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參數(shù)資料
| 型號(hào)： | LM4867MTX/NOPB |
| 廠商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | 2 CHANNEL, AUDIO AMPLIFIER, PDSO20 |
| 封裝： | 4.40 MM, PLASTIC, TSSOP-20 |
| 文件頁數(shù)： | 5/26頁 |
| 文件大?。?/td> | 763K |
| 代理商： | LM4867MTX/NOPB |
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Application Information (Continued)
the bridge mode is four times that of a single-ended ampli-
fier. From Equation (3), assuming a 5V power supply and an
4
load, the maximum single channel power dissipation is
1.27W or 2.54W for stereo operation.
P
DMAX =4 * (VDD)
2/(2
π2R
L):
Bridge Mode
(3)
The LM4867’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)/θJA
(4)
The LM4867’s TJMAX = 150C. In the LQ package soldered
to a DAP pad that expands to a copper area of 5in
2 on a
PCB, the LM4867’s
θ
JA is 20C/W. In the MTE package
soldered to a DAP pad that expands to a copper area of 2in
2
on a PCB, the LM4867’s
θ
JA is 41C/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 LM4867’s maximum junction
temperature.
T
A =TJMAX 2XPDMAX θJA
(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 LQ
package and 45C for the MTE package.
T
JMAX =PDMAX θJA +TA
(6)
Equation (6) gives the maximum junction temperature
T
JMAX. If the result violates the LM4867’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 θJC, θCS, and θSA.(θJC is the
junctiontocase
thermal
impedance,
θ
CS
is
the
casetosink
thermal
impedance,
and
θ
SA
is
the
sinktoambient thermal impedance.) Refer to the Typical
Performance Characteristics curves for power dissipation
information 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 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
presence does not eliminate the need for a local 1.0F
tantalum bypass capacitance connected between the
LM4867’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 LM4867’s power supply pin and
ground as short as possible. Connecting a 1F 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 increase as the bypass pin
capacitor value increases. Too large, however, increases
turnon time and can compromise the amplifier’s click and
pop performance. The selection of bypass capacitor values,
especially C
B, depends on desired PSRR requirements,
click and pop performance (as explained in the section,
Proper Selection of External Components), system cost,
and size constraints.
MICROPOWER SHUTDOWN
The voltage applied to the SHUTDOWN pin controls the
LM4867’s shutdown function. Activate micropower shut-
down by applying V
DD to the SHUTDOWN pin. When active,
the LM4867’s micropower shutdown feature turns off the
amplifier’s bias circuitry, reducing the supply current. The
logic threshold is typically V
DD/2. The low 0.7A typical
shutdown 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.
Table 1 shows the logic signal levels that activate and deac-
tivate micropower shutdown and headphone amplifier op-
eration. To ensure that the output signal remains
transientfree, do not cycle the shutdown function
faster than 1Hz.
There are a few ways to control the micropower shutdown.
These include using a singlepole, single, throw switch, a
microprocessor, or a microcontroller. When using a switch,
connect an external 100k
pullup resistor between the
SHUTDOWN pin and V
DD. Connect the switch between the
SHUTDOWN pin and ground. Select normal amplifier opera-
tion by closing the switch. Opening the switch connects the
SHUTDOWN pin to V
DD through the pullup resistor, acti-
vating micropower shutdown. The switch and resistor guar-
antee that the SHUTDOWN pin will not float. This prevents
unwanted state changes. In a system with a microprocessor
or a microcontroller, 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.
LM4867
www.national.com
13
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