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AN38 Implementing Temperature-Based Variable Fan Speed Control in NLX Power Supplies Author: 1. Abid Hussain and Danny Allred, Microchip Technology, Inc. INTRODUCTION The NLX Power Supply Specification released by Intel Corp. defines the requirements for next-generation PC system power supplies. There are several enhancements outlined in this specification as compared to the “old” PS/2 power supply form factor. One of these features is control and monitoring of the cooling fan(s) inside the NLX power supply. The NLX specification designates two interface fan control signals: 1. 2. FanM signal is an output from the NLX-compliant power supply. This signal allows the host (typically a system management ASIC) to monitor fan RPM. FanM is an open-collector signal consisting of two pulses per fan rotation. FanC is an input to the NLX-compliant power supply used by the motherboard to regulate fan speed and to shut the fan down. FanC is a 0V to 12V analog signal. BRUSHLESS DC FAN BASICS 2. APPLICATION CIRCUIT The application circuit using the Microchip TC646 is shown in Figure 2. Please refer to the TC646 datasheet for details on pin descriptions. The overall circuit can be broken down into five basic functional blocks plus the TC646 itself: 1. Brushless DC (BDC) fans are popular for cooling electronics and come in many voltage, current, and CFM ratings. The most common versions in PCs are +5V and +12V. The nominal voltage rating is typically the input voltage at which the fan runs at approximately 100% RPM. Some fans have a third terminal that outputs pulses as a tachometer signal. 2. 100 RPM % 3. 50 PWM Drive 4. Linear Drive 0 0 6 VFAN (Avg.) Volts 12 FIGURE 1: Linear voltage control vs. pulse-width modulation. © 2001 Microchip Technology, Inc. Linear Voltage Control: This method varies the DC voltage at the VDD terminal of the fan in a linear manner. For example, a +12V nominal fan modulated linearly to +8V would theoretically run at 66.7% RPM. Though this seems relatively simple, there are drawbacks. All BDC fans have a “stall voltage” that can vary from 35% to 60% of nominal voltage, making wide range speed control impossible. Additionally, there is considerable start-up hysteresis after a stall, complicating matters further. Lastly, linear speed control is inefficient, generating excessive heat. Pulse Width Modulation: This method applies the full DC nominal voltage to the fan, but only for a duty cycle that corresponds to the desired speed. The average energy delivered to the fan is, therefore, variable over a wide range, and stall voltage is not an issue. 5. FanC “Y-Network” Input From Motherboard: This section accepts the 0V–12VDC signal from an NLX-compliant motherboard. The Y-network formed by R12, R1, and R2 performs level shifting and range compression to match the 1.25V–2.65V analog input range of the TC646. The FanC signal indicates a shutdown request at or below +1V DC. Above that, the signal is used for proportional fan speed control. If FanC becomes disconnected or is not present, the fan will be driven at full speed via the R13 pull-up resistor. Thermistor-Controlled Fan Override: T1 (thermistor 10 KΩ at 25°C), R6 and Q1 form the “thermal override function.” This circuit provides a local override in the event of high ambient temperature. Under normal operation, Q1 is off. Output Stage: This section consists of the fan and its drive components: R5, Q2. The signal output from the VOUT pin is a 30Hz nominal 5Vp-p PWM waveform. Q2 is a 2N2222A small-signal BJT. The fan is driven to a full +12V (minus Q2 saturation voltage). FanM Signal: To fully comply with NLX specification, the power supply must provide a signal back to the motherboard (FanM). The NLX specification requires this signal to be an open collector output from the tachometer of a 3-wire fan. Minimum Speed and Auto-Shutdown: R3 and R4 form a divider