PDF《APPLICATION NOTE》

Thermal Design Considerations―EL75XX APPLICATION NOTE AN1096 Rev 0.

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7 Mar 20,

1998 AN1096 Rev 0.00 Mar 20,

1998 Elantec'

s EL7560/EL7561/EL7556 series of voltage regulators are highly integrated, simple to use and the most effective switching mode designs to power microprocessors such as Intel Corporation'

s Pentium? Pro, and Pentium. The EL7560/EL7561 is designed for classes of processors such as the Pentium Pro. EL7556 is designed primarily for Pentium class desktop models. These devices are packaged in a 28-pin power SO package (28 PSOP 2) developed by Amkor Corporation. The

28 PSOP

2 is a 28-lead surface mount package with

50 mil lead centers. It includes a specially designed copper slug on which the die is attached. The copper slug provides the excellent power dissipation capability required for these devices. Two versions, Slug up and Slug down , are available for various board mounting and heatsink options. This application note describes the various package and heatsink combinations, which can be used with the EL75XX series. Typical Applications Power Requirement The typical power applications requirement for microprocessors is either a VRM (Voltage Regulator Module) or a direct mother board solution, both options operate under certain power dissipation, environmental, physical, and, of course, cost constraints. Table 1, below, summarizes the majority of typical applications requirements. Several other variations in ambient temperature and airflow conditions may exist in practice, depending upon the user'

s board layout and system configurations. Structure and Characteristics of the

28 PSOP

2 Figure

1 illustrates the structure of the Amkor PSOP

2 package. The copper slug is

535 x

170 mils in dimension and its thickness is optimized in order to arrive at a good compromise between biggest size possible for maximum thermal performance and the smallest foot print for the package. The die can either be soft soldered to the slug or attached using a thermally conductive epoxy. TABLE 1. APPLICATION THERMAL REQUIREMENT TYPICAL ENVIRONMENT THERMAL RESISTANCE REQUIRED VRM-Pentium Pro Pdiss = 3.4W, TJ = 105°C TA = 60°C,

100 LFM ?JX = 13°C/W VRM-Pentium P54/55 Pdiss = 2.4W, TJ = 105°C TA = 50°C,

100 LFM ?JX = 23°C/W Laptop-Pentium P54 Pdiss = 1.6W, TJ = 105°C TA = 50°C, No Airflow ?JX = 34.3 C/W Thermal Design Considerations― EL75XX AN1096 Rev 0.00 Page

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7 Mar 20,

1998 The slug provides low thermal resistance from the die to the external heatsink and also through the leads. At the same time, it provides a large thermal capacitance to absorb power peaks during switching. Thermal resistance (?JC―junction to case) for a typical

100 x

100 mil die is 45°C/W. The electrical lead resistance is in the range (5) mmhos;

inductance is typically (2nH) per lead and capacitance is about 1pF. The package comes in two different lead stand-offs: normal stand-off is

9 mils and low stand-off is

2 mils. The two board mounting options are shown in Figure 2. Components of Thermal Resistance and Heat Dissipation Long term reliability of the semiconductor device is enhanced by keeping the junction temperature (TJ) as low as possible consistent with the given application. (See Elantec Reliability and the Electronic Engineer Tutorial #1.) TJ―Junction temperature is a function of the amount of heat dissipated, the thermal resistance between the junction and the ambient air, and the ambient air temperature. Integrated thermal resistance (?IT) is the sum of the following individual thermal resistances. ?JC―Thermal resistance from junction to case (die size, d/a, package dependent). ?CH―Thermal resistance from case to heatsink (interface― package to heatsink dependent). ?HA―Thermal resistance from heatsink to ambient (heatsink dependent). Thus, ?IT = ?JC + ?CH + ?HA (°C/W) As stated in section 2, the thermal resistance required is the total integrated thermal resistance ?IT under the In Situ applications condition. ?JC―Thermal resistance from junction to case depends on the die size, Cu slug size, the die attach material and the lead frame. In the case of PSOP