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Thermal/Mechanical Specifications and Design Guidelines 71
Sensor Based Thermal Specification Design Guidance
8.4.1 DTS 1.1 A New Fan Speed Control Algorithm without
T
AMBIENT
Data
In most system designs incorporating processor ambient inlet data in fan speed control
adds design and validation complexity with a possible BOM cost impact to the system.
A new fan speed control methodology is introduced to improve system acoustics
without needing the processor inlet ambient information.
The DTS 1.1 implementation consists of two parts, a Ψ
CA
requirement at T
CONTROL
and
a Ψ
CA
point at DTS = -1.
The Ψ
CA
point at DTS = -1 defines the minimum Ψ
CA
required at TDP considering the
worst case system design Tambient design point:
Ψ
CA
= (T
CASE_max
- T
Ambient target
) / TDP
For example, for a 95 TDP part, the Tcase max is 72.6C and at a worst case design
point of 40C local ambient this will result in
Ψ
CA
= (72.6 - 40) / 95 = 0.34 C/W
Similarly for a system with a design target of 45 C ambient the Ψ
CA
at DTS = -1 needed
will be 0.29 C/W.
The second point defines the thermal solution performance (Ψ
CA
) at T
CONTROL
.
Figure 8-1 lists the required Ψ
CA
for various 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 programed such that the resulting
Ψ
CA
is better than or equivalent to the required Ψ
CA
listed in Table 8-1. Similarly the fan
speed should be set at DTS = -1 such that the thermal solution performance is better
than or equivalent to the
Ψ
CA
requirements at Tambient_Max. Based on the processor
temperature, the fan speed controller must linearly change the fan speed from DTS =
T
CONTROL
to DTS = -1 between these points. Figure 8-3 gives a visual description on
DTS 1.1.