general description the max6691 four-channel thermistor temperature-to- pulse-width converter measures the temperatures of up to four thermistors and converts them to a series of out- put pulses whose widths are related to the thermistors temperatures. each of the four thermistors and an external fixed resistor (r ext ) form a voltage-divider that is driven by the max6691? internal voltage reference (v ref ). v ref and the voltage across r ext are mea- sured and converted to a pulse. the max6691 has a single open-drain i/o pin that can be readily connected to a variety of microcontrollers. the microcontroller initiates a conversion by pulling the i/o pin low and releasing it. when conversion is done, the max6691 signals the end of conversion by pulling the i/o pin low once again. the pulse corresponding to the first thermistor is sent immediately after the release of the i/o pin. the on-chip power-management circuitry reduces the average thermistor current to minimize errors due to thermistor self-heating. between conversions, the max6691 falls into a 10? (max) sleep mode, where the voltage reference is disabled and the supply cur- rent is at its minimum. the max6691 is available in a 10-pin ?ax package and is specified from -55? to +125? temperature range. applications hvac home appliances medical devices features ? simple single-wire interface ? measures up to four thermistor temperatures ? low-average thermistor current minimizes self- heating errors ? internal voltage reference isolates thermistor from power-supply noise ? accommodates any thermistor temperature range max6691 four-channel thermistor temperature-to-pulse- width converter ________________________________________________________________ maxim integrated products 1 1 2 3 4 5 10 9 8 7 6 v cc i/o n.c. gnd t4 t3 t2 t1 max6691 max top view r+ r- pin configuration 1 2 3 4 5 10 9 8 7 6 v cc i/o n.c. gnd t4 t3 t2 t1 max6691 top view r+ r- t4 t3 t2 t1 micro- controller 10k ? v cc r ext typical application circuit 19-2304; rev 1; 2/07 for pricing, delivery, and ordering information, please contact maxim/dallas direct! at 1-888-629-4642, or visit maxim? website at www.maxim-ic.com. ordering information part temp range pin- package pkg code MAX6691MUB -55 c to +125 c 10 ?ax u10-2
max6691 four-channel thermistor temperature-to-pulse- width converter 2 _______________________________________________________________________________________ absolute maximum ratings stresses beyond those listed under ?bsolute maximum ratings?may cause permanent damage to the device. these are stress rating s only, and functional operation of the device at these or any other conditions beyond those indicated in the operational sections of the specificatio ns is not implied. exposure to absolute maximum rating conditions for extended periods may affect device reliability. note 1: specification limits over temperature are guaranteed by design, not production tested. v cc to gnd ...........................................................-0.3v to +6.0v all other pins to gnd.................................-0.3v to (v cc + 0.3v) i/o, r+, r-, t1?4 current................................................?0ma esd protection (human body model) .............................?000v continuous power dissipation (t a = +70?) 10-pin ?ax (derate 5.6mw/? above +70?) ........444.4mw operating temperature range .........................-55? to +125? junction temperature ......................................................+150? storage temperature range .............................-65? to +150? lead temperature (soldering, 10s) .................................+300? electrical characteristics (v cc = 3.0v to 5.5v, t a = -55? to +125?, unless otherwise noted. typical values are specified at v cc = 3.3v and t a = +25?.) (note1) parameter symbol conditions min typ max units t a = +25 c, v cc = 3.3v 0.5 t high /t low accuracy v rext t a = t min to t max 1.0 % fs supply voltage range v cc 3.0 5.5 v supply current i cc during conversion, no load 300 600 ? sleep-mode supply current i standby 3.5 10 ? input leakage current i leakage 1.0 ? reference voltage output v ref i ref = 1ma, t a = +25 c 1.19 1.24 1.32 v reference load regulation 0 < i ref < 2ma 0.1 0.2 % reference supply rejection 0.2 % logic input low voltage v il 0.3 ? ? timing characteristics (v cc = 3.0v to 5.5v, t a = -55? to +125?, unless otherwise noted. typical values are specified at v cc = 3.3v and t a = +25?.) (figure 1) (note1) parameter symbol conditions min typ max units glitch immunity on i/o input 500 ns conversion time t conv 86 102 156 ms nominal pulse width t low 4.0 4.9 7.5 ms start pulse width t start 5 s data ready pulse width t ready 103 122 188 ? error pulse width t error 103 122 188 ? rise time t rise c l = 15pf, r l = 10k ? = = ?
