Good one Joe; I love it when a chip (like a regulator) gets used for something the maker never imagined.
Linear RegulationAs the term implies, "linear regulation" adjusts the DC voltage across the fan by using a linear regulator. When using this method, it is important to make sure the fan is specified to operate over a wide range of voltages. One major advantage linear regulation has over PWM is that it allows the use of speed and alarm sensors. Unfortunately, linear regulation also has its drawbacks: mainly power dissipation in the pass element, as well as startup and stalling issues.Linear regulators control the DC voltage across the fan. They do this by dissipating power in the form of heat. It probably seems silly to generate heat in order to cool something down. But it is not as ridiculous as you might think. During maximum and minimum cooling, power dissipation will ideally be zero. During maximum cooling, the pass element is fully on, so the voltage across it is nearly zero. Zero volts means zero power dissipation. During minimum cooling, the pass element is off (zero current flows), so again power dissipation is zero. As previously discussed, the current draw of the fan can be approximated as a linear function of the voltage applied, making it look resistive. With this in mind, worst-case power dissipation occurs roughly when the voltage across the fan is one-half its maximum operating voltage. See Figure 5. This means worst-case power dissipation in the pass element can be estimated by the following equation: P = 1/4(VMAX × IMAX), where IMAX and VMAX are the rated voltages and currents of the fan, respectively. For example, a 1.2W fan (12V at 98mA) will have worst-case power dissipation across the pass element of only 300mW when running at 6V with a 12V supply. It is comforting to note that maximum heat dissipation in the fan circuit occurs during minimal cooling requirements. Also, even though a power-dissipating device is being used, there is still an overall power savings when fan speed is reduced. See Figure 6.
It's not like the series-pass element is getting hottest when the fan is stopped, so there is a degree of self-regulation here if the series-pass element is also exposed to the fan (and why would you not?).