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参数资料
| 型号： | LM4914MH/NOPB |
| 厂商： | NATIONAL SEMICONDUCTOR CORP |
| 元件分类： | 音频／视频放大 |
| 英文描述： | 0.085 W, 2 CHANNEL, AUDIO AMPLIFIER, PDSO10 |
| 封装： | TSSOP-10 |
| 文件页数： | 2/16页 |
| 文件大小： | 767K |
| 代理商： | LM4914MH/NOPB |
Application Information (Continued)
P
DMAX-MONOBTL = 2(VDD)
2 /2
π2R
L:
Bridge Mode
(3)
The maximum power dissipation point given by Equation (3)
must not exceed the power dissipation given by Equation
(4):
P
DMAX’ =(TJMAX -TA)/
θ
JA
(4)
The LM4914’s T
JMAX = 150C. In the MH package, the
LM4914’s
θ
JA is 46C/W. At any given ambient temperature
TA, 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 mono BTL power
dissipation without violating the LM4914’s maximum junction
temperature.
T
A =TJMAX -PDMAX-MONOBTL
θ
JA
(5)
For a typical application with a 5V power supply and an 8
load, the maximum ambient temperature that allows maxi-
mum BTL power dissipation without exceeding the maximum
junction temperature is approximately 134C for the IBL
package.
T
JMAX =PDMAX-MONOBTL
θ
JA +TA
(6)
Equation (6) gives the maximum junction temperature T
J-
MAX.
If the result violates the LM4914’s 150C T
JMAX, reduce
the maximum junction temperature by decreasing the power
supply voltage or increasing the load resistance. Further
allowance should be made for increased ambient tempera-
tures.
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 (3) is
greater than that of Equation (4), 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 con-
nections 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 Perfor-
mance Characteristics curves for power dissipation informa-
tion at lower output power levels.
EXPOSED-DAP PACKAGE PCB MOUNTING
CONSIDERATIONS
The LM4914’s exposed-DAP (die attach paddle) package
provides a low thermal resistance between the die and the
PCB to which the part is mounted and soldered. This low
thermal resistance is achieved by soldering the DAP to a
copper pad on the PCB. The copper pad’s dimensions
should match the DAP’s. The copper pad should then con-
nect to a larger copper area. This area can be on the
component side, in an inner layer in a multi-layer board, or
on the board’s back side. This connection from the DAP, to
the DAP pad, and finally to a larger copper area allows rapid
heat transfer away from the die to the surrounding air. The
result is a low voltage audio power amplifier that produces
1W at = 1% THD+N with an 8
load. This high power is
achieved through careful consideration of necessary thermal
design. Failing to optimize thermal design may compromise
the LM4914’s high power performance and activate un-
wanted, though necessary, thermal shutdown protection.
The MH package must have its DAP soldered to a copper
pad on the PCB. The DAP’s PCB copper pad is connected to
a large plane of continuous unbroken copper. This plane
forms a thermal mass, and heat sink, and radiation area.
Place the heat sink area on either outside plane in the case
of a two-sided PCB, or on an inner layer of a board with more
than two layers. Connecting to a ground plane is permis-
sible. Connect the DAP copper pad to the inner layer or
backside copper heat sink area with 4(2x2) vias. The via
diameter should be 0.012in-0.013in with a 1.27mm pitch.
Ensure efficient thermal conductivity by plating-through and
solder-filling the vias.
Best thermal performance is achieved with the largest prac-
tical copper heatsink area. If the heatsink and amplifier share
the same PCB layer, a nominal 2.5in
2 (min) area is neces-
sary for 5V operation with an 8
load. The heatsink area
should be 5in
2 (min) when placed on a layer different from
that used by the LM4914. The last two area recommenda-
tions apply for 25C ambient temperature. Increase the area
to compensate for ambient temperatures above 25C. In all
circumstances and conditions, the junction temperature
must be held below 150C to prevent activating the
LM4914’s thermal shutdown protection. The LM4914’s
power de-rating curve in the Typical Performance Character-
istics shows the maximum power dissipation versus tem-
perature. An example PCB layout for the LM4914’s exposed-
DAP package is shown in the Demonstration Board Layout
section.
PCB LAYOUT AND SUPPLY REGULATION
CONSIDERATIONS FOR DRIVING 4
LOADS
Power dissipated by a load is a function of the voltage swing
across the load and the load’s impedance. As load imped-
ance decreases, load dissipation becomes increasingly de-
pendent on the interconnect (PCB trace and wire) resistance
between the amplifier output pins and the load’s connec-
tions. Residual trace resistance causes a voltage drop,
which results in power dissipated in the trace and not in the
load as desired. For example, 0.1
trace resistance reduces
the output power dissipated by an 8
load from 1W to 0.9W.
This problem of decreased load dissipation is exacerbated
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, make the power supply traces
as wide as possible to maintain full output voltage swing.
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 0.47F
tantalum bypass capacitance connected between the
LM4914
www.national.com
10
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