max6691 four-channel thermistor temperature-to-pulse- width converter _______________________________________________________________________________________ 3 sleep-mode supply current vs. supply voltage max6691 toc01 supply voltage (v) sleep-mode supply current ( a) 5.0 4.5 4.0 3.5 3.0 3.5 4.0 4.5 2.5 3.0 5.5 t high /t low error vs. power-supply noise frequency max6691 toc02 power-supply noise frequency (mhz) t high /t low full-scale error (%) 20 15 10 5 -0.5 0 0.5 1.0 -1.0 025 v cc = 5.0v v cc = 3.3v v in = square wave applied to v cc with no vcc bypass capacitor v in = 250mv p-p t high /t low error vs. power-supply noise frequency max6691 toc03 power-supply noise frequency (mhz) t high /t low full-scale error (%) 20 15 10 5 -0.5 0 0.5 1.0 -1.0 025 t a = +25 c t a = -55 c t a = +85 c t a = +125 c v in = square wave applied to v cc with no vcc bypass capacitor v in = 250mv p-p __________________________________________typical operating characteristics (v cc = 5v, r ext = 7.5k ? , r th = 12.5k ? , t a = +25?, unless otherwise noted.) pin name function 1 t1 thermistor 1. connect to external thermistor 1. 2 t2 thermistor 2. connect to external thermistor 2. 3 t3 thermistor 3. connect to external thermistor 3. 4 t4 thermistor 4. connect to external thermistor 4. 5 r- external resistor low side. connect r ext between r- and r+. 6 r+ reference voltage output. connect r ext between r- and r+. 7 gnd ground. ground connection for max6691 and ground return for external thermistor(s). 8 n.c. no connection. do not make a connection to this pin. 9 i/o i/o connection to microcontroller. connect a 10k ? pullup resistor from i/o pin to v cc . 10 v cc supply voltage. bypass v cc to gnd with a capacitor of at least 0.1?. pin description
max6691 detailed description the max6691 is an interface circuit that energizes up to four thermistors and converts their temperatures to a series of output pulses. the max6691 powers the ther- mistors only when a measurement is being made. this minimizes the power dissipation in the thermistors, virtu- ally eliminating self-heating, a major component of ther- mistor error. the simple i/o allows the initiation of conversion and delivery of output pulses or a single pin. temperature measurement when it is not performing conversions or transmitting output pulses, the max6691 is in a low-power sleep mode and the i/o pin is held at v cc by the external pullup resistor (typically 10k ? ). to initiate measurement of up to four thermistor temperatures, the external microcontroller pulls the i/o pin low for at least 5 s (figure 1). when the microcontroller releases the i/o pin, the max6691 applies the reference voltage (v ref ) to the external resistor (r ext ), which is connected sequentially to each of the four external thermistors (t1 through t4). when the measurements are complete (after a period equal to t conv ), the max6691 pulls the i/o pin low for 125?. the i/o pin remains high for a period proportion- al to the first v ext measurement (corresponding to the first thermistor). the max6691 then pulls the i/o pin low for a period proportional to v ref . three more high/low pulse pairs follow, corresponding to t2 through t4, after which the i/o pin is released. the relationship between pulse width, r ext , and ther- mistor resistance (r th ) can be described as: the relationship between v ext and the temperature of a thermistor is determined by the values of r ext and the thermistor? characteristics. if the relationship between r th and the temperature is known, a micro- controller with no on-chip adc can measure t high and t low and accurately determine the temperature at the corresponding thermistor. for each operation, the max6691 generates four puls- es on the i/o pin. in the case of an open or short con- nection on the thermistor, the corresponding pulse (t high ) is a short pulse of less than 5% of t low . applications information thermistors and thermistor selection either ntc or ptc thermistors can be used with the max6691, but ntc thermistors are more commonly used. ntc thermistors are resistive temperature sen- sors whose resistance decreases with increasing tem- perature. they are available in a wide variety of packages that are useful in difficult applications such as measurement of air or liquid temperature. some can operate over temperature ranges beyond that of most ics. the relationship between temperature and resis- tance in an ntc thermistor is very nonlinear and can be described by the following approximation: where t is absolute temperature, r is the thermistor? resistance, and a, b, c are coefficients that vary with manufacturer and material characteristics. the general shape of the curve is shown in figure 2. 1 3 t c inr =+ a b(inr) + () t t v v .0002 = r r+r high low ext ref ext ext th =? ? 0 0 0002 . four-channel thermistor temperature-to-pulse- width converter 4 _______________________________________________________________________________________ t start conv request, pulled low by c t ready data ready, pulled low by max6691 t error thermistor is either open or short t conv t high1 t high2 t high4 thermistor 1 data t low t low t low t low thermistor 2 data thermistor 3 data thermistor 4 data figure 1. timing diagram
