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参数资料
| 型号： | LM4836MTX/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频控制 |
| 英文描述： | 2 CHANNEL(S), TONE CONTROL CIRCUIT, PDSO28 |
| 封装： | TSSOP-28 |
| 文件页数： | 5/27页 |
| 文件大小： | 836K |
| 代理商： | LM4836MTX/NOPB |
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Application Information (Continued)
However, a direct consequence of the increased power
delivered to the load by a bridge amplifier is higher internal
power dissipation for the same conditions.
The LM4836 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
amplifier. From Equation (3), assuming a 5V power supply
anda4
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 LM4836’s power dissipation is twice that given by
Equation (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 exceed the power dissipation given by Equation
(4):
P
DMAX'=(TJMAX TA)/θJA
(4)
The LM4836’s T
JMAX = 150C. In the LQ package soldered
to a DAP pad that expands to a copper area of 5in
2 on a
PCB, the LM4836’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 LM4836’s
θ
JA is 41C/W. At any given ambient
temperature T
A,
use Equation (4) to find the maximum
internal power dissipation supported by the IC packaging.
Rearranging Equation (4) and substituting P
DMAX for PDMAX'
results in Equation (5). This equation gives the maximum
ambient temperature that still allows maximum stereo power
dissipation without violating the LM4836’s maximum junction
temperature.
T
A =TJMAX – 2*PDMAX θJA
(5)
For a typical application with a 5V power supply and an 4
load, the maximum ambient temperature that allows
maximum stereo power dissipation without exceeding the
maximum 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 LM4836’s 150C, reduce the
maximum junction temperature by reducing the power
supply voltage or increasing the load resistance. Further
allowance
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
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
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 10 F in parallel with a 0.1 F 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 1.0 F
tantalum bypass capacitance connected between the
LM4836’s supply pins and ground. Do not substitute a
ceramic capacitor for the tantalum. Doing so may cause
oscillation. Keep the length of leads and traces that connect
capacitors between the LM4836’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
turn-on time and can compromise 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.
PROPER SELECTION OF EXTERNAL COMPONENTS
Optimizing the LM4836’s performance requires properly
selecting
external
components.
Though
the
LM4836
operates well when using external components with wide
tolerances, best performance is achieved by optimizing
component values.
The LM4836 is unity-gain stable, giving a designer maximum
design flexibility. The gain should be set to no more than a
given application requires. This allows the amplifier to
achieve minimum THD+N and maximum signal-to-noise
ratio.
These
parameters
are
compromised
as
the
closed-loop gain increases. However, low gain demands
input signals with greater voltage swings to achieve
maximum output power. Fortunately, many signal sources
such as audio CODECs have outputs of 1V
RMS (2.83VP-P).
Please refer to the Audio Power Amplifier Design section
for more information on selecting the proper gain.
Input Capacitor Value Selection
Amplifying the lowest audio frequencies requires high value
input coupling capacitor (0.33F in
Figure 1). A high value
capacitor can be expensive and may compromise space
efficiency in portable designs. In many cases, however, the
speakers used in portable systems, whether internal or
external, have little ability to reproduce signals below 150Hz.
Applications using speakers with this limited frequency
response reap little improvement by using large input
capacitor.
Besides effecting system cost and size, the input coupling
capacitor has an affect on the LM4835’s click and pop
performance. When the supply voltage is first applied, a
LM4836
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