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參數(shù)資料
| 型號： | LM4851LQ/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 1.5 W, 3 CHANNEL, AUDIO AMPLIFIER, QCC24 |
| 封裝： | LLP-24 |
| 文件頁數(shù)： | 8/22頁 |
| 文件大?。?/td> | 1126K |
| 代理商： | LM4851LQ/NOPB |
Application Information (Continued)
as load impedance decreases. Therefore, to maintain the
highest load dissipation and widest output voltage swing,
PCB traces that connect the output pins to a load must be as
wide as possible.
Poor power supply regulation adversely affects maximum
output power. A poorly regulated supply’s output voltage
decreases with increasing load current. Reduced supply
voltage causes decreased headroom, output signal clipping,
and reduced output power. Even with tightly regulated sup-
plies, trace resistance creates the same effects as poor
supply regulation. Therefore, making the power supply
traces as wide as possible helps maintain full output voltage
swing.
BRIDGE CONFIGURATION EXPLANATION
As shown in Figure 1, the LM4851 consists of three pairs of
output amplifier blocks (A4-A6). A4, A5, and A6 consist of
bridged-tied amplifier pairs that drive LOUT, ROUT, and
SPKROUT respectively. The LM4851 drives a load, such as
a speaker, connected between outputs, SPKROUT+ and
SPKROUT-. In the amplifier block A6, the output of the
amplifier that drives SPKROUT- serves as the input to the
unity gain inverting amplifier that drives SPKROUT+.
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 SPKROUT- and
SPKROUT+ and driven differentially (commonly referred to
as ’bridge mode’). Assuming R
F =RI, this results in a differ-
ential or BTL gain of:
A
VD = 2(Rf/Ri)=2
(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. 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
SPKROUT- and SPKROUT+ outputs at half-supply. This
eliminates the coupling capacitor that single supply, single-
ended amplifiers require. Eliminating an output coupling ca-
pacitor in a typical single-ended configuration forces a
single-supply amplifier’s half-supply bias voltage across the
load. This 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 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 LM4851 has a pair of bridged-tied amplifiers
driving a handsfree speaker, SPKROUT. The maximum in-
ternal 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
SPKROUT power dissipation is 634mW.
P
DMAX-SPKROUT = 4(VDD)
2/(2
π2 R
L): Bridge Mode
(2)
The LM4851 also has 2 pairs of bridged-tied amplifiers driv-
ing stereo headphones, ROUT and LOUT. The maximum
internal power dissipation for ROUT and LOUT 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 LOUT and ROUT is 158mW, or 316mW total.
P
DMAX-LOUT = 4(VDD)
2/(2
π2 R
L): Bridge Mode
(3)
P
DMAX-ROUT = 4(VDD)
2/(2
π2 R
L): Bridge Mode
(4)
The maximum internal power dissipation of the LM4851
occurs when all 3 amplifiers pairs are simultaneously on; and
is given by Equation (5).
P
DMAX-TOTAL =
P
DMAX-SPKROUT +PDMAX-LOUT +PDMAX-ROUT
(5)
The maximum power dissipation point given by Equation (5)
must not exceed the power dissipation given by Equation
(6):
P
DMAX’= (TJMAX -TA)/
θ
JA
(6)
The LM4851’s TJMAX = 150C. In the IBL and ITL packages,
the LM4851’s
θ
JA is 48C/W. In the LQ package soldered to
a DAP pad that expands to a copper area of 2.5in
2 on a
PCB, the LM4851’s
θ
JA is 42C/W. At any given ambient
temperature T
A, use Equation (6) to find the maximum inter-
nal power dissipation supported by the IC packaging. Rear-
ranging Equation (6) and substituting P
DMAX-TOTAL for PD-
MAX
’ results in Equation (7). This equation gives the
maximum ambient temperature that still allows maximum
stereo power dissipation without violating the LM4851’s
maximum junction temperature.
T
A =TJMAX -PDMAX-TOTAL
θ
JA
(7)
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
IBL package.
T
JMAX =PDMAX-TOTAL
θ
JA +TA
(8)
Equation (8) gives the maximum junction temperature T
J-
MAX
. If the result violates the LM4851’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
LM4851
www.national.com
16
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