Specifications

Thermal/Mechanical Design Guide 35
Sensor Based Thermal Specification Design Guidance
5.4.2 Fan Speed Control Algorithm with T
AMBIENT
Data
In a system where the FSC algorithm has access to the T
AMBIENT
information and is
capable of using the data the benefits of the DTS thermal specification become more
striking.
As will be demonstrated below, there is still over cooling of the processor, even when
compared to a nominally ambient aware thermal solution equipped with a thermistor.
An example of these thermal solutions are the RCFH5 or the boxed processor thermal
solutions. This over cooling translates into acoustic margin that can be used in the
overall system acoustic budget.
In this example the following assumptions are made:
•T
AMBIENT
= 35 °C
• Thermal Solution designed / validated to a 39 °C environment
•T
CONTROL
= -20
• FSC device has access to T
AMBIENT
• Reference processor thermal solution (RCFH5)
•Below T
CONTROL
the fan speed is slowed down as in prior products
For a processor specification based on a T
CASE
thermal profile, when the DTS value is
equal to or greater than T
CONTROL
, the fan speed is accelerated to maximum fan speed
for the T
AMBIENT
as controlled by the thermistor in thermal solution. For the RCFH5, this
would be about 2500 RPM at 35 °C. This is graphically displayed as the dashed line in
Figure 5-7.
This is an improvement over the ambient unaware system but is not fully optimized for
acoustic benefit. The DTS thermal specification required Ψ
CA
and therefore the fan
speed in this scenario is 1450 RPM. This is less than thermistor controlled speed of
2500 RPM - even if the assumption is a T
AMBIENT
= 35 °C. This is graphically displayed
in Figure 5-7.
The shaded area displayed in Figure 5-7 is where DTS values are less than T
CONTROL
.
For simplicity, the graph shows a linear acceleration of the fans from T
CONTROL
- 10 to
T
CONTROL
as has been Intel’s guidance for simple fan speed control algorithms.
As the processor workload continues to increase, the DTS value will increase and the
FSC algorithm will linearly increase the fan speed from the 1450 RPM at DTS = -20 to
2250 RPM at DTS value = -1.
Figure 5-7. Fan Response with T
AMBIENT
Aware FSC