MAX31740 Ultra-Simple Fan-Speed Controller General Description Features The MAX31740 is a sophisticated, yet easy-to-use fanspeed controller. It monitors the temperature of an external NTC thermistor and generates a PWM signal that can be used to control the speed of a 2-, 3-, or 4-wire fan. The fan control characteristics are set using external resistors, thereby eliminating the need for an external microcontroller. Controllable characteristics include the starting temperature for fan control, PWM frequency, fan speed at low temperatures, and slope of the temperature-dutycycle transfer function. ● Self-Contained PWM Fan Control—No Micro Needed ● Controls Speed of 2-, 3-, or 4-Wire Fans ● Resistors Set Fan Control Characteristics ● Smooth, Linearly Varying PWM Duty Cycle Minimizes Audibility of Fan Noise ● Accurately Monitors External Thermistor Temperature ● 3.0V to 5.5V Operating Voltage Range ● -40°C to +125°C Operating Temperature Range Because the operating characteristics are selected by hardwired passive components, a simple, low-cost fanspeed controller can be implemented without the need for firmware development. This can dramatically reduce development time for the fan control function. Applications ● ● ● ● The MAX31740 is available in a 2mm x 3mm, 8-pin TDFN package. Consumer Equipment Communications Equipment Computing Equipment Industrial Equipment Ordering Information appears at end of data sheet. Typical Application Circuits 2-WIRE FAN-SPEED CONTROLLER 4-WIRE FAN-SPEED CONTROLLER VDD RST CB RB RD1 VDD RST VDD SENSE D0 FREQ MAX31740 DMIN RD2 GND VFAN PWM_OUT SLOPE RSLOPE RD1 VDD SENSE D0 FREQ CF 33Hz CB RB CF MAX31740 N DMIN RD2 GND PWM_OUT SLOPE 25kHz RSLOPE For related parts and recommended products to use with this part, refer to www.maximintegrated.com/MAX31740.related. 19-6697; Rev 0; 5/13 VFAN TACH OR LOCKED ROTOR MAX31740 Ultra-Simple Fan-Speed Controller Absolute Maximum Ratings (All voltages relative to ground.) Voltage Range on VDD............................................... -0.3V to +6.0V Voltage Range on Any Non-Power Pin..... -0.3V to (VDD + 0.3V) Operating Temperature Range.......................... -40°C to +125°C Storage Temperature Range............................. -55°C to +125°C Junction Temperature Maximum......................................+150°C Soldering Temperature (reflow)........................................+260°C Stresses beyond those listed under “Absolute Maximum Ratings” may cause permanent damage to the device. These are stress ratings only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specifications is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability. Package Thermal Characteristics (Note 1) TDFN Junction-to-Ambient Thermal Resistance (θJA)...........60°C/W Junction-to-Case Thermal Resistance (θJC)................11°C/W Note 1: Package thermal resistances were obtained using the method described in JEDEC specification JESD51-7, using a four-layer board. For detailed information on package thermal considerations, refer to www.maximintegrated.com/thermal-tutorial. Recommended Operating Conditions (TA = -40°C to +125°C, unless otherwise noted.) (Note 2) PARAMETER SYMBOL CONDITIONS MIN TYP 3.3 MAX UNITS Supply Voltage VDD 3.0 5.5 V Logic 1 (D0) VIH VDD x 0.7 VDD + 0.3 V Logic 0 (D0) VIL -0.3 VDD x 0.3 V Electrical Characteristics (VDD = VDDMIN to VDDMAX, TA = -40°C to +125°C, unless otherwise noted.) (Notes 2, 3) PARAMETER Supply Current (Note 4) SYMBOL IDD PWM Start Voltage (Note 5) VSTART Input Bias Current (SENSE) IBIAS Internal DO Pulldown Resistor Internal SLOPE Feedback