Combining a switch-mode dc-dc controller with two low-cost temperature monitor ICs produces a three-speed fan controller (see the figure). This useful circuit reduces both noise and power consumption in computers, temperature controllers, and alarm systems.
The idea is made possible by utilizing IC3’s pin-selectable shutdown and output-voltage capabilities. The logic levels applied to those inputs (—3V/5V and SHDN), along with properly selected feedback resistors (R2 and R3), set the output-voltage levels to 0 V, 8 V, or 12 V.
In general, the lower voltage VOUT1 (which is 8 V in this case) is determined by the R2/R3 divider, and the higher voltage VOUT2 (which is 12 V in this case) is determined by the product of VOUT, and an internal ratio:
VOUT1 = 3.3 ((R2 +R3)/R3)
VOUT2 = VOUT1(5/3.3)
The temperature monitors (IC1 and IC2) have open-drain outputs (—TOVER) that are driven low when the ambient temperature exceeds a factory-programmed internal threshold. The monitors come in tiny SOT23-5 packages with dedicated thresholds in the range 35°C to 115°C.
When the temperature exceeds the threshold of IC2, 45°C in this example, IC2 turns on IC3 by pulling its SHDN terminal low. IC3’s —3V/5V input remains low, producing 3.3 V at OUT (and 8 V at the fan), until the temperature rises to 65°C. At that time, the IC1 output will pull low. This turns off Q2 and allows R6 to pull the —3V/5V input high, which applies 12 V to the fan. Q2 is necessary for signal inversion and for meeting the —3V/5V input’s logic-high threshold (V+−0.5 V).
The ability of IC3 to produce 100% duty cycles enables a very low dropout voltage for this application of approximately 150 mV at 1-A load. The conversion efficiency is independent of output voltage but varies with output current, ranging from 85% and 96% for fan output currents between 10 mA and 1 A.
The average efficiency of the fan controller is 90%. At low temperatures when a fan isn’t required (below 45°C), the switching regulator shuts down and lowers the supply current in this circuit to about 100 µA.