network which defines the shutdown threshold of the circuit. This corresponds to the +1V NLXspecification for fan shutdown. DS00764A-page 1 AN38 System operation is straight-forward: fan speed ranges from approximately 30% to 90% for a FanC voltage range of 1V to 12V. The fan will be held in shutdown when FanC is less than 1V. FanM is returned to the motherboard as prescribed in the NLX specification. FanM also is monitored by the TC646. FAULT is asserted if the fan fails to operate (see TC646 datasheet for details). Additionally, C4 sets the 30Hz PWM frequency, and R7 is the pull-up resistor for the FAULT output. SUMMARY The Microchip TC646 Fan Speed Controller allows a computer designer to implement a robust system cooling design and be fully NLX-compliant. Additionally, the PWM control mode is superior to traditional linear control methods. The low cost of the TC646 and supporting components make it the ideal choice for highvolume applications. +12V R8 10kΩ Q3 2N3906 +5V FANM (To Motherboard) R10 Q4 2N2222A R9 2.7kΩ (TACH) 4.7kΩ (+) (23CFM) F1 (∆AFB0912M-FOO) R11 36kΩ C9 0.47µF (-) Override +5V +5V T1 NTC 10kΩ C1 10µF Q1 2N2222A R6 4.7kΩ C6 0.1µF SENSE +12V R13 10kΩ FANC 0V-12.0V (From Motherboard) VDD 30kΩ +5V R12 400kΩ R2 51KΩ VIN + C2 0.01µF – TC646 +5V 1µF R7 C4 FAULT R3 33kΩ R4 16kΩ R5 C7 0.47-1µF (Optional) CF + +5V Q2 2N2222A VOUT D1 1N4148 R1 100kΩ C8 0.01µF VAS 10kΩ System Shutdown Gnd C3 .01µF FIGURE 2: BDC fan control for NLX power supply (3-wire fan). DS00764A-page 2 © 2001 Microchip Technology, Inc. AN38 Information contained in this publication regarding device applications and the like is intended through suggestion only and may be superseded by updates. It is your responsibility to ensure that your application meets with your specifications. No representation or warranty is given and no liability is assumed by Microchip Technology Incorporated with respect to the accuracy or use of such information, or infringement of patents or other intellectual property rights arising from such use or otherwise. Use of Microchip’s products as critical components in life support systems is not authorized except with express written approval by Microchip. No licenses are conveyed, implicitly or otherwise, under any intellectual property rights. Trademarks The Microchip name and logo, the Microchip logo, PIC, PICmicro, PICMASTER, PICSTART, PRO MATE, KEELOQ, SEEVAL, MPLAB and The Embedded Control Solutions Company are registered trademarks of Microchip Technology Incorporated in the U.S.A. and other countries. Total Endurance, ICSP, In-Circuit Serial Programming, FilterLab, MXDEV, microID, FlexROM, fuzzyLAB, MPASM, MPLINK, MPLIB, PICC, PICDEM, PICDEM.net, ICEPIC, Migratable Memory, FanSense, ECONOMONITOR, Select Mode and microPort are trademarks of Microchip Technology Incorporated in the U.S.A. Serialized Quick Term Programming (SQTP) is a service mark of Microchip Technology Incorporated in the U.S.A. All other trademarks mentioned herein are property of their respective companies. © 2001, Microchip Technology Incorporated, Printed in the U.S.A., All Rights Reserved. Printed on recycled paper. Microchip received QS-9000 quality system certification for its worldwide headquarters, design and wafer fabrication facilities in Chandler and Tempe, Arizona in July 1999. The Company’s quality system processes and procedures are QS-9000 compliant for its PICmicro® 8-bit MCUs, KEELOQ® code hopping devices, Serial EEPROMs and microperipheral products. In addition, Microchip’s quality system for the design and manufacture of development systems is ISO 9001 certified. 2001 Microchip Technology Inc. DS00764A-page 3 25/':,'($/(6$1'(59,&( $0(5,&$6 $6,$3$&,),& -#"'" #&" '',23&>> !$ $ #2'9' + >' '9%>%?'9' + # >,*3& ! " ' ! " #"$%&&' ( &))***+#"'" #&"'+ ! $%,-#.'$%# / ! 0' 1#, *2 %##,3$%# %#4 ! 56#$%# 7'- ! 8#-#(''92''+##%+ 5'*.' 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