Resistance (Note 6) Sawtooth Peak Voltage Offset (Note 7) Sawtooth Peak Voltage RSLOPE Capacitive Load (Note 8) CONDITIONS TYP MAX VDD = 3.3V 500 800 VDD = 5.5V 750 1100 -40 +10 VDD = 3.3V -80 TA = +25°C to +125°C DORLOAD RFBK MIN 19 20 VDD = 3.3V, TA = +25°C VFSOFFSET VFS 0.4925 VOL ISINK = 6mA PWM Output High VoH ISOURCE = -6mA 60 100 www.maximintegrated.com kΩ kΩ ±12 mV 0.5 0.5075 x VDD 10 pF 0.4 V VDD - 0.4 V 10.5455 /CF PWMFREQ mV 22 ± 2.4 Hz -6 PWM Frequency µA nA CSLOPE PWM Output Low UNITS TA = +25°C to +125°C -10 +10 TA = -40°C to +125°C -20 +20 % Maxim Integrated │ 2 MAX31740 Ultra-Simple Fan-Speed Controller Capacitance (TA = +25°C, unless otherwise noted.) PARAMETER SYMBOL CONDITIONS MIN TYP MAX UNITS Input Capacitance CI (Note 9) 10 pF Output Capacitance CO (Note 9) 15 pF Note 2: All voltages referenced to ground. Note 3: Limits are production tested at TA = +25°C. Limits over the operating temperature range and relevant supply voltage range are guaranteed by design and characterization. Typical values are not guaranteed. Note 4: SENSE = VDD/2. Note 5: VSTART specifies the voltage change relative to VDD/2 that is required to start PWM. Negative value indicates lower than VDD/2. Note 6: The typical (TYP) column indicates ±3 sigma distribution of a trimmed resistance. Note 7: VFSOFFSET is specified relative to VDD/2. The total error equals VFS + VFSOFFSET. Note 8: For stable PWM operation, the maximum external capacitance connected to RSLOPE from all sources must be less than 10pF. Note 9: Guaranteed by design; not 100% production tested. www.maximintegrated.com Maxim Integrated │ 3 MAX31740 Ultra-Simple Fan-Speed Controller Typical Operating Characteristics (TA = +25°C, unless otherwise noted.) +125°C 550 -40°C 500 450 3.0 3.5 4.0 4.5 5.0 5.5 40 35 30 25 20 6.0 VDD = 3.3V, CF = 330nF, CL = 10pF 45 -40 1.E+03 1.E+02 1.E-07 1.E-06 TA = +25°C 1.E+04 1.E+03 1.E+02 1.E+01 1.E-10 40 20 15% 30% DMIN BIAS (%VDD) www.maximintegrated.com 45% PWM_OUT OUTPUT VOLTAGE HIGH vs. OUTPUT CURRENT 3.30 MAX31740 toc06 MAX31740 toc05 VDD = 3.3V, TA = +25°C 1.E-09 1.E-08 1.E-07 1.E-06 FREQ INPUT CAPACITANCE (F) VDD = 3.3V, TA = +25°C 3.35 3.20 3.15 3.10 3.05 3.00 -10 -8 -6 -4 OUTPUT CURRENT (mA) -2 0 PWM_OUT OUTPUT VOLTAGE LOW vs. OUTPUT CURRENT 0.4 MAX31740 toc07 1.E-08 1.E+05 PWM_OUT OUTPUT VOLTAGE LOW (V) 1.E-09 PWM_OUT OUTPUT VOLTAGE HIGH (V) PWM DUTY CYCLE (%) 110 MAX31740 toc04 1.E+04 MEASURED PWM OUTPUT FREQUENCY (Hz) TA = +25°C PWM DUTY CYCLE vs. DMIN INPUT BIAS 0% 80 MEASURED PWM OUTPUT FREQUENCY vs. FREQ INPUT CAPACITANCE 60 0 50 THEROETICAL PWM OUTPUT FREQUENCY vs. FREQ INPUT CAPACITANCE FREQ INPUT CAPACITANCE (F) 80 20 TEMPERATURE (°C) 1.E+01 1.E-10 100 10 POWER-SUPPLY VOLTAGE (V) 1.E+05 PWM OUTPUT FREQUENCY (Hz) PWM_OUT OUTPUT FREQUENCY (Hz) 600 50 MAX31740 toc01 CF = 330nF, DUTY CYCLE = 50% MAX31740 toc03 POWER-SUPPLY CURRENT (µA) 650 PWM_OUT OUTPUT FREQUENCY vs. TEMPERATURE MAX31740 toc02 POWER-SUPPLY CURRENT vs. POWER-SUPPLY VOLTAGE VDD = 3.3V, TA = +25°C 0.3 0.2 0.1 0 0 5 10 15 20 OUTPUT CURRENT (mA) Maxim Integrated │ 4 MAX31740 Ultra-Simple Fan-Speed Controller Pin Configuration TOP VIEW 1 SLOPE 2 SENSE 3 GND 4 + DMIN MAX31740 EP 8 VDD 7 PWM_OUT 6 D0 5 FREQ TDFN Pin Description PIN NAME FUNCTION 1 DMIN 2 SLOPE Connect to an external resistor to set the slope of the temperature-PWM curve. 3 SENSE Thermistor Voltage Input. External NTC thermistor senses temperature. Thermistor and external resistor form a voltage divider with a negative temperature coefficient. 4 GND Ground 5 FREQ Connect to external capacitor CF to set PWM frequency. 