Precision temperature sensing boosts PC reliability

High-performance PCs have one drawback—their high-speed fans used for cooling usually create a constant background noise that can be quite disturbing. However, according to Bill Sheppard, the president and CEO of Andigilog, by precisely monitoring and maintaining the computer's internal temperature, fan noise can be reduced, system efficiency can be improved, and overall system reliability can also be enhanced.

The intelligent thermal management circuits developed by Andigilog, Figure 1 , minimizes the guardbanding that system designers must include when setting the turn-on and turn-off points.The less accurate the sensing of the temperature the larger the guardband must be and thus the cooling fans will have to run longer, generating more noise and potentially wearing out faster.

To minimize the guardbands, Andigilog has created a pair of chips—the aSC7512 system controller (Figure 2 ), which integrates remote temperature sensing with automatic fan speed control, and the aMC8500 motor controller (Figure 3 )for brushless DC fan applications. Embedded on the aSC7512 are high-accuracy circuits that perform temperature sensing with accuracies to within ±1 degree Celsius. That precision sensing capability allows for more-accurate monitoring so the fan is used more efficiently, turning on only when really needed and shutting off a little faster once the desired temperature has been reached. When coupled with the aMC8500 fan motor controller, which includes a selectable-slope pulse-width modulator (PWM) for efficient speed control with analog and digital control signal compatibility, and programmable minimum speed setting, the two chips form a complete thermal management system.

By narrowing the guardband, the fan doesn't run as long, and thus it may not wear out as fast as a non-managed fan. Additionally, when applied to laptops and other battery-operated systems, the reduced run time can also conserve power. Furthermore, the variable-speed capability incorporated in the motor controller chip reduces the operating noise since the fan speed can be ramped to just the right speed to adequately cool the system.

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The aSC7512 can control 3-wire or 4-wire fans and uses a two-wire digital control interface, compatible with the system management bus standard (SMBus 2.0). A tachometer input on the chip captures the fan's motor speed information to form a feedback control loop. To sense the temperature, the controller connects to either a remotely located diode-connected transistor (such as the temperature diode embedded in a CPU chip) as well as its own chip.

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A PWM on the aSC7512 helps set the fan speed by using either of the two temperatures (the remote sensor or on-chip sensor) and can thus control a CPU cooler fan based on either temperature diode. A digital filter on the controller chip can help smooth temperature readings for better control of the fan speed. Limit and status registers on the chip hold all measured values. The controller is housed in an 8-lead MSOP or SOIC package.

The aMC8500 two-phase variable speed fan motor controller ties seamlessly into the aSC7512 and can drive brushless DC motors. Designed to work in tandem with a Hall-effect sensor, the aMC8500 packs proprietary noise immunity scheme to enhance drive sequencing, and adaptive non-overlapping commutation logic to reduce supply current spiking.

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Dual on-chip, 0.5 Ω power MOSFETs with thermal protection provide direct motor drive, while programmable cycle-by-cycle current limiting prevents overloads from burning out the drivers. Additional features include an internal fault timer with auto-start retry, a motor kick-start timer to insure startup, a combined frequency generator/rotor lock-up output, and an independent operational amplifier with a voltage reference for speed control signal scaling. For portable applications, the motor controller includes an automatic low-current power-down mode.

Samples of both the aSC7512 and aMC8500 will be available this month, with production anticipated in June. In 1k quantities, the aSC7512 and aMC8500 sell for $1.50 and $1.45 apiece. Details at

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