Closed loop control is often referred to as servo control. Agave Semiconductor offers both open loop and closed loop pre-drivers. The block diagrams illustrate the main difference between the two; and show how both can be used in a closed loop system.
The control signal from a system controller to the pre-driver IC embedded within the fan represents the desired fan speed. On a PC or server motherboard, the desired fan speed may be a function of both the CPU and ambient temperatures. Knowledge of the actual rotational speed of the fan is acquired from a Hall sensor and is reported on Tach. An error amplifier compares the actual fan speed against the desired speed. In turn, a speed corrected control signal continuously adjusts the H-Bridge PWM ensuring the actual fan speed matches the desired speed under varying load and voltage supply conditions.
Open loop pre-drivers do not contain the error amplifier which compares the desired speed of the fan to the actual speed. In some applications it may be more desirable to implement the closed loop algorithm on the motherboard as illustrated in the lower block diagram. In other applications, closed loop servo control may not be required at all.