6 D0 7 PWM_OUT 8 VDD — EP Connect to an external resistor divider to set the minimum active PWM duty cycle. (Typically between 0.05VDD to 0.2VDD depending on desired minimum duty cycle.) Duty Cycle Input. Sets the duty cycle below tMIN to either DMIN or 0%. Connect to GND for 0% or to VDD for DMIN. D0 has an internal 60kΩ (typ) pulldown resistor. PWM CMOS output signal. 3.0V to 5.5V Supply Voltage Input. Bypass with at least a 0.01µF capacitor. Exposed Pad. Connect to ground, but do not use as the sole ground connection point or leave unconnected. www.maximintegrated.com Maxim Integrated │ 5 MAX31740 Ultra-Simple Fan-Speed Controller Block Diagram VDD VFS VDD VL FREQ P OSCILLATOR CF RFBK PWM_OUT N SLOPE RSLOPE VFS VDD RST ∑ RB SENSE MAX31740 CB 60kΩ (TYP) THERMISTOR VDD RD1 DMIN D0 VDD RD2 Detailed Description The MAX31740 monitors the temperature of an external NTC thermistor and generates a PWM signal that can be used to control the speed of a 2-, 3-, or 4-wire fan. The fan control characteristics are set using external resistors and capacitors, thereby eliminating the need for an external microcontroller. Controllable characteristics include the starting temperature for fan control, PWM frequency, fan speed at low temperatures, and slope of the temperatureduty-cycle transfer function. Controlling Fan Speed The device generates a PWM signal and varies the duty cycle of that signal to control the speed of one or more fans. If the fan has a PWM speed control input (typically this is a “4-wire” fan), the recommended PWM frequency is usually in the 20kHz to 30kHz range. www.maximintegrated.com PWM_OUT is a CMOS output that can be connected directly to most fans’ speed control input as shown in the 4-Wire Fan-Speed Controller Typical Application Circuits. If the fan has no speed control input (this is the case for all 2-wire fans and most 3-wire fans), there are two options for controlling the fan’s speed. The first option is to use a low-frequency (typically 33Hz) PWM signal to modulate the fan’s power supply as shown in the 2-Wire Fan-Speed Controller Typical Application Circuits. The advantage of using PWM to modulate the fan’s power supply is that it is inexpensive. Note, however, that some fan manufacturers recommend against this approach for their fans. Before using this approach, be sure to verify that the fan is compatible with pulse-width modulation of the power supply. Also, modulating the power-supply voltage in this manner can cause an increase in the perceived noise level when the duty cycle is not equal to 100% or 0%. Maxim Integrated │ 6 MAX31740 Ultra-Simple Fan-Speed Controller Another option for fans with no speed control input is to convert the PWM signal to a DC voltage. This can be done using a simple two-transistor buffer circuit, a linear low-dropout voltage regulator, or a switch-mode voltage regulator. Always use a high PWM frequency (20kHz or higher recommended) in this case to ease filtering. Figure 1 shows an example of a two-transistor buffer circuit. Fan Control Profile Figure 2 shows three general curves of PWM duty cycle vs. temperature for the device. The important parameters are listed as follows: • TSTART is the temperature that corresponds to the intersection of the diagonal portion of the curve, including the dashed portion in (b) and (c), with 0% duty cycle. It is selected by setting resistor RST equal to the resistance of the thermistor at temperature TSTART. • DMIN is the PWM duty cycle at the lower left end of the solid diagonal portion of the curve. It is selected using a resistor-divider to set the voltage