Some designs require the use of higher-voltage fans than those for which the MAX6650 and MAX6651 were designed. Adapting these ICs to higher-voltage fans is largely a matter of ensuring that their input pins are driven with correctly scaled voltages within the acceptable operating ranges. This can be done by adding a handful of external resistors.
Figure 1 shows a typical MAX6651 circuit with a 12V BLDC fan. (The MAX6651 is shown in the schematics throughout this application note. The same pin names apply for the MAX6650.) OUT drives the gate of the external N-channel MOSFET, and FB is the input that closes the feedback loop at the MOSFET's drain. This voltage will vary from 0V to VFAN (12V in this example). The fan's tachometer output is usually open-drain, and is pulled up to VFAN through an external resistor. TACH0 accepts the tachometer signal and adjusts its logic level according to the voltage on FB; the logic level therefore follows the voltage at the fan's tachometer output as it changes with the fan's drive level. The FB and TACH inputs can accept voltages up to 13.2V, so VFAN can be as large as 12V nominal.
Figure 1. Normal 12V operating circuit for the MAX6651.
To control a 24V fan with the MAX6650, the circuit in Figure 2 may be used. The only change from the circuit in Figure 1 is the addition of two pairs of resistors to attenuate the voltages at the TACH0 and FB inputs to 12V or less. Similarly, the circuit in Figure 3 provides further attenuation to reduce the input voltage from 48V to 12V or less.
Figure 2. Driving a 24V fan with the MAX6651.
Figure 3. Driving a 48V fan with the MAX6651.