Audio amp makes efficient fan controller
Wallace Ly, National Semiconductor Corp, Santa Clara, CA -- EDN, November 9, 2000
You can use discrete transistors to vary power to a fan to control its speed. However, with a simple modification, you can use an audio-amplifier IC to control a fan module (Figure 1). The LM4872 "Boomer" is an audio amplifier capable of delivering 1W maximum output power. A COP8SAC µC connects to the audio amplifier and the fan module. The µC's T1A output pin delivers a PWM signal to an RC network, which produces a dc signal. The dc output of the RC network is proportional to the duty cycle of the PWM signal. The dc signal drives the audio amplifier, which powers the fan via its bridge-configuration outputs. Listing 1 demonstrates how to control the PWM signal from the µC. For standby operation, set the D0 bit; this operation puts the LM4872 in shutdown mode. We also recommend that you put the µC in shutdown mode. In shutdown mode, the total quiescent current is approximately 4 µA, so the circuit in Figure 1 is ideal for battery-powered applications.