at the DMIN input. • TMIN is the temperature at which the duty cycle begins to increase from DMIN. • D0 is the value of the PWM duty cycle for temperatures below TMIN. This value is equal to either DMIN or 0% in curves (b) and (c), depending upon whether D0 is connected to VDD or GND. • The slope of the diagonal portion of the curve is selected by the value of the resistor at the SLOPE input. VFAN (5V OR 12V) 3.3V 100kΩ P 2N3904 MAX31740 PWM_OUT 33kΩ 100kΩ 2.2µF 9.1kΩ 10µF Figure 1. Two-Transistor Buffer DMIN TSTART TEMPERATURE (°C) 100 c) DMIN > 0%, D0 = VDD DUTY CYCLE (%) 100 b) DMIN > 0%, D0 = GND DUTY CYCLE (%) DUTY CYCLE (%) a) DMIN = 0%, D0 = GND DMIN TSTART TMIN TEMPERATURE (°C) 100 DMIN TSTART TMIN TEMPERATURE (°C) Figure 2. PWM Duty Cycle vs. Temperature www.maximintegrated.com Maxim Integrated │ 7 MAX31740 Operation Referring to the Block Diagram, CF sets the frequency of the internal saw-tooth oscillator that is used to generate the PWM speed control signal. The oscillator’s output voltage swings from near-zero to VFS (VDD/2). The external NTC thermistor and resistor (RST) form a voltage-divider whose output voltage is approximately linear and has a negative temperature coefficient. This voltage is subtracted from VFS to create a voltage with a positive temperature coefficient at the input to the amplifier. The amplifier’s closed-loop gain is set by an external resistor (RSLOPE) and an internal 25kΩ resistor (RFBK). The value of RSLOPE therefore determines the slope of the duty cycle as a function of temperature. The temperature at which the thermistor’s resistance is equal to RST is the nominal value of TSTART. The voltage at DMIN, derived by the voltage-divider between VDD and GND, determines the minimum duty cycle. The logic level at D0 determines whether the lowtemperature duty cycle will be 0% or equal to DMIN. Component Selection Before picking component values, be sure that you have determined target values for the important parameters such as PWM frequency, TSTART, DMIN, D0, and the Slope of the duty cycle vs. temperature curve. Most of these parameters are defined in the Fan Control Profile section. PWM Frequency If the fan has a speed control input, the most common recommended PWM frequency is 25kHz, although some fans require different frequencies. If the fan has no PWM input and will be controlled by applying the PWM signal directly to a power-supply modulation transistor (as in the typical 2-wire fan-speed controller circuit), the PWM frequency should normally be in the 25Hz to 35Hz range. A good starting point is 33Hz. CF sets the PWM frequency according to the equation: CF = 10.5455E-6/FREQ (Hz) The most common values of CF are 330nF for fPWM = 33Hz and 430pF for fPWM = 25kHz. TSTART Select RST equal to the resistance of the thermistor at the desired value of TSTART. www.maximintegrated.com Ultra-Simple Fan-Speed Controller DMIN and D0 Select DMIN and D0 based on the system requirements and the type of fan to be used. For example, in some systems, the optimum cooling strategy requires that the fan stop spinning when the temperature is below a specific value (TMIN). Such a scheme can be achieved with the fan profile shown in Figure 2(c). The voltage VMIN at the DMIN input selects the minimum duty cycle using the following equation: VMIN/VDD = R2/(R1+R2) = DMIN (%)/200 where DMIN is the minimum duty cycle (in percent). For example, if a minimum duty cycle of 30% is desired, the voltage at the DMIN input should be 15% of VDD. When the temperature drops below TMIN in the profile shown in Figure 2(b), the duty cycle