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參數(shù)資料
| 型號(hào)： | LM4818M/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 0.35 W, 1 CHANNEL, AUDIO AMPLIFIER, PDSO8 |
| 封裝： | SOP-8 |
| 文件頁(yè)數(shù)： | 16/16頁(yè) |
| 文件大小： | 483K |
| 代理商： | LM4818M/NOPB |
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Application Information
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4818 consist of two operational
amplifiers. External resistors, R
i and RF set the closed-loop
gain of the first amplifier (and the amplifier overall), whereas
two internal 20k
resistors set the second amplifier’s gain at
-1. The LM4818 is typically used to drive a speaker con-
nected between the two amplifier outputs.
Figure 1 shows that the output of Amp1 servers as the input
to Amp2, which results in both amplifiers producing signals
identical in magnitude but 180 out of phase. Taking advan-
tage of this phase difference, a load is placed between V
01
and V
02 and driven differentially (commonly referred to as
"bridge mode"). This results in a differential gain of
A
VD= 2 *(Rf/Ri)
(1)
Bridge mode is different from single-ended amplifiers that
drive loads connected between a single amplifier’s output
and ground. For a given supply voltage, bridge mode has a
distinct advantage over the single-ended configuration: its
differential output doubles the voltage swing across the load.
This results in four times the output power when compared
to a single-ended amplifier under the same conditions. This
increase in attainable output assumes that the amplifier is
not current limited or the output signal is not clipped. To
ensure minimum output signal clipping when choosing an
amplifier’s closed-loop gain, refer to the Audio Power Am-
plifier Design Example section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This results from biasing V
01
and V
02 at half-supply. This eliminates the coupling capacitor
that single supply, single-ended amplifiers require. Eliminat-
ing an output coupling capacitor in a single-ended configu-
ration forces a single supply amplifier’s half-supply bias volt-
age across the load. The current flow created by the half-
supply bias voltage increases internal IC power dissipation
and may permanently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful bridged or single-ended amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified load.
P
DMAX =(VDD)
2 /(2
π2R
L ) (W) Single-ended
(2)
However, a direct consequence of the increased power de-
livered to the load by a bridged amplifier is an increase in the
internal power dissipation point for a bridge amplifier oper-
ating at the same given conditions. Equation (3) states the
maximum power dissipation point for a bridged amplifier
operating at a given supply voltage and driving a specified
load.
P
DMAX = 4(VDD)
2/(2
π2 R
L ) (W) Bridge Mode
(3)
The LM4818 has two operational amplifiers in one package
and the maximum internal power dissipation is four times
that of a single-ended amplifier. However, even with this
substantial increase in power dissipation, the LM4818 does
not require heatsinking. From Equation (3), assuming a 5V
power supply and an 8
load, the maximum power dissipa-
tion point is 633mW. The maximum power dissipation point
obtained from Equation (3) must not exceed the power dis-
sipation predicted by Equation (4):
P
DMAX =(TJMAX -TA)/
θ
JA (W)
(4)
For the M08A package,
θ
JA = 170C/W and TJMAX = 150C
for the LM4818. For a given ambient temperature, T
A, Equa-
tion (4) can be used to find the maximum internal power
dissipation supported by the IC packaging. If the result of
Equation (3) is greater than the result of Equation (4), then
decrease the supply voltage, increase the load impedance,
or reduce the ambient temperature. For a typical application
using the M08A packaged LM4818 with a 5V power supply
and an 8
load, the maximum ambient temperature that
does not violate the maximum junction temperature is ap-
proximately 42C. It is assumed that a device is a surface
mount part operating around the maximum power dissipation
point. The assumption that the device is operating around
the maximum power dissipation point is incorrect for an 8
load. The maximum power dissipation point occurs when the
output power is equal to the maximum power dissipation or
50% efficiency. The LM4818 is not capable of the output
power level (633mW) required to operate at the maximum
power dissipation point for an 8
load. To find the maximum
power dissipation, the graph Power Dissipation vs. Output
Power must be used. From the graph, the maximum power
dissipation for an 8
load and a 5V supply is approximately
575mW. Substituting this value back into equation (4) for
P
DMAX and using
θ
JA = 170C/W for the M08A package, the
maximum ambient temperature is 52C. Refer to the Typical
Performance Characteristics curves for power dissipation
information for lower output powers and maximum power
dissipation for each package at a given ambient tempera-
ture.
POWER SUPPLY BYPASSING
As with any power amplifier, proper supply bypassing is
critical for low noise performance and high power supply
rejection. The capacitors connected to the bypass and power
supply pins should be placed as close to the LM4818 as
possible. The capacitor connected between the bypass pin
and ground improves the internal bias voltage’s stability,
producing improved PSRR. The improvements to PSRR
increase as the bypass pin capacitor value increases. Typi-
cal applications employ a 5V regulator with 10F and 0.1F
filter capacitors that aid in supply stability. Their presence,
however, does not eliminate the need for bypassing the
supply nodes of the LM4818. The selection of bypass ca-
pacitor values, especially C
B , depends on desired PSRR
requirements, click and pop performance as explained in the
section, Proper Selection of External Components, as
well as system cost and size constraints.
SHUTDOWN FUNCTION
The voltage applied to the LM4818’s SHUTDOWN pin con-
trols the shutdown function. Activate micro-power shutdown
by applying V
DD to the SHUTDOWN pin. When active, the
LM4818’s micro-power shutdown feature turns off the ampli-
fier’s bias circuitry, reducing the supply current. The logic
threshold is typically 1/2V
DD. The low 0.7A typical shut-
down current is achieved by applying a voltage that is as
near as V
DD as possible to the SHUTDOWN pin. A voltage
that is less than V
DD may increase the shutdown current.
Avoid intermittent or unexpected micro-power shutdown by
ensuring that the SHUTDOWN pin is not left floating but
connected to either V
DD or GND.
There are a few ways to activate micro-power shutdown.
These included using a single-pole, single-throw switch, a
microcontroller, or a microprocessor. When using a switch,
connect an external 10k
to 100k pull-up resistor between
the SHUTDOWN pin and V
DD. Connect the switch between
the SHUTDOWN pin and ground. Select normal amplifier
LM4818
www.national.com
9
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相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4818MX | 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 |
| LM4818MX/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 |
| LM4819 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:350mW Audio Power Amplifier with Shutdown Mode |
| LM4819LD | 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 |
| LM4819LD/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 |
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