Sensor Based Thermal Specification Design Guidance

Note: This graph is provided as a reference, the complete thermal specification is in Chapter 6.

8.2.2Specification When DTS value is Greater than TCONTROL

The product specification provides a table of ΨCA values at DTS = TCONTROL and DTS = -1 as a function of TAMBIENT (inlet to heatsink). Between these two defined points, a linear interpolation can be done for any DTS value reported by the processor.

The fan speed control algorithm has enough information using only the DTS value and

TAMBIENT to command the thermal solution to provide just enough cooling to keep the part on the thermal profile.

In the prior thermal specifications this region, DTS values greater than TCONTROL, was defined by the processor thermal profile. This required the user to estimate the

processor power and case temperature. Neither of these two data points are accessible in real time for the fan speed control system. As a result, the designer had to assume the worst case TAMBIENT and drive the fans to accommodate that boundary condition.

8.3Thermal Solution Design Process

Thermal solution design guidance for this specification is the same as with previous products. The initial design needs to take into account the target market and overall product requirements for the system. This can be broken down into several steps:

•Boundary condition definition

•Thermal design / modelling

•Thermal testing.

8.3.1Boundary Condition Definition

Using the knowledge of the system boundary conditions (such as inlet air temperature, acoustic requirements, cost, design for manufacturing, package and socket mechanical specifications and chassis environmental test limits) the designer can make informed thermal solution design decisions.

For the thermal boundary conditions for system are as follows:

•TEXTERNAL = 35 °C. This is typical of a maximum system operating environment

•TRISE = 5 °C.

•TAMBIENT = 40 °C (TAMBIENT = TEXTERNAL + TRISE)

Based on the system boundary conditions, the designer can select a TAMBIENT and ΨCA to use in thermal modelling. The assumption of a TAMBIENT has a significant impact on the required ΨCA needed to meet TTV TCASEMAX at TDP. A system that can deliver lower assumed TAMBIENT can utilize a design with a higher ΨCA, which can have a lower cost.

Note: If the assumed TAMBIENT is inappropriate for the intended system environment, the thermal solution performance may not be sufficient to meet the product requirements.

The results may be excessive noise from fans having to operate at a speed higher than intended. In the worst case this can lead to performance loss with excessive activation of the Thermal Control Circuit (TCC).

68

Thermal/Mechanical Specifications and Design Guidelines