNAL COMPONENTS
Optimizing the LM4981’s performance requires properly se-
lecting external components. Though the LM4981 operates
well when using external components with wide tolerances,
best performance is achieved by optimizing component val-
ues
Charge Pump Capacitor Selection
Use low ESR (equivalent series resistance) (
<
100m
) ce-
ramic capacitors with an X7R dielectric for best perfor-
mance. Low ESR capacitors keep the charge pump output
impedance to a minimum, extending the headroom on the
negative supply. Higher ESR capacitors result in reduced
output power from the audio amplifiers.
Charge pump load regulation and output impedance are
affected by the value of the flying capacitor (C
). A larger
valued C
(up to 3.3uF) improves load regulation and mini-
mizes charge pump output resistance. Beyond 3.3uF, the
switch-on resistance dominates the output impedance for
capacitor values above 2.2uF.
The output ripple is affected by the value and ESR of the
output capacitor (C
). Larger capacitors reduce output
ripple on the negative power supply. Lower ESR capacitors
minimize the output ripple and reduce the output impedance
of the charge pump.
The LM4981 charge pump design is optimized for 2.2uF, low
ESR, ceramic, flying, and output capacitors.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitors (C
in
A and C
in
B in Figure 1). 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 high value
input and output capacitors.
Besides affecting system cost and size, the input capacitor
has an effect on the LM4981’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.
As shown in Figure 1, the internal input resistor, R
and the
input capacitor, C
, produce a -3dB high pass filter cutoff
frequency that is found using Equation (3). Conventional
headphone amplifiers require output capacitors; Equation (3)
can be used, along with the value of R
L
, to determine to-
wards the value of output capacitor needed to produce a
–3dB high pass filter cutoff frequency.
f
i-3dB
= 1 / 2
π
R
i
C
i
(3)
L
www.national.com
13
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| LM4981SQ/NOPB | 功能描述:IC AMP AUDIO .083W STER AB 16LLP RoHS:是 類別:集成電路 (IC) >> 線性 - 音頻放大器 系列:Boomer® 產(chǎn)品培訓(xùn)模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標(biāo)準(zhǔn)包裝:2,500 系列:DirectDrive® 類型:H 類 輸出類型:耳機(jī)，2-通道（立體聲） 在某負(fù)載時(shí)最大輸出功率 x 通道數(shù)量:35mW x 2 @ 16 歐姆 電源電壓:1.62 V ~ 1.98 V 特點(diǎn):I²C，麥克風(fēng)，靜音，短路保護(hù)，音量控制 安裝類型:表面貼裝 供應(yīng)商設(shè)備封裝:25-WLP（2.09x2.09） 封裝/外殼:25-WFBGA，WLCSP 包裝:帶卷 (TR) |
| LM4981SQX/NOPB | 功能描述:IC AMP AUDIO .083W STER AB 16LLP RoHS:是 類別:集成電路 (IC) >> 線性 - 音頻放大器 系列:Boomer® 產(chǎn)品培訓(xùn)模塊:Lead (SnPb) Finish for COTS Obsolescence Mitigation Program 標(biāo)準(zhǔn)包裝:2,500 系列:DirectDrive® 類型:H 類 輸出類型:耳機(jī)，2-通道（立體聲） 在某負(fù)載時(shí)最大輸出功率 x 通道數(shù)量:35mW x 2 @ 16 歐姆 電源電壓:1.62 V ~ 1.98 V 特點(diǎn):I²C，麥克風(fēng)，靜音，短路保護(hù)，音量控制 安裝類型:表面貼裝 供應(yīng)商設(shè)備封裝:25-WLP（2.09x2.09） 封裝/外殼:25-WFBGA，WLCSP 包裝:帶卷 (TR) |
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