10.4 Thermal Considerations

The 8-pin PDIP (P) package and the 14-pin SOIC (NPA) package are molded plastic packages with solid copper lead frames. The copper lead frame conducts the majority of the heat from the die, through the leads, to the printed-circuit board copper, which acts as the heat sink. For best thermal performance, wide copper traces must be used, and all ground and unused pins must be soldered to generous amounts of printed-circuit board copper, such as a ground plane. Large areas of copper provide the best transfer of heat (lower thermal resistance) to the surrounding air, and even double-sided or multilayer boards provide better heat paths to the surrounding air. Unless the power levels are small, using a socket for the 8-pin package is not recommended because of the additional thermal resistance it introduces, and the resultant higher junction temperature.

Because of the 0.5-A current rating of the LM2574, the total package power dissipation for this switcher is quite low, ranging from approximately 0.1 W up to 0.75 W under varying conditions. In a carefully engineered printed-circuit board, both the P and the NPA package can easily dissipate up to 0.75 W, even at ambient temperatures of 60°C, and still keep the maximum junction temperature less than 125°C.

A curve, Figure 14, displaying thermal resistance versus PCB area for the two packages is shown in Typical Characteristics.

These thermal resistance numbers are approximate, and there can be many factors that affect the final thermal resistance. Some of these factors include board size, shape, thickness, position, location, and board temperature. Other factors are, the area of printed-circuit copper, copper thickness, trace width, multi-layer, single- or double-sided, and the amount of solder on the board. The effectiveness of the PCB to dissipate heat also depends on the size, number and spacing of other components on the board. Furthermore, some of these components, such as the catch diode and inductor generate some additional heat. Also, the thermal resistance decreases as the power level increases because of the increased air current activity at the higher power levels, and the lower surface to air resistance coefficient at higher temperatures.

The data sheet thermal resistance curves can estimate the maximum junction temperature based on operating conditions. ln addition, the junction temperature can be estimated in actual circuit operation by using Equation 15.

With the switcher operating under worst case conditions and all other components on the board in the intended enclosure, measure the copper temperature (T_cu ) near the IC. This can be done by temporarily soldering a small thermocouple to the PCB copper near the IC, or by holding a small thermocouple on the PCB copper using thermal grease for good thermal conduction.

The thermal resistance (θ_j-cu) for the two packages is:

θ_j-cu = 42°C/W for the P-8 package

θ_j-cu = 52°C/W for the NPA-14 package

The power dissipation (P_D) for the IC could be measured, or it can be estimated by using Equation 16.

Equation 16.

where

• I_S is obtained from the typical supply current curve (adjustable version use the supply current vs duty cycle curve)