tance of these packages below 100˚C/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

TABLE 1. Thermal Resistance with Area of Cu

Package

L (in)

H (in)

θ

SOIC 16

1

0.5

83

SOIC 16

2

1

70

SOIC 16

3

1.5

67

From

Table 1 it is apparent that two areas of 1oz copper at

each end of the package, each 2 in

2600mm

temperature under 120˚C with an 85˚C 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

the LLP package with various areas and weights of copper

are tabulated below.

TABLE 2. Thermal Resistance of LLP Package

Copper

Area

0.5 in

2

1.0 in

2

2.0 in

2

Top

Layer

Only

0.5 oz

1.0 oz

2.0 oz

115

91

74

105

79

60

102

72

52

Bottom

Layer

Only

0.5 oz

1.0 oz

2.0 oz

102

92

85

88

75

66

81

65

54

Top And

Bottom

0.5 oz

1.0 oz

2.0 oz

83

71

63

70

57

48

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

thermal resistance of 72˚C/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 2 is 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 (81˚C/W)

as a competitive thermally enhanced SO-8 package which

needs two layers of 2 oz copper, each 4 in

5000 mm

as 54˚C/W are possible with bottom side heatsinking only,

substantially improving the long term reliability since the

maximum junction temperature is held to less than 110˚C,

even with an ambient temperature of 85˚C. If both top and

bottom copper planes are used, the thermal resistance can

be brought to under 40˚C/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

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

www.national.com

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