Specifications

Sensor Based Thermal Specification Design Guidance
34 Thermal/Mechanical Design Guide
5.4.1 Fan Speed Control Algorithm without T
AMBIENT
Data
In a system that does not provide the FSC algorithm with the T
AMBIENT
information, the
designer must make the following assumption:
• When the DTS value is greater than T
CONTROL
the T
AMBIENT
is at boundary condition
derived in Section 5.3.1.
This is consistent with our previous FSC guidance to accelerate the fan to full speed
when the DTS value is greater than T
CONTROL
. As will be shown below, the DTS thermal
specification at DTS = T
CONTROL
can reduce some of the over cooling of the processor
and provide an acoustic noise reduction from the processor thermal solution.
In this example the following assumptions are made:
•T
AMBIENT
= 39 °C
• Thermal Solution designed / validated to a 39 °C environment
•T
CONTROL
= -20
• 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 must be accelerated to full speed. For
the reference thermal solution full speed is 3500 RPM (dashed line in Figure 5-6). The
DTS thermal specification defines a required Ψ
CA
and therefore the fan speed is
2500 RPM. This is much less than full speed even if the assumption is a
T
AMBIENT
= 39 °C (solid line in Figure 5-6). The shaded area displayed in Figure 5-6 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 2500 RPM at DTS = -20 to
full speed at DTS value = -1.
Figure 5-6. Fan Response Without T
AMBIENT
Data