should drop to zero. This is accomplished by connecting the D0 input to GND. If the system requires a profile like the one in Figure 2(c), where the duty cycle remains at D MIN when the temperature drops below T MIN , simply connect D0 to V DD . Some fans with speed control inputs (these are typically 4-wire fans) are designed to keep spinning at a reduced speed even when the duty cycle is equal to zero. For such fans, a profile like that of Figure 2(a) is usually appropriate. With this profile, the duty cycle decreases linearly to zero as temperature decreases. To achieve this profile, connect D0 to GND. Thermistor Use a standard NTC thermistor. A +25°C resistance in the 10kΩ to 50kΩ range works well. An NTC’s resistancetemperature curve is generally very nonlinear, but when combined with RST in a voltage-divider, the resulting curve is reasonably linear over the temperature range of interest. RST First determine TSTART. In Figure 2(a), TSTART is the temperature at which the duty-cycle curve intersects the horizontal axis. In Figure 2(b) and Figure 2(c), TSTART can be determined by continuing the diagonal line until it crosses the horizontal axis, and the point at which it intersects the horizontal axis is TSTART. Now choose RST equal to the resistance of the thermistor at TSTART. Maxim Integrated │ 8 MAX31740 Ultra-Simple Fan-Speed Controller Slope RSLOPE sets the slope of the duty cycle vs. temperature curve. Pick the value based on the thermistor characteristics and the desired range of temperatures between TMIN and the point where the duty cycle reaches 100%. As an example, assume that a typical NTC thermistor + RST combination will provide a slope of about 1% of VDD per °C. Since VFS = VDD/2, this is equivalent to 2% of VFS per °C at the input to the internal amplifier. Therefore, the range of duty cycles from 0% to 100% would correspond to about a 50°C range of temperatures when the amplifier gain is equal to one. In most implementations, you would want a smaller temperature range (for example, 15°C) to cause the duty cycle to cover the full 0% to 100% range. Doing so requires an amplifier gain of: AV = 50°C/15°C = 3.33 The closed-loop gain of the internal amplifier is: AV = (1 + RFBK/RSLOPE). Therefore: RSLOPE = RFBK/(AV – 1) = 25kΩ/(3.33 – 1) = 10.7kΩ RST and RSLOPE Example Values Table 1 gives example values of RST and RSLOPE for three values of TSTART and three fan control temperature spans. Values are given for two standard thermistor products, one rated at 10kΩ and the other rated at 15kΩ at +25°C. CB and RB One of the most common reasons for controlling fan speed is to reduce the audible noise perceived by users in the vicinity of the equipment. The audibility of fan noise increases significantly when the fan speed undergoes rapid changes. When the thermistor is in contact with a significant mass, such as a heat sink or a printed circuit board, the thermal mass of the object being measured will often limit the rate of change of the voltage at the SENSE input so that any fan speed changes are slow and no additional filtering is needed. In such cases, RB and CB are not necessary. In some cases, the thermistor could be in contact with an object whose temperature changes relatively rapidly, or a low-mass thermistor can be suspended in an area where air flow could cause its temperature to undergo Table 1. RST and RSLOPE Resistor Options THERMISTOR BetaTHERM 10K3A1 TSTART RST (kΩ) CONTROL RANGE (TSTART to T100%) RSLOPE (kΩ) +10°C 6.65 25 10 +15°C 11 +20°C 16.2 +10°C 6.49 +15°C 10.5 30 35 25 Murata NCP15XW153J03RC 30 35 www.maximintegrated.com 