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參數(shù)資料
| 型號： | LM4816MT/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 1.5 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO20 |
| 封裝： | 4.40 MM, TSSOP-20 |
| 文件頁數(shù)： | 13/13頁 |
| 文件大?。?/td> | 432K |
| 代理商： | LM4816MT/NOPB |
Application Information (Continued)
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4816 consists of two pairs of
operational amplifiers, forming a two-channel (channel A and
channel B) stereo amplifier. (Though the following discusses
channel A, it applies equally to channel B.) External resistors
R
f and Ri set the closed-loop gain of Amp1A, whereas two
internal 20k
resistors set Amp2A’s gain at -1. The LM4816
drives a load, such as a speaker, connected between the two
amplifier outputs, -OUTA and +OUTA.
Figure 1 shows that Amp1A’s output serves as Amp2A’s
input. This results in both amplifiers producing signals iden-
tical in magnitude, but 180 out of phase. Taking advantage
of this phase difference, a load is placed between -OUTA
and +OUTA and driven differentially (commonly referred to
as "bridge mode"). This results in a differential gain of
A
VD =2x(Rf /Ri)
(1)
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. This produces four times the output power
when compared to a single-ended amplifier under the same
conditions. This increase in attainable output power as-
sumes that the amplifier is not current limited or that the
output signal is not clipped. To ensure minimum output sig-
nal clipping when choosing an amplifier’s closed-loop gain,
refer to the Audio Power Amplifier Design section.
Another advantage of the differential bridge output is no net
DC voltage across the load. This is accomplished by biasing
channel A’s and channel B’s outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a single-ended configuration forces a single-supply
amplifier’s half-supply bias voltage across the load. This
increases internal IC power dissipation and may perma-
nently damage loads such as speakers.
POWER DISSIPATION
Power dissipation is a major concern when designing a
successful single-ended or bridged amplifier. Equation (2)
states the maximum power dissipation point for a single-
ended amplifier operating at a given supply voltage and
driving a specified output load
P
DMAX =(VDD)
2 /(2
π2 R
L) Single-Ended
(2)
However, a direct consequence of the increased power de-
livered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4816 has two operational amplifiers per channel. The
maximum internal power dissipation per channel operating in
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and an
8
load, the maximum single channel power dissipation is
0.633W or 1.27W for stereo operation.
P
DMAX =4x(VDD)
2 /(2
π2 R
L) Bridge Mode
(3)
The LM4816’s power dissipation is twice that given by Equa-
tion (2) or Equation (3) when operating in the single-ended
mode or bridge mode, respectively. Twice the maximum
power dissipation point given by Equation (3) must not ex-
ceed the power dissipation given by Equation (4):
P
DMAX’= (TJMAX TA)/
θ
JA
(4)
The LM4816’s T
JMAX = 150C. In the MT (TSSOP) package,
the LM4816’s
θ
JA is 80C/W. At any given ambient tempera-
ture T
J\A, use Equation (4) to find the maximum internal
power dissipation supported by the IC packaging. Rearrang-
ing Equation (4) and substituting PDMAX for PDMAX’ results
in Equation (5). This equation gives the maximum ambient
temperature that still allows maximum stereo power dissipa-
tion without violating the LM4816’s maximum junction tem-
perature.
T
A =TJMAX 2xPDMAX
θ
JA
(5)
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 48C.
T
JMAX =PDMAX
θ
JA +TA
(6)
Equation (6) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4816’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 twice the value given by Equation (3) exceeds the value
given by Equation (4), then decrease the supply voltage,
increase the load impedance, or reduce the ambient tem-
perature.
OUTPUT VOLTAGE LIMITER
The LM4816’s adjustable output voltage limiter can be used
to set a maximum and minimum output voltage swing mag-
nitude. The voltage applied to the V
LIM input (pin 20) controls
the amount voltage limit magnitude.
Without the limiter’s influence (V
LIM = 0V), the LM4816’s
maximum BTL output swing is nominally
2xV
DD
When the limiter input voltage is greater than 0V, the BTL
output voltage swing is
V
OUT-BTL =(2xVDD)-(4xVLIM)
with a tolerance of ±800 mV.
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
LM4816
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