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參數(shù)資料
| 型號： | L7372 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | High Speed, High Output Current, Dual Operational Amplifier |
| 中文描述： | 高速，高輸出電流，雙路運算放大器 |
| 文件頁數(shù)： | 12/15頁 |
| 文件大?。?/td> | 417K |
| 代理商： | L7372 |
Application Notes
(Continued)
tance of these packages below 100C/W by these methods
and several manufacturers, including National, offer pack-
age modifications to enhance the thermal characteristics.
Improved removal of internal heat can be achieved by di-
rectly connecting bond wires to the lead frame inside the
package. Since this lead frame supports the die attach
paddle, heat is transferred directly from the substrate to the
outside copper by these bond wires. For an 8 pin package,
this enhancement is somewhat limited since only the V-bond
wire can be used, because it is the only lead at the same
voltage as the substrate and there is an electrical connection
as well as a thermal connection.
The LM7372 is available in the SOIC-16 package. Since only
8 pins are needed for the two operational amplifiers, the
remaining pins are used for heat sink purposes. Each of the
end pins, 1,8,9 & 16 are internally bonded to the lead frame
and form an effective means of transferring heat to external
copper. This external copper can be either electrically iso-
lated or be part of the topside ground plane in a single supply
application.
Figure 2 shows a copper pattern which can be used to
dissipate internal heat from the LM7372. Table 1 gives some
values of
θ
JA
for different values of L and H with 1oz copper.
TABLE 1. Thermal Resistance with Area of Cu
Package
SOIC 16
SOIC 16
SOIC 16
L (in)
1
2
3
H (in)
0.5
1
1.5
θ
JA
(C/W)
83
70
67
From Table 1 it is apparent that two areas of 1oz copper at
each end of the package, each 2 in
2
in area (for a total of
2600mm
2
) will be sufficient to hold the maximum junction
temperature under 120C with an 85C ambient temperature.
An even better package for removing internally generated
heat is a package with an exposed die attach paddle. The
LM7372 is also available in the 8 lead LLP and PSOP
packages. For these packages the entire lower surface of
the paddle is not covered with plastic, which would otherwise
act as a thermal barrier to heat transfer. Heat is transferred
directly from the die through the paddle rather than through
the small diameter bonding wires. Values of
θ
in C/W for
the LLP package with various areas and weights of copper
are tabulated below.
TABLE 2. Thermal Resistance of LLP Package
Copper
Top
Layer
Only
Bottom
Layer
Only
Top And
Bottom
Area
0.5 oz
1.0 oz
2.0 oz
0.5 oz
1.0 oz
2.0 oz
0.5 oz
1.0 oz
2.0 oz
0.5 in
2
115
91
74
102
92
85
83
71
63
1.0 in
2
105
79
60
88
75
66
70
57
48
2.0 in
2
102
72
52
81
65
54
63
47
37
Table 2 clearly demonstrates the superior thermal qualities
of the exposed pad package. For example, using the topside
copper only in the same way as shown for the SOIC package
(Figure 2), with the L dimension held at 1 inch, the LLP
requires half the area of 1 oz copper at each end of the
package (1 in
2
, for a total of 1300mm
2
), for a comparable
thermal resistance of 72C/Watt. This gives considerably
more flexibility in the pcb layout aside from using less cop-
per.
The shape of the heat sink shown in Figure 2is necessary to
allow external components to be connected to the package
pins. If thermal vias are used beneath the LLP to the bottom
side ground plane, then a square pattern heat sink can be
used and there is no restriction on component placement on
the top side of the board. Even better thermal characteristics
are obtained with bottom layer heatsinking. A 2 inch square
of 0.5oz copper gives the same thermal resistance (81C/W)
as a competitive thermally enhanced SO-8 package which
needs two layers of 2 oz copper, each 4 in
2
(for a total of
5000 mm
2
). With heavier copper, thermal resistances as low
as 54C/W are possible with bottom side heatsinking only,
substantially improving the long term reliability since the
maximum junction temperature is held to less than 110C,
even with an ambient temperature of 85C. If both top and
bottom copper planes are used, the thermal resistance can
be brought to under 40C/W.
Power Supplies
The LM7372 is fabricated on a high voltage, high speed
process. Using high supply voltages ensures adequate
headroom to give low distortion with large signal swings. In
Figure 1 a single 24V supply is used. To maximize the
output dynamic range the non-inverting inputs are biassed to
half supply voltage by the resistive divider R1, R2. The input
signals are AC coupled and the coupling capacitors (C1, C2)
can be scaled with the bias resistors (R3, R4) to form a high
pass filter if unwanted coupling from the POTS signal oc-
curs.
Supply decoupling is important at both low and high frequen-
cies. The 10μF Tantalum and 0.1μF Ceramic capacitors
should be connected close to the supply Pin 14. Note that
the V
pin (pin 6), and the PCB area associated with the
heatsink (Pins 1,8,9 & 16) are at the same potential. Any
layout should avoid running input signal leads close to this
ground plane, or unwanted coupling of high frequency sup-
ply currents may generate distortion products.
Although this application shows a single supply, conversion
to a split supply is straightforward. The half supply resistive
20004925
FIGURE 2. Copper Heatsink Patterns
L
www.national.com
12
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