Fan Speed Control Scheme with DTS
With Digital Thermal Sensor (DTS) 1.1
To correctly use DTS 1.1, the designer must first select a worst case scenario TAMBIENT, and ensure that the Fan Speed Control (FSC) can provide a Ψ CA that is equivalent or greater than the Ψ CA specification.
- a Ψ CA at T CONTROL
- a Ψ CA at DTS = -1
The Ψ CA point at DTS = -1 defines the minimum Ψ CA required at Processor Base Power (a.k.a TDP) considering the worst case system design T AMBIENT design point:
Ψ CA = (T CASE-MAX - T AMBIENT-TARGET - 1 ) / TDP
For example, for a 125 W Processor Base Power (a.k.a TDP) part, the T CASE maximum is 62.0 °C and at a worst case design point of 40 °C local ambient this will result in:
Ψ CA = (62.0 - 40 - 1) / 125 = 0.168 °C/W
Similarly for a system with a design target of 45 °C ambient, the Ψ CA at DTS = -1 needed will be 0.128 °C/W.
The second point defines the thermal solution performance (Ψ CA ) at T CONTROL . The following table lists the required Ψ CA for the various Processor Base Power (a.k.a TDP) processors.
These two points define the operational limits for the processor for DTS 1.1 implementation. At T CONTROL the fan speed must be programmed such that the resulting Ψ CA is better than or equivalent to the required Ψ CA listed in the following table. Similarly, the fan speed should be set at DTS = -1 such that the thermal solution performance is better than or equivalent to the ΨCA requirements at T AMBIENT-MAX .
The fan speed controller must linearly ramp the fan speed from processor DTS = T CONTROL to processor DTS = -1.
With Digital Thermal Sensor (DTS) 2.0
To simplify processor thermal specification compliance, the processor calculates the DTS Thermal Profile from TCONTROL Offset, TCC Activation Temperature, Processor Base Power (a.k.a TDP), and the Thermal Margin Slope provided in the following table.
Using the DTS Thermal Profile, the processor can calculate and report the Thermal Margin, where a value less than 0 indicates that the processor needs additional cooling, and a value greater than 0 indicates that the processor is sufficiently cooled.