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参数资料
| 型号： | AN-1192 |
| 厂商： | National Semiconductor Corporation |
| 英文描述： | broad portfolio of monolithic power integrated circuits covering power levels |
| 中文描述： | 丰富的产品组合覆盖功率级别单片功率集成电路 |
| 文件页数： | 3/21页 |
| 文件大小： | 548K |
| 代理商： | AN-1192 |
4.0 Thermal Background
(Continued)
forward. Therefore, the designer must understand the IC’s
power dissipation capabilities before using the IC in a
booster configuration.
4.1 TYPICAL CHARACTERISTIC DATA
The power dissipation capabilities of a power IC are either
specified in the datasheet or can be derived from its guar-
anteed output power specification. While the power dissipa-
tion rating for the LM3886T is 125W, this number can be
misleading. Its power dissipation specification is derived
from the IC’s junction-to-case thermal resistance,
θ
=
1C/W, the maximum junction temperature, T
= 150C, and
the ambient air, T
= 25C. As stated in the datasheet, the
device must be derated based on these parameters while
operating at elevated temperatures. The heat sinking re-
quirements for the application are based on these param-
eters so that the IC will not go into Thermal Shutdown (TSD).
The real problem for Overture ICs, however, comes from the
sensitivity of the output stage’s unique SPiKe
Protection
which dynamically monitors the output transistor’s tempera-
ture. While the thermal shutdown circuitry is enabled at T
=
150C, SPiKe circuitry is enabled at T
= 250C for instanta-
neous power spikes in the output stage transistor. As the
overall temperature of the IC increases, SPiKe circuitry be-
comes even more sensitive causing it to turn on before the
125W limit is reached. TSD circuitry will continue to function
globally for the IC in conjunction with the SPiKe circuitry.
However, protection circuitry should not be activated under
normal operating conditions. The question then becomes,
what is the power dissipation limit for the IC such that SPiKe
circuitry is not enabled Knowing the power dissipation limit
and keeping the case temperature of the IC as cool as
possible will expand the output power capability without
activating SPiKe Protection.
The other way to determine IC power dissipation capabilities
is to analyze the output power specification in the datasheet.
In the case of the LM3886T, there are two output power
specification guarantees: 60W (min) into a 4
load using
±
28V supplies and 50W(typ) into an 8
load from
±
35V
supplies. Using these two conditions and the theoretical
maximum power dissipation equation shown below, results
in the following maximum power dissipations:
4.2 SINGLE-ENDED AMPLIFIER Pdmax EQUATION:
Pdmax = V
CCtot
2
/2
π
2
R
L
Non-Isolated LM3886T:
1. V
CC
=
±
28V, R
L
= 4
Pdmax = V
CCtot
2
/2
π
2
R
L
= (
±
28V)
2
/2
π
2
(4
) = 39.7W
2. V
CC
=
±
35V, R
L
= 8
Pdmax = V
CCtot
2
/2
π
2
R
L
= (
±
35V)
2
/2
π
2
(8
) = 31.0W
These results show that the IC can handle a maximum of
≈
40W of continuous power dissipation without SPiKe Pro-
tection being turned on under continuous sinusoidal input
with proper heat sinking. The same theory applies to other
Overture ICs as well, like the LM3876T, which is capable of
dissipating 31W with proper heat sinking. It should be noted
that the results shown above are for the non-isolated power
package, where the back of the package is tied to the silicon
substrate, or Vee. The isolated power package has over-
molded plastic on the back keeping the package electrically
isolated from the silicon substrate. This extra amount of
plastic increases the package thermal resistance from
1C/W for the non-isolated version to
≈
2C/W for the iso-
lated version. The result of increased thermal resistance is
higher die temperature under the same conditions even
though the heat sink temperature will not change.
There are two major points to note:
1.
The maximum power dissipation analysis was taken into
account using regulated power supplies. The IC for the
whole analysis is being tested at the worst case power
dissipation point for a constant full-load power supply
voltage. When using an unregulated power supply, the
no-load voltage will be somewhat higher (15%–35%)
causing the overall maximum power dissipation to be
higher than expected.
2.
In the real “audio” application, the average music power
dissipation is much less than the maximum power dissi-
pation created by a sinusoidal input. Therefore, the IC
will run cooler than expected due to the lower power
dissipation.
However, when you put these two points together, they
mostly cancel out, but only for music stimulus. Product quali-
fications may go through worse case power dissipation sce-
narios which implies that sinusoids will be used with unregu-
lated power supplies. Therefore, when doing the thermal
portion of the design, the higher supply voltages will increase
the IC power dissipation and must be taken into account.
4.3 BRIDGED-OUTPUT AMPLIFIER Pdmax EQUATION
To determine the Pdmax equation for a bridged amplifier
solution, the single-ended Pdmax equation is used as a
starting point. A bridged amplifier solution requires two am-
plifiers and each amplifier will see 1/2 the total impedance.
Adding these factors of 2 and 1/2 into the single-ended
Pdmax equation results in the total Pdmax equation for a
bridged amplifier.
Pdmax
BTL
= 2*[V
CCtot
2
/2
π
2
(/
1
2
R
L
)] = 4*(V
CCtot
2
/2
π
2
R
L
)
The bridged-output Pdmax equation represents the bridged
amplifier solution. If a dual amplifier IC is used, then the total
Pdmax would need to be dissipated in the single IC package.
However, if two individual ICs are used, then the total power
dissipation is divided between each IC.
Two Non-Isolated LM3886Ts:
V
CC
=
±
28V, R
L
= 4
Pdmax = 4V
CCtot
2
/2
π
2
R
L
= 4(
±
28V)
2
/2
π
2
(4
) = 158.8W
Pdmax = 158.8W
Pdmax/IC = 79.4W
Therefore, using a bridged configuration, V
cc
would have to
be equal to
±
20V to keep the IC’s power dissipation within
A
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3
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