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參數(shù)資料
| 型號： | LM4888SQ/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 3 W, 2 CHANNEL, AUDIO AMPLIFIER, QCC24 |
| 封裝： | LLP-24 |
| 文件頁數(shù)： | 7/23頁 |
| 文件大?。?/td> | 1181K |
| 代理商： | LM4888SQ/NOPB |
Application Information (Continued)
The LM4888’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 LM4888’s T
JMAX = 150C. In the SQ package soldered
to a DAP pad that expands to a copper area of 5in
2 on a
PCB, the LM4888’s
θ
JA is 20C/W. At any given ambient
temperature T
A, use Equation (4) to find the maximum inter-
nal power dissipation supported by the IC packaging. Rear-
ranging Equation (4) and substituting P
DMAX for PDMAX' re-
sults in Equation (5). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4888’s maximum junction
temperature.
T
A =TJMAX – 2*PDMAX
θ
JA
(5)
For a typical application with a 5V power supply and an 4
load, the maximum ambient temperature that allows maxi-
mum stereo power dissipation without exceeding the maxi-
mum junction temperature is approximately 99C for the SQ
package.
T
JMAX =PDMAX
θ
JA +TA
(6)
Equation (6) gives the maximum junction temperature
T
JMAX. If the result violates the LM4888’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 (2) is greater than that of Equation
(3), 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 connections 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
JA is the sum of
θ
JC,
θ
CS, and
θ
SA.(
θ
JC is the
junction-to-case thermal impedance,
θ
CS is the case-to-sink
thermal impedance, and
θ
SA is the sink-to-ambient thermal
impedance.) Refer to the Typical Performance Character-
istics curves for power dissipation information at lower out-
put 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 10 F in parallel with a 0.1 F filter capacitor 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 1.0 F
tantalum bypass capacitance connected between the
LM4888’s supply pins and ground. Do not substitute a ce-
ramic capacitor for the tantalum. Doing so may cause oscil-
lation. Keep the length of leads and traces that connect
capacitors between the LM4888’s power supply pin and
ground as short as possible.
MICRO-POWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4888’s shutdown function. Activate micro-power shut-
down by applying GND to the SHUTDOWN pin. When ac-
tive, the LM4888’s micro-power shutdown feature turns off
the amplifier’s bias circuitry, reducing the supply current. The
low 0.04 A typical shutdown current is achieved by applying
a voltage that is as near as GND as possible to the SHUT-
DOWN pin. A voltage that is more than GND may increase
the shutdown current. Table 1 shows the logic signal levels
that activate and deactivate micro-power shutdown and
headphone amplifier operation.
There are a few ways to control the micro-power shutdown.
These include using a single-pole, single-throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 100k resistor between the SHUTDOWN
pin and Ground. Connect the switch between the SHUT-
DOWN pin V
DD. Select normal amplifier operation by closing
the switch. Opening the switch sets the SHUTDOWN pin to
ground through the 100k resistor, which activates the micro-
power shutdown. The switch and resistor guarantee that the
SHUTDOWN pin will not float. This prevents unwanted state
changes. In a system with a microprocessor or a microcon-
troller, use a digital output to apply the control voltage to the
SHUTDOWN pin. Driving the SHUTDOWN pin with active
circuitry eliminates the pull up resistor.
LM4888
www.national.com
15
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