A similar idea appeared in the February 22, 1999 issue of Electronic Design
Combining a switch-mode DC-DC controller with two low-cost temperature-monitor ICs produces a 3-speed fan controller (Figure 1
). Useful in many applications, this circuit cuts noise and power consumption in computers, temperature controllers, and alarm systems.
Figure 1. Controlled by the temperature monitors IC1 and IC2, this switch-mode DC-DC controller (IC3) applies either 0V, 8V, or 12V to the fan.
The idea is made possible by IC3's pin-selectable shutdown and output-voltage capabilities. The logic levels applied to those inputs (active-low 3/5 and SHDN), along with properly valued feedback resistors (R2 and R3) set the output-voltage levels (available one at a time) at 0V, 8V, and 12V. In general, the lower voltage (VOUT1
, which equals 8V in this case) is determined by the R2/R3 divider, and the higher voltage (VOUT2
) (which equals 12V in this case) is determined by the product of VOUT1
and an internal ratio:
= 3.3 [(R2+R3)/R3]
The temperature monitors (IC1 and IC2) have open-drain outputs (active-low TOVER) that are pulled low when the ambient temperature exceeds a factory-programmed internal threshold. The monitors come in tiny SOT23-5 packages, with dedicated thresholds in the +35°C to +115°C range. When the temperature exceeds the threshold of IC2 (+45°C in this example), that device turns on IC3 by pulling its SHDN terminal low. IC3's active-low 3/5 input remains low, producing 3.3V at OUT (and 8V at the fan), until the temperature rises to +65°C. At that time, the IC1 output pulls low, turning off Q2 and allowing R6 to pull the active-low 3/5 input high, which applies 12V to the fan. Q2 is necessary for signal inversion and for meeting the active-low 3/5 input's logic-high threshold (V+ - 0.5V).
IC3's ability to produce 100% duty cycles enables a very low dropout voltage for this application—about 150mV at 1A load. The conversion efficiency is independent of output voltage but varies with output current, ranging from 85% and 96% for currents between 10mA and 1A. The average efficiency is 90%. At low temperatures for which a fan is not required (below +45°C), the switching regulator shuts down and lowers the supply current in this circuit to about 100µA.