The first member of a new thermal management family of ICs from Micrel Semiconductor, the MIC502 fan controller, addresses the problem of keeping electronic systems cool. The device meets the needs of NLX and ATX power supplies, where there is a growing trend to deliver systems that are "always on" but remain quiet and efficient. It is also suitable for a broad range of other applications requiring intelligent, cost-effective fan control.

The MIC502 has two control inputs that can be used as thermistor or voltage control inputs. The higher of the two sets the duty cycle of the signal from the PWM output, which has sufficient drive to allow the use of very low-cost transistors to drive the fan. In addition, the IC can operate from the same 12V supply used by the fan and is designed for fully ratio metric temperature sensing, eliminating power supply drift as an error source.

"The Instantly Available PC specification by Intel and Microsoft has created a big demand for more intelligent fan control," says Larry Mazer, product marketing manager at Micrel. "The function of the MIC502 is to ensure that the fan runs only as fast as required to keep the system cool."

"The part has the right mix of external programmability and intelligently design system protection, as required in new PCs implementing ACPI power management," says Mazer. "It allows full control of the thermal environment by the operating system when the PC is fully operational. It also ensures reliable fan control when the PC is in the sleep mode, when no external control signals will be available."

The MIC502 implements pulse width modulation (PWM) to control fan speed, which is more efficient than traditional linear control because it allows the fan to operate reliably at lower speeds. PWM control substantially reduces the power dissipation in the drive transistor, eliminating the need for a heat sink in most cases. The PWM frequency is in the 30Hz to 60Hz (Hertz) range, allowing the use of standard brushless DC fans.

A simple idea taken to a practical product. The need for fan control is increasing as the demands on the PC increase and the specifications for the "whole" PC become more rigid than ever before. The most intelligent controls, especially thermal and at the "right" price, will win the day, whether they are for the microprocessor or for peripherals. This part from Micrel allows for a variety of sensor/control methods with the higher "OR" input taking charge. The drive level from the part (at 10 mA output) means that the external drive transistors can be extremely small -- and cheap-- while the use of PWM ensures that the overall dissipation is also low. Using the low "mains" frequency for the modulation is one of those "of courses" that we all wish we could have thought of, so allowing the fan to be the cheapest of the cheap.

I also like the supply range of 4.5 to 13.2 V which will enable the part to be used across the whole spectrum of the PC world, the integrated start-up timer for the fan, and the automatic sleep mode. Although clearly aimed at the NLX and ATX power supply market I would expect to see the part make headway in other office-type equipment, particularly upcoming (non-domestic/consumer) printers that will have a lot of smarts built-in to them.