參數(shù)資料
型號(hào)： LM4924MM
廠商： NATIONAL SEMICONDUCTOR CORP
元件分類： 音頻／視頻放大
英文描述： 2 Cell Battery, 40mW Per Channel Output Capacitor-Less(OCL) Stereo Headphone Audio Amplifier
中文描述： 0.04 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO10
封裝： MSOP-10
文件頁(yè)數(shù)： 10/16頁(yè)
文件大?。?/td> 425K
代理商： LM4924MM
Application Information
ELIMINATING OUTPUT COUPLING CAPACITORS
Typical single-supply audio amplifiers that drive single-
ended (SE) headphones use a coupling capacitor on each
SE output. This output coupling capacitor blocks the half-
supply voltage to which the output amplifiers are typically
biased and couples the audio signal to the headphones. The
signal return to circuit ground is through the headphone
jack’s sleeve.
The LM4924 eliminates these output coupling capacitors.
V
o
C is internally configured to apply a 1/2V
DD
bias voltage to
a stereo headphone jack’s sleeve. This voltage matches the
quiescent voltage present on the V
o
A and V
o
B outputs that
drive the headphones. The headphones operate in a manner
similar to a bridge-tied-load (BTL). The same DC voltage is
applied to both headphone speaker terminals. This results in
no net DC current flow through the speaker.AC current flows
through a headphone speaker as an audio signal’s output
amplitude increases on the speaker’s terminal.
The headphone jack’s sleeve is not connected to circuit
ground. Using the headphone output jack as a line-level
output will place the LM4924’s bandgap 1/2V
bias on a
plug’s sleeve connection. This presents no difficulty when
the external equipment uses capacitively coupled inputs. For
the very small minority of equipment that is DC-coupled, the
LM4924 monitors the current supplied by the amplifier that
drives the headphone jack’s sleeve. If this current exceeds
500mA
, the amplifier is shutdown, protecting the LM4924
and the external equipment.
BYPASS CAPACITOR VALUE SELECTION
Besides minimizing the input capacitor size, careful consid-
eration should be paid to value of C
BYPASS
, the capacitor
connected to the BYPASS pin. Since C
BYPASS
determines
how fast the LM4924 settles to quiescent operation, its value
is critical when minimizing turn-on pops. The slower the
LM4924’s outputs ramp to their quiescent DC voltage (nomi-
nally V
DD
/2), the smaller the turn-on pop. Choosing C
B
equal
to 4.7μF along with a small value of C
i
(in the range of 0.1μF
to 0.47μF), produces a click-less and pop-less shutdown
function. As discussed above, choosing C
no larger than
necessary for the desired bandwidth helps minimize clicks
and pops. This ensures that output transients are eliminated
when power is first applied or the LM4924 resumes opera-
tion after shutdown.
OPTIMIZING CLICK AND POP REDUCTION
PERFORMANCE
The LM4924 contains circuitry that eliminates turn-on and
shutdown transients ("clicks and pops"). For this discussion,
turn-on refers 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 LM4924’s internal amplifiers are configured
as unity gain buffers. 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
BYPASS
alters the device’s
turn-on time. There is a linear relationship between the size
of C
and the turn-on time. Here are some typical
turn-on times for various values of C
BYPASS
.
AMPLIFIER CONFIGURATION EXPLANATION
As shown in
Figure 1
, the LM4924 has three operational
amplifiers internally. Two of the amplifier’s have externally
configurable gain while the other amplifier is internally fixed
at the bias point acting as a unity-gain buffer. The closed-
loop gain of the two configurable amplifiers is set by select-
ing the ratio of R
f
to R
i
. Consequently, the gain for each
channel of the IC is
A
V
= -(R
f
/R
i
)
By driving the loads through outputs V
and V
with V
acting as a buffered bias voltage the LM4924 does not
require output coupling capacitors. The typical single-ended
amplifier configuration where one side of the load is con-
nected to ground requires large, expensive output coupling
capacitors.
A configuration such as the one used in the LM4924 has a
major advantage over single supply, single-ended amplifiers.
Since the outputs V
, V
, and V
are all biased at 1/2
V
, no net DC voltage exists across each load. This elimi-
nates the need for output coupling capacitors that are re-
quired in a single-supply, single-ended amplifier configura-
tion. Without output coupling capacitors in a typical single-
supply, single-ended amplifier, the bias voltage is placed
across the load resulting in both increased internal IC power
dissipation and possible loudspeaker damage.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful amplifier. A direct consequence of the increased
power delivered to the load by a bridge amplifier is an
increase in internal power dissipation. The maximum power
dissipation for a given application can be derived from the
power dissipation graphs or from Equation 1.
P
DMAX
= 4(V
DD
)
2
/ (
π
2
R
L
)
(1)
It is critical that the maximum junction temperature T
of
150C is not exceeded. Since the typical application is for
headphone operation (16
impedance) using a 3.3V supply
the maximum power dissipation is only 138mW. Therefore,
power dissipation is not a major concern.
POWER SUPPLY BYPASSING
As with any amplifier, proper supply bypassing is important
for low noise performance and high power supply rejection.
The capacitor location on the power supply pins should be
as close to the device as possible.
Typical applications employ a 3.0V regulator with 10μF tan-
talum or electrolytic capacitor and a ceramic bypass capaci-
tor which aid in supply stability. This does not eliminate the
need for bypassing the supply nodes of the LM4924. A
bypass capacitor value in the range of 0.1μF to 1μF is
recommended for C
S
.
MICRO POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4924’s shutdown function. Activate micro-power shut-
down by applying a logic-low voltage to the SHUTDOWN
L
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相關(guān)代理商/技術(shù)參數(shù)
參數(shù)描述
LM4924MM/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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel
LM4924MMBD 功能描述:BOARD EVALUATION LM4924MM RoHS:否 類別:編程器，開(kāi)發(fā)系統(tǒng) >> 評(píng)估板 - 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:1 系列:-
LM4924MMX 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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel
LM4924MMX/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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel
LM4924SD 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(fēng)格:SMD/SMT 封裝 / 箱體:TQFP-64 封裝:Reel
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