the relationship between temperature and resistance of an ntc thermistor is highly nonlinear. however, by connecting the thermistors in series with a properly chosen resistor (r ext ) and using the max6691 to mea- sure the voltage across the resistor, a reasonably linear transfer function can be obtained over a limited temper- ature range. linearity improves for smaller temperature ranges. figures 3 and 4 show typical t high /t low curves for a standard thermistor in conjunction with values of r ext chosen to optimize linearity over two series resistors chosen to optimize linearity over two different tempera- ture ranges. ntc thermistors are often described by the resistance at +25?. therefore, a 10k ? thermistor has a resis- tance of 10k ? at +25?. when choosing a thermistor, ensure that the thermistor? minimum resistance (which occurs at the maximum expected operating tempera- ture) in series with r ext does not cause the voltage ref- erence output current to exceed about 1ma. some standard 10k ? thermistors with similar characteristics are listed in table 1. choosing r ext choose r ext to minimize nonlinearity errors from the thermistor: 1) decide on the temperature range of interest (for example 0? to +70?). 2) find the thermistor values at the limits of the tem- perature range. r min is the minimum thermistor value (at the maximum temperature) and r max is the maximum thermistor value (at the minimum tem- perature). also find r mid , the thermistor resistance in the middle of the temperature range (+35? for the 0? to +70? range). 3) find r ext using the equation below: power-supply considerations the max6691 accuracy is relatively unaffected by power-supply coupled noise. in most applications, r rr rr rr r min max min max min max mid ext mid = r + () ? +? 2 2 max6691 four-channel thermistor temperature-to-pulse- width converter _______________________________________________________________________________________ 5 figure 2. thermistor resistance vs. temperature 0 20 40 60 80 100 120 -40 0 -20 20406080100120 thermistor resistance vs. temperature temperature ( c) thermistor resistance (k ? ) figure 3. t high /t low vs. temperature, r ext = 5110 ? t high /t low vs. temperature for betatherm 10k3a1 thermistor with r ext = 5110 ? temperature ( c) t high /t low 120 100 80 60 40 20 0.2 0.4 0.6 0.8 1.0 1.2 0 0 140 figure 4. t high /t low vs. temperature, r ext = 7680 ? t high /t low vs. temperature for betatherm 10k3a1 thermistor with r ext = 7680 ? temperature ( c) t high /t low 100 80 40 60 0 20 -20 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 0 -40 120
max6691 bypass v cc to gnd by placing a 0.1 f to 1.0? ceramic bypass capacitor close to the supply pin of the devices. thermal considerations self-heating degrades the temperature measurement accuracy of thermistors. the amount of self-heating depends on the power dissipated and the dissipation constant of the thermistor. dissipation constants depend on the thermistor? package and can vary con- siderably. a typical thermistor might have a dissipation constant equal to 1mw/?. for every milliwatt the thermistor dis- sipates, its temperature rises by 1?. for example, con- sider a 10k ? (at +25?) ntc thermistor in series with a 5110 ? resistor operating +40? with a constant 5v bias. if it is one of the standard thermistors previously mentioned, its resistance is 5325 ? at this temperature. the power dissipated in the thermistor is: (5v) 2 (5325 ? ) / (5325 ? + 5110 ? ) 2 = 1.22mw this thermistor therefore has a self-heating error at +40? of 1.22?. because the max6691 uses a small reference voltage and energizes each thermistor for only about 25ms per conversion cycle, the self-heating of the thermistor under the same conditions when used with the max6691 is far less. assuming one conversion cycle every 5s, each thermistor is energized only 0.5% of the time: (1.22) 2 (5325)(0.005) / (5325 + 5110) 2 = 0.364?, or only about 0.00036? self-heating error. chip information transistor count: 7621 process: bicmos four-channel thermistor temperature-to-pulse- width converter 6 _______________________________________________________________________________________ table 1. standard thermistors manufacturer part website betatherm 10k3a1 www.betatherm.com/indexna.htm dale 1m1002 www.vishay.com/brands/dale/main.html thermometrics c100y103j www.thermometrics.com t1 t2 t3 t4 reference voltage-to-pwm converter r- r+ i/o max6691 functional diagram
max6691 four-channel thermistor temperature-to-pulse- width converter maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a maxim product. no circu it patent licenses are implied. maxim reserves the right to change the circuitry and specifications without notice at any time. maxim integrated products, 120 san gabriel drive, sunnyvale, ca 94086 408-737-7600 _____________________ 7 2007 maxim integrated products is a registered trademark of maxim integrated products, inc. package information 10lumax.eps package outline, 10l umax/usop 1 1 21-0061 rev. document control no. approval proprietary information title: top view front view 1 0.498 ref 0.0196 ref s 6 side view bottom view 0 0 6 0.037 ref 0.0078 max 0.006 0.043 0.118 0.120 0.199 0.0275 0.118 0.0106 0.120 0.0197 bsc inches 1 10 l1 0.0035 0.007 e c b 0.187 0.0157 0.114 h l e2 dim 0.116 0.114 0.116 0.002 d2 e1 a1 d1 min - a 0.940 ref 0.500 bsc 0.090 0.177 4.75 2.89 0.40 0.200 0.270 5.05 0.70 3.00 millimeters 0.05 2.89 2.95 2.95 - min 3.00 3.05 0.15 3.05 max 1.10 10 0.60.1 0.60.1 ? 0.500.1 h 4x s e d2 d1 b a2 a e2 e1 l l1 c gage plane a2 0.030 0.037 0.75 0.95 a1 revision history pages changed at rev 1: 1, 5, 7
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