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general description the MAX31740 is a sophisticated, yet easy-to-use fan- speed controller. it monitors the temperature of an exter - nal 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 microcon - troller. controllable characteristics include the starting temperature for fan control, pwm frequency, fan speed at low temperatures, and slope of the temperature-duty- cycle transfer function. because the operating characteristics are selected by hardwired passive components, a simple, low-cost fan- speed controller can be implemented without the need for firmware development. this can dramatically reduce development time for the fan control function. the MAX31740 is available in a 2mm x 3mm, 8-pin tdfn package. features self-contained pwm fan controlno 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 -40c to +125c operating temperature range applications consumer equipment communications equipment computing equipment industrial equipment 19-6697; rev 0; 5/13 ordering information appears at end of data sheet. for related parts and recommended products to use with this part, refer to www.maximintegrated.com/MAX31740.related . typical application circuits 2-wire f an-speed controlle r n v dd d0 sense dmin freq pwm_out slope gnd 33hz r slope c f v dd MAX31740 vf an c b r b r st r d1 r d2 4-wire f an-speed controlle r v dd d0 sense dmin freq pwm_out slope gnd 25khz r slope c f vf an v dd MAX31740 ta ch or locked roto r c b r b r st r d1 r d2 MAX31740 ultra-simple fan-speed controller
maxim integrated 2 recommended operating conditions (t a = -40c to +125c, unless otherwise noted.) (note 2) electrical characteristics (v dd = v ddmin to v ddmax , t a = -40c to +125c, unless otherwise noted.) (notes 2, 3) 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 . (all voltages relative to ground.) voltage range on v dd .............................................. -0.3v to +6.0v voltage range on any non-power pin .... -0.3v to (v dd + 0.3v) operating temperature range ......................... -40c to +125c storage temperature range ............................ -55c to +125c junction temperature maximum ..................................... +150c soldering temperature (reflow) ....................................... +260c 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 ab solute maximum rating conditions for extended periods may affect device reliability. package thermal characteristics (note 1) tdfn junction-to-ambient thermal resistance ( ja ) .......... 60c/w junction-to-case thermal resistance ( jc ) ............... 11c/w absolute maximum ratings parameter symbol conditions min typ max units supply current (note 4) i dd v dd = 3.3v 500 800 a v dd = 5.5v 750 1100 pwm start voltage (note 5) v start v dd = 3.3v -80 -40 +10 mv input bias current (sense) i bias t a = +25c to +125c 19 na internal do pulldown resistor do rload 20 60 100 k? internal slope feedback resistance (note 6) r fbk v dd = 3.3v, t a = +25c 22 2.4 k? sawtooth peak voltage offset (note 7) v fsoffset 12 mv sawtooth peak voltage v fs 0.4925 0.5 0.5075 x v dd r slope capacitive load (note 8) c slope 10 pf pwm output low v ol i sink = 6ma 0.4 v pwm output high vo h i source = -6ma v dd - 0.4 v pwm frequency pwm freq 10.5455 -6 /c f hz t a = +25c to +125c -10 +10 % t a = -40c to +125c -20 +20 parameter symbol conditions min typ max units supply voltage v dd 3.0 3.3 5.5 v logic 1 (d0) v ih v dd x 0.7 v dd + 0.3 v logic 0 (d0) v il -0.3 v dd x 0.3 v MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 3 capacitance (t a = +25c, unless otherwise noted.) note 2: all voltages referenced to ground. note 3: limits are production tested at t a = +25c. 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 = v dd /2. note 5: v start specifies the voltage change relative to v dd /2 that is required to start pwm. negative value indicates lower than v dd /2. note 6: the typical (typ) column indicates 3 sigma distribution of a trimmed resistance. note 7: v fsoffset is specified relative to v dd /2. the total error equals v fs + v fsoffset . note 8: for stable pwm operation, the maximum external capacitance connected to r slope from all sources must be less than 10pf. note 9: guaranteed by design; not 100% production tested. parameter symbol conditions min typ max units input capacitance c i (note 9) 10 pf output capacitance c o (note 9) 15 pf MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 4 typical operating characteristics (t a = +25c, unless otherwise noted.) pwm_out output frequency vs. temperature MAX31740 toc02 temperature (c) pwm_out output frequency (hz) 110 80 50 20 10 25 30 35 40 45 50 20 -40 v dd = 3.3v, c f = 330nf, c l = 10pf power-supply current vs. power-supply voltage MAX31740 toc01 power-supply voltage (v) power-supply current (a) 5.0 5.5 4.5 4.0 3.5 500 550 600 650 450 3.0 6.0 c f = 330nf, duty cycle = 50% -40c +125c measured pwm output frequency vs. freq input capacitance MAX31740 toc04 freq input capacitance (f) measured pwm output frequency (hz) 1.e-07 1.e-08 1.e-09 1.e+02 1.e+03 1.e+04 1.e+05 1.e+01 1.e-10 1.e-06 