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參數(shù)資料
| 型號： | LM4845 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | Output Capacitor-less Audio Subsystem with Programmable National 3D |
| 中文描述： | 輸出電容器的音頻子系統(tǒng)，具有可編程全國三維 |
| 文件頁數(shù)： | 25/29頁 |
| 文件大?。?/td> | 1392K |
| 代理商： | LM4845 |
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Application Information
(Continued)
BRIDGE CONFIGURATION EXPLANATION
The LM4845 drives a load, such as a speaker, connected
between outputs, MONO+ and MONO-.
This results in both amplifiers producing signals identical in
magnitude, but 180 out of phase. Taking advantage of this
phase difference, a load is placed between MONO- and
MONO+ and driven differentially (commonly referred to as
”bridge mode”). This results in a differential or BTL gain of:
A
VD
= 2(R
f
/ R
i
) = 2
(7)
Bridge mode amplifiers are different from single-ended am-
plifiers that drive loads connected between a single amplifi-
er’s output and ground. For a given supply voltage, bridge
mode has a distinct advantage over the single-ended con-
figuration: its differential output doubles the voltage swing
across the load. Theoretically, this produces four times the
output power when compared to a single-ended amplifier
under the same conditions. This increase in attainable output
power assumes that the amplifier is not current limited and
that the output signal is not clipped.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
MONO- and MONO+ outputs at half-supply. This eliminates
the coupling capacitor that single supply, single-ended am-
plifiers require. Eliminating an output coupling capacitor in a
typical single-ended configuration forces a single-supply am-
plifier’s half-supply bias voltage across the load. This in-
creases internal IC power dissipation and may permanently
damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier.
A direct consequence of the increased power delivered to
the load by a bridge amplifier is higher internal power dissi-
pation. The LM4845 has a pair of bridged-tied amplifiers
driving a handsfree speaker, MONO. The maximum internal
power dissipation operating in the bridge mode is twice that
of a single-ended amplifier. From Equation (2), assuming a
5V power supply and an 8
load, the maximum MONO
power dissipation is 634mW.
P
DMAX-SPKROUT
= 4(V
DD
)
2
/
(2
π
2
R
L
): Bridge Mode (8)
The LM4845 also has a pair of single-ended amplifiers driv-
ing stereo headphones, R
and L
. The maximum in-
ternal power dissipation for R
OUT
and L
OUT
is given by
equation (3) and (4). From Equations (3) and (4), assuming
a 5V power supply and a 32
load, the maximum power
dissipation for L
OUT
and R
OUT
is 40mW, or 80mW total.
P
DMAX-LOUT
= (V
DD
)
2
/ (2
π
2
R
L
): Single-ended Mode (9)
P
DMAX-ROUT
= (V
DD
)
2
/ (2
π
2
R
L
): Single-ended Mode10)
The maximum internal power dissipation of the LM4845
occurs when all 3 amplifiers pairs are simultaneously on; and
is given by Equation (5).
P
=
P
DMAX-SPKROUT
+ P
DMAX-LOUT
+ P
DMAX-ROUT
(11)
The maximum power dissipation point given by Equation (5)
must not exceed the power dissipation given by Equation
(6):
P
DMAX
= (T
JMAX
- T
A
) /
θ
JA
(12)
The LM4845’s T
JMAX
= 150C. In the ITL package, the
LM4845’s
θ
JA
is 65C/W. At any given ambient temperature
T
A
, use Equation (6) to find the maximum internal power
dissipation supported by the IC packaging. Rearranging
Equation (6) and substituting P
DMAX-TOTAL
for P
DMAX
’ results
in Equation (7). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4845’s maximum junction tem-
perature.
T
A
= T
JMAX
- P
DMAX-TOTAL
θ
JA
(13)
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
ITL package.
T
JMAX
= P
DMAX-TOTAL
θ
JA
+ T
A
(14)
Equation (8) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4845’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 1μF in parallel with a 0.1μF filter capacitors to stabilize
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 1.1μF
tantalum bypass capacitance connected between the
LM4845’s supply pins and ground. Keep the length of leads
and traces that connect capacitors between the LM4845’s
power supply pin and ground as short as possible. Connect-
ing a 2.2μF 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
L
www.national.com
25
相關(guān)PDF資料 |
PDF描述 |
|---|---|
| LM4845ITL | Output Capacitor-less Audio Subsystem with Programmable National 3D |
| LM4850 | Mono 1.5 W / Stereo 300 mW Power Amplifier |
| LM4850LD | Mono 1.5 W / Stereo 300 mW Power Amplifier |
| LM4850MM | Mono 1.5 W / Stereo 300 mW Power Amplifier |
| LM4850MT | Mono 1.5 W / Stereo 300 mW Power Amplifier |
相關(guān)代理商/技術(shù)參數(shù) |
參數(shù)描述 |
|---|---|
| LM4845ITL | 制造商:Texas Instruments 功能描述:Audio Amp Headphone/Speaker 1-CH Mono/2-CH Stereo 1.15W Class-AB 25-Pin uSMD T/R |
| LM4845ITL/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 |
| LM4845ITLX/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 |
| LM4846 | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Output Capacitor-less Audio Subsystem with Programmable National 3D |
| LM4846TL | 制造商:NSC 制造商全稱:National Semiconductor 功能描述:Output Capacitor-less Audio Subsystem with Programmable National 3D |
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