8.06 6.49 15 12.4 10.5 +20°C 15.4 +10°C 6.04 +15°C 10 +20°C 14.7 +10°C 5.49 +15°C 8.87 +20°C 13 +10°C 5.23 +15°C 8.45 +20°C 12.4 +10°C 5.36 +15°C 8.45 +20°C 12.4 Maxim Integrated │ 9 MAX31740 Ultra-Simple Fan-Speed Controller fast changes. In these cases, the temperature changes can be fast enough to cause audible fan speed variations. To minimize this effect, the rate at which the duty cycle can change can be slowed down using an external RC network consisting of RB and CB and connected to the SENSE input. Typical values for these components are 5MΩ and 1µF, although they can be easily adjusted to conform to the requirements of the system. CB can be connected to GND, VDD, or an intermediate voltage depending on the desired startup characteristics. When connected to VDD, CB initially holds the SENSE input high upon application of VDD, which delays the onset of the PWM signal when D0 is grounded and the temperature on application of VDD is greater than TSTART. The delay time is related to the time constant CBRB. When connected to GND, CB briefly keeps the SENSE input low upon application of VDD, providing a “spin-up” function on power-up that can be useful in some cases (but is generally not necessary). Connecting CB to a voltage-divider that produces an output of VDD/2 can be used to minimize any spin-up or delay time. Ordering Information Power-Supply Decoupling To achieve the best results when using the device, decouple the VDD power supply with a (minimum) 0.01µF capacitor. Use a high-quality, ceramic, surface-mount capacitor if possible. Surface-mount components minimize lead inductance, which improves performance, and ceramic capacitors tend to have adequate high-frequency response for decoupling applications. Handling, PCB Layout, and Assembly The lead-free/RoHS package can be soldered using a reflow profile that complies with JEDEC J-STD-020. Moisture-sensitive packages are shipped from the factory dry-packed. Handling instructions listed on the package label must be followed to prevent damage during reflow. Refer to the IPC/JEDEC J-STD-020 standard for moisture-sensitive device (MSD) classifications. Package Information PART TEMP RANGE PIN-PACKAGE MAX31740ATA+ -40°C to +125°C 8 TDFN-EP* +Denotes a lead(Pb)-free/RoHS-compliant package. *EP = Exposed pad. Chip Information Applications Information SUBSTRATE CONNECTED TO GROUND For the latest package outline information and land patterns (footprints), go to www.maximintegrated.com/packages. Note that a “+”, “#”, or “-” in the package code indicates RoHS status only. Package drawings may show a different suffix character, but the drawing pertains to the package regardless of RoHS status. PACKAGE TYPE PACKAGE CODE OUTLINE NO. LAND PATTERN NO. 8 TDFN-EP T823+1 21-0174 90-0091 PROCESS: CMOS www.maximintegrated.com Maxim Integrated │ 10 MAX31740 Ultra-Simple Fan-Speed Controller Revision History REVISION NUMBER REVISION DATE 0 5/13 DESCRIPTION Initial release PAGES CHANGED — For pricing, delivery, and ordering information, please contact Maxim Direct at 1-888-629-4642, or visit Maxim’s website at www.maximintegrated.com. Maxim Integrated cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim Integrated product. No circuit patent licenses are implied. Maxim Integrated reserves the right to change the circuitry and specifications without notice at any time. The parametric values (min and max limits) shown in the Electrical Characteristics table are guaranteed. Other parametric values quoted in this data sheet are provided for guidance. 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