t a = +25c pwm_out output voltage high vs. output current MAX31740 toc06 output current (ma) pwm_out output voltage high (v) -4 -6 -8 3.05 3.10 3.15 3.20 3.35 3.30 3.00 -10 0 -2 v dd = 3.3v, t a = +25c pwm duty cycle vs. d min input bias MAX31740 toc05 d min bias (%v dd ) pwm duty cycle (%) 45% 30% 15% 20 40 60 80 100 0 0% v dd = 3.3v, t a = +25c pwm_out output voltage low vs. output current MAX31740 toc07 output current (ma) pwm_out output voltage low (v) 15 10 5 0.1 0.2 0.3 0.4 0 0 20 v dd = 3.3v, t a = +25c theroetical pwm output frequency vs. freq input capacitance MAX31740 toc03 freq input capacitance (f) pwm output frequency (hz) 1.e-07 1.e-08 1.e-09 1.e+02 1.e+03 1.e+04 1.e+05 1.e+01 1.e-10 1.e-06 t a = +25c MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 5 pin description pin confguration pin name function 1 dmin connect to an external resistor divider to set the minimum active pwm duty cycle. (typically between 0.05v dd to 0.2v dd depending on desired minimum duty cycle.) 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 coeffcient. 4 gnd ground 5 freq connect to external capacitor c f to set pwm frequency. 6 d0 duty cycle input. sets the duty cycle below t min to either d min or 0%. connect to gnd for 0% or to v dd for d min . d0 has an internal 60k? (typ) pulldown resistor. 7 pwm_out pwm cmos output signal. 8 v dd 3.0v to 5.5v supply voltage input. bypass with at least a 0.01f capacitor. ep exposed pad. connect to ground, but do not use as the sole ground connection point or leave unconnected. 1 + 3 4 8 6 5 v dd d0 freq MAX31740 27 pwm_out dmin sense ep gnd slope tdfn top view MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 6 block diagram 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 temperature- duty-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. 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 fans speed. the first option is to use a low-frequency (typically 33hz) pwm signal to modulate the fans power supply as shown in the 2-wire fan-speed controller typical application circuits . the advantage of using pwm to modulate the fans 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%. p n v dd v dd v dd v fs v dd thermis to r pwm_out d0 dmin MAX31740 r fbk oscilla to r freq slope sense v fs v l v dd c b r b r st r slope r d2 60k (typ) r d1 c f MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 7 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 profle figure 2 shows three general curves of pwm duty cycle vs. temperature for the device. the important parameters are listed as follows: ? t start is the temperature that corresponds to the inter - section 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 r st equal to the resis - tance of the thermistor at temperature t start. ? d min 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. ? t min is the temperature at which the duty cycle begins to increase from d min . ? d0 is the value of the pwm duty cycle for tempera - tures below t min . this value is equal to either d min or 0% in curves (b) and (c), depending upon whether d0 is connected to v dd or gnd. ? the slope of the diagonal portion of the curve is select - ed by the value of the resistor at the slope input. figure 1. two-transistor buffer figure 2. pwm duty cycle vs. temperature pwm_out MAX31740 9.1k 100k 100k 33k 2.2f 2n3904 10f p v fa n (5v or 12v) 3.3v 100 d min t st ar t tempera ture (c) a) d min = 0%, d0 = gnd b) d min > 0%, d0 = gnd c) d min > 0%, d0 = v dd duty cycle (%) 100 d min t st ar t t min tempera ture (c) duty cycle (%) 100 d min t st ar t t min tempera ture (c) duty cycle (%) MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 8 operation referring to the block diagram, c f sets the frequency of the internal saw-tooth oscillator that is used to generate the pwm speed control signal. the oscillators output voltage swings from near-zero to v fs (v dd /2). the external ntc thermistor and resistor (r st ) form a voltage-divider whose output voltage is approximately linear and has a negative temperature coefficient. this voltage is subtracted from v fs to create a voltage with a positive temperature coefficient at the input to the amplifier. the amplifiers closed-loop gain is set by an external resistor (r slope ) and an internal 25k? resistor (r fbk ). the value of r slope therefore determines the slope of the duty cycle as a function of temperature. the temperature at which the thermistors resistance is equal to r st is the nominal value of t start . the voltage at d min , derived by the voltage-divider between v dd and gnd, determines the minimum duty cycle. the logic level at d0 determines whether the low- temperature duty cycle will be 0% or equal to d min . component selection before picking component values, be sure that you have determined target values for the important parameters such as pwm frequency, t start , d min , 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. c f sets the pwm frequency according to the equation: c f = 10.5455e -6 /freq (hz) the most common values of c f are 330nf for f pwm = 33hz and 430pf for f pwm = 25khz. t start