Designers of today’s electronic products are packing more performance and functionality into smaller and smaller systems. The increased performance often generates additional heat, which degrades the optimal performance of the system. Many products targeted for consumer and business applications need a thermal management strategy that includes actively removing heat from the enclosure with one or more fans.
A simple fan control approach is to simply turn the fan ON when the temperature exceeds a set-point, however, the acoustic penalty for this method is severe. ON/OFF control sufficed for early product generations, but consumers are now demanding that products are both capable and quiet. There are several speed fan control circuits that can be implemented using digital temperature sensors with two temperature programmable interrupt outputs. Digital temperature sensors provide high accuracy and ease-of-use at a very low cost and can support three speed fan controls with just a few additional components.
LCD, plasma and projection TVs, set-top boxes and projectors are examples of products with high heat-generating components that require active cooling. Consumers expect new products to outperform their predecessors and quiet operation is now a required feature. Products that previously had no fan speed control now must incorporate more sophisticated thermal management at little or no cost increase.
Many product designs include microcontrollers (MCUs) with built-in I2C masters that can interface to a wide variety of peripherals, including I2C and SMBus thermal monitoring ICs.
When ON/OFF fan control is used, the maximum fan noise is generated whenever the fan is ON. And worse, cycling from OFF to full ON fan speed produces an abrupt change that is the most noticeable form of acoustic noise. In many products, a low or medium speed will be adequate to remove heat during normal operation, allowing maximum speeds to be reserved for high ambient or high power dissipation conditions.
The circuits described below use linear voltage control, which involves reducing the fan speed (and thus noise) by operating the fan at a DC voltage lower than the full manufacturer’s rating.
Figure 1 illustrates the relationship between temperature and fan speed as controlled by these circuits. For temperatures below the Limit1 set-point, the fan will either be OFF or at low speed depending on the circuit. Temperatures exceeding Limit1 will cause output Out1# to assert, resulting in a medium fan speed. Temperatures in excess of Limit2 will assert Out2# resulting in high fan speed. For more information on the function of Out#1 and Out#2, see the section titled, “Intelligent Control” below.
Figure 1 - Temperature vs. Fan Speed
Note: Digital temperature sensors typically have a programmable hysteresis value that ensures the fan doesn’t turn ON and OFF rapidly at a Limit set-point.