19-2304; Rev 1; 2/07 Four-Channel Thermistor Temperature-to-PulseWidth Converter The MAX6691 four-channel thermistor temperature-topulse-width converter measures the temperatures of up to four thermistors and converts them to a series of output pulses whose widths are related to the thermistors’ temperatures. Each of the four thermistors and an external fixed resistor (REXT) form a voltage-divider that is driven by the MAX6691’s internal voltage reference (VREF). VREF and the voltage across REXT are measured 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 10µA (max) sleep mode, where the voltage reference is disabled and the supply current is at its minimum. The MAX6691 is available in a 10-pin µMAX package and is specified from -55°C to +125°C temperature range. Features ♦ Simple Single-Wire Interface ♦ Measures Up to Four Thermistor Temperatures ♦ Low-Average Thermistor Current Minimizes SelfHeating Errors ♦ Internal Voltage Reference Isolates Thermistor from Power-Supply Noise ♦ Accommodates Any Thermistor Temperature Range Ordering Information PART TEMP RANGE MAX6691MUB -55°C to +125°C PINPACKAGE PKG CODE 10 µMAX U10-2 Applications HVAC Home Appliances Medical Devices Pin Configuration Typical Application Circuit VCC TOP VIEW TOP VIEW T1 T2 T1 T3 T2 T4 T3 RT4 1 10 2 9 3 8 MAX6691 4 7 5 6 REXT VCC I/O N.C. GND R+ 10kΩ T1 1 MICROCONTROLLER T2 10 VCC 2 MAX6691 9 I/O T3 3 8 N.C. T4 4 7 GND R- 5 6 R+ µMAX ________________________________________________________________ Maxim Integrated Products For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at 1-888-629-4642, or visit Maxim’s website at www.maxim-ic.com. 1 MAX6691 General Description MAX6691 Four-Channel Thermistor Temperature-to-PulseWidth Converter ABSOLUTE MAXIMUM RATINGS VCC to GND ...........................................................-0.3V to +6.0V All Other Pins to GND.................................-0.3V to (VCC + 0.3V) I/O, R+, R-, T1–T4 Current................................................±20mA ESD Protection (Human Body Model) .............................±2000V Continuous Power Dissipation (TA = +70°C) 10-Pin µMAX (derate 5.6mW/°C above +70°C) ........444.4mW Operating Temperature Range .........................-55°C to +125°C Junction Temperature ......................................................+150°C Storage Temperature Range .............................-65°C to +150°C Lead Temperature (soldering, 10s) .................................+300°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. ELECTRICAL CHARACTERISTICS (VCC = 3.0V to 5.5V, TA = -55°C to +125°C, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25°C.) (Note1) PARAMETER SYMBOL THIGH/TLOW Accuracy VREXT Supply Voltage Range VCC Supply Current ICC Sleep-Mode Supply Current ISTANDBY Input Leakage Current ILEAKAGE Reference Voltage Output VREF Reference Load Regulation CONDITIONS MIN TYP MAX TA = +25°C, VCC = 3.3V 0.5 TA = TMIN to TMAX 1.0 3.0 During conversion, no load IREF = 1mA, TA = +25°C 1.19 0 < IREF < 2mA Reference Supply Rejection VIL Logic Input High Voltage VIH % FS 5.5 V 300 600 µA 3.5 10 µA 1.0 µA 1.24 1.32 V 0.1 0.2 % 0.2 Logic Input Low Voltage UNITS % 0.3 ✕ VCC 0.7 ✕ VCC V V TIMING CHARACTERISTICS (VCC = 3.0V to 5.5V, TA = -55°C to +125°C, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25°C.) (Figure 1) (Note1) PARAMETER SYMBOL CONDITIONS MIN Glitch Immunity on I/O Input TYP UNITS ns Conversion Time tCONV 86 102 156 ms Nominal Pulse Width tLOW 4.0 4.9 7.5 ms Start Pulse Width tSTART 5 Data Ready Pulse Width tREADY 103 122 188 µs µs Error Pulse Width tERROR 103 122 188 µs Rise Time tRISE CL = 15pF, RL = 10kΩ 600 ns Fall Time tFALL CL = 15pF, RL = 10kΩ 600 ns Note 1: Specification limits over temperature are guaranteed by design, not production tested. 2 MAX 500 _______________________________________________________________________________________ Four-Channel Thermistor Temperature-to-PulseWidth Converter 3.5 3.0 2.5 MAX6691 toc02 VIN = SQUARE WAVE APPLIED TO VCC WITH NO VCC BYPASS CAPACITOR VIN = 250mVP-P 0.5 0 VCC = 5.0V VCC = 3.3V -0.5 1.0 THIGH/TLOW FULL-SCALE ERROR (%) 4.0 1.0 THIGH/TLOW FULL-SCALE ERROR (%) MAX6691 toc01 SLEEP-MODE SUPPLY CURRENT (µA) 4.5 THIGH/TLOW ERROR vs. POWER-SUPPLY NOISE FREQUENCY THIGH/TLOW ERROR vs. POWER-SUPPLY NOISE FREQUENCY 3.5 4.0 4.5 5.0 SUPPLY VOLTAGE (V) 5.5 0.5 0 TA = +85°C TA = +125°C -0.5 -1.0 -1.0 3.0 VIN = SQUARE WAVE APPLIED TO VCC WITH NO VCC BYPASS CAPACITOR VIN = 250mVP-P TA = -55°C TA = +25°C MAX6691 toc03 SLEEP-MODE SUPPLY CURRENT vs. SUPPLY VOLTAGE 0 5 10 15 20 25 POWER-SUPPLY NOISE FREQUENCY (MHz) 5 0 10 15 20 25 POWER-SUPPLY NOISE FREQUENCY (MHz) Pin Description 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 REXT between R- and R+. 6 R+ Reference Voltage Output. Connect REXT 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 10kΩ pullup resistor from I/O pin to VCC. 10 VCC Supply Voltage. Bypass VCC to GND with a capacitor of at least 0.1µF. _______________________________________________________________________________________ 3 MAX6691 __________________________________________Typical Operating Characteristics (VCC = 5V, REXT = 7.5kΩ, RTH = 12.5kΩ, TA = +25°C, unless otherwise noted.) MAX6691 Four-Channel Thermistor Temperature-to-PulseWidth Converter THERMISTOR 1 DATA THIGH1 THERMISTOR 2 DATA THERMISTOR 3 DATA THIGH2 TLOW THERMISTOR 4 DATA THIGH4 TLOW TLOW TLOW tCONV tSTART CONV REQUEST, PULLED LOW BY µC tREADY DATA READY, PULLED LOW BY MAX6691 tERROR THERMISTOR IS EITHER OPEN OR SHORT Figure 1. Timing Diagram 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 thermistors only when a measurement is being made. This minimizes the power dissipation in the thermistors, virtually eliminating self-heating, a major component of thermistor 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 VCC by the external pullup resistor (typically 10kΩ). To initiate measurement of up to four thermistor temperatures, the external microcontroller pulls the I/O pin low for at least 5µs (Figure 1). When the microcontroller releases the I/O pin, the MAX6691 applies the reference voltage (VREF) 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 TCONV), the MAX6691 pulls the I/O pin low for 125µs. The I/O pin remains high for a period proportional to the first VEXT measurement (corresponding to the first thermistor). The MAX6691 then pulls the I/O pin low for a period proportional to VREF. 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, REXT, and thermistor resistance (RTH) can be described as: THIGH V REXT = EXT − 0.0002 = − 0.0002 TLOW VREF REXT +RTH 4 The relationship between VEXT and the temperature of a thermistor is determined by the values of REXT and the thermistor’s characteristics. If the relationship between RTH and the temperature is known, a microcontroller with no on-chip ADC can measure THIGH and TLOW and accurately determine the temperature at the corresponding thermistor. For each operation, the MAX6691 generates four pulses on the I/O pin. In the case of an open or short connection on the thermistor, the corresponding pulse (THIGH) is a short pulse of less than 5% of TLOW. 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 sensors whose resistance decreases with increasing temperature. 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 resistance in an NTC thermistor is very nonlinear and can be described by the following approximation: 1 = A + B(InR) + C(InR)3 T Where T is absolute temperature, R is the thermistor’s 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. _______________________________________________________________________________________ Four-Channel Thermistor Temperature-to-PulseWidth Converter MAX6691 THERMISTOR RESISTANCE vs. TEMPERATURE THIGH/TLOW vs. TEMPERATURE FOR BETATHERM 10K3A1 THERMISTOR WITH REXT = 7680