select r st equal to the resistance of the thermistor at the desired value of t start . d min and d0 select d min 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 (t min ). such a scheme can be achieved with the fan profile shown in figure 2 (c). the voltage v min at the dmin input selects the minimum duty cycle using the following equation: v min /v dd = r2/(r1+r2) = d min (%)/200 where d min 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 v dd . when the temperature drops below t min 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 10k? to 50k? range works well. an ntcs resistance- temperature curve is generally very nonlinear, but when combined with r st in a voltage-divider, the resulting curve is reasonably linear over the temperature range of interest. r st first determine t start . in figure 2 (a), t start is the temperature at which the duty-cycle curve intersects the horizontal axis. in figure 2 (b) and figure 2 (c), t start 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 t start . now choose r st equal to the resistance of the thermistor at t start . MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 9 slope r slope 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 t min and the point where the duty cycle reaches 100%. as an example, assume that a typical ntc thermistor + r st combination will provide a slope of about 1% of v dd per c. since v fs = v dd /2, this is equivalent to 2% of v fs per c at the input to the internal amplifier. therefore, the range of duty cycles from 0% to 100% would correspond to about a 50c range of temperatures when the amplifier gain is equal to one. in most implementations, you would want a smaller temperature range (for example, 15c) to cause the duty cycle to cover the full 0% to 100% range. doing so requires an amplifier gain of: a v = 50c/15c = 3.33 the closed-loop gain of the internal amplifier is: a v = (1 + r fbk /r slope ). therefore: r slope = r fbk /(a v C 1) = 25k?/(3.33 C 1) = 10.7k? r st and r slope example values table 1 gives example values of r st and r slope for three values of t start and three fan control temperature spans. values are given for two standard thermistor products, one rated at 10k? and the other rated at 15k? at +25c. c b and r b 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. r st and r slope resistor options thermistor t start r st (k?) control range (t start to t 100%) r slope (k?) betatherm 10k3a1 25 10 +10c 6.65 +15c 11 +20c 16.2 30 8.06 +10c 6.49 +15c 10.5 +20c 15.4 35 6.49 +10c 6.04 +15c 10 +20c 14.7 murata ncp15xw153j03rc 25 15 +10c 5.49 +15c 8.87 +20c 13 30 12.4 +10c 5.23 +15c 8.45 +20c 12.4 35 10.5 +10c 5.36 +15c 8.45 +20c 12.4 MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
maxim integrated 10 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 r b and c b and connected to the sense input. typical values for these components are 5m? and 1f, although they can be easily adjusted to conform to the requirements of the system. c b can be connected to gnd, v dd , or an intermediate voltage depending on the desired startup characteristics. when connected to v dd , c b initially holds the sense input high upon application of v dd , which delays the onset of the pwm signal when d0 is grounded and the temperature on application of v dd is greater than t start . the delay time is related to the time constant c b r b . when connected to gnd, c b briefly keeps the sense input low upon application of v dd , providing a spin-up function on power-up that can be useful in some cases (but is generally not necessary). connecting c b to a voltage-divider that produces an output of v dd /2 can be used to minimize any spin-up or delay time. applications information power-supply decoupling to achieve the best results when using the device, decouple the v dd power supply with a (minimum) 0.01f capacitor. use a high-quality, ceramic, surface-mount capacitor if possible. surface-mount components mini - mize 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 mois - ture-sensitive device (msd) classifications. package information 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. + denotes a lead(pb)-free/rohs-compliant package. * ep = exposed pad. ordering information chip information substrate connected to ground process: cmos package type package code outline no. land pattern no. 8 tdfn-ep t823+1 21-0174 90-0091 part temp range pin-package MAX31740ata+ -40c to +125c 8 tdfn-ep* MAX31740 ultra-simple fan-speed controller www.maximintegrated.com
? 2013 maxim integrated products, inc. 11 revision history revision number revision date description pages changed 0 5/13 initial release 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 specifcations 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. maxim integrated and the maxim integrated logo are trademarks of maxim integrated products, inc. MAX31740 ultra-simple fan-speed controller for pricing, delivery, and ordering information, please contact maxim direct at 1-888-629-4642, or visit maxims website at www.maximintegrated.com.

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