The DDV package is designed to interface directly to a heat sink using a thermal interface compound in between, (for example, Ceramique from Arctic Silver, TIMTronics 413, etc.). The heat sink absorbs heat from the DRV8462 and transfers it to the air. With proper thermal management this process can reach equilibrium and heat can be continually transferred from the device. A concept digram of the heatsink on top of the DDV package is shown in Figure 9-11.

Care must be taken when mounting the heatsinks, ensuring good contact with thermal pads and not exceeding the mechanical stress capability of the parts to avoid breakage. Even though the DDV package is capable of tolerating up to 90 Newton load, in production it is recommended to apply less than 45 Newton load.

R_θJA is a system thermal resistance from junction to ambient air. As such, it is a system parameter with the following components:

• R_θJC of the DDV Package (thermal resistance from junction to exposed pad)

• Thermal resistance of the thermal interface material

• Thermal resistance of the heat sink

R_θJA = R_θJC + thermal interface resistance + heat sink resistance

The thermal resistance of the thermal interface material can be determined from the area of the exposed metal package and manufacturer’s value for the area thermal resistance (expressed in °Cmm²/W). For example, a typical white thermal grease with a 0.0254 mm (0.001 inch) thick layer has 4.52 °Cmm²/W thermal resistance. The DDV package has an exposed area of 28.7 mm². By dividing the area thermal resistance by the exposed metal area determines the thermal resistance for the interface material as 0.157°C/W.

Heat sink thermal resistance is predicted by the heat sink vendor, modeled using a continuous flow dynamics (CFD) model, or measured. The following are the various important parameters in selecting a heatsink.

1. Thermal resistance
2. Airflow
3. Volumetric resistance
4. Fin density
5. Fin spacing
6. Width
7. Length

The thermal resistance is one parameter that changes dynamically depending on the airflow available.

Airflow is typically measured in LFM (linear feet per minute) or CFM (cubic feet per minute). LFM is a measure of velocity, whereas CFM is a measure of volume. Typically, fan manufacturers use CFM because fans are rated according to the quantity of air it can move. Velocity is more meaningful for heat removal at the board level, which is why the derating curves provided by most power converter manufacturers use this.

Typically, airflow is either classified as natural or forced convection.

• Natural convection is a condition with no external induced flow and heat transfer depends on the air surrounding the heatsink. The effect of radiation heat transfer is very important in natural convection, as it can be responsible for approximately 25% of the total heat dissipation. Unless the component is facing a hotter surface nearby, it is imperative to have the heatsink surfaces painted to enhance radiation.

• Forced convection occurs when the flow of air is induced by mechanical means, usually a fan or blower.

Limited thermal budget and space make the choice of a particular type of heatsink very important. This is where the volume of the heatsink becomes relevant. The volume of a heatsink for a given flow condition can be obtained by using the following equation:

Volume_(heatsink) = volumetric resistance (Cm³ °C/W)/thermal resistance θ_SA (°C/W)

An approximate range of volumetric resistance is given in the following table:

The next important criterion for the performance of a heatsink is the width. It is linearly proportional to the performance of the heatsink in the direction perpendicular to the airflow. An increase in the width of a heatsink by a factor of two, three, or four increase the heat dissipation capability by a factor of two, three, or four. Similarly, the square root of the fin length used is approximately proportional to the performance of the heatsink in the direction parallel to the airflow. In case of an increase in the length of the heatsink by a factor of two, three, or four only increases the heat dissipation capability by a factor of 1.4, 1.7, or 2.

If the board has sufficient space, it is always beneficial to increase the width of a heatsink rather than the length of the heatsink. This is only the beginning of an iterative process before the correct and the actual heatsink design is achieved.

The heat sink must be supported mechanically at each end of the IC. This mounting ensures the correct pressure to provide good mechanical, thermal and electrical contact. The heat sink should be connected to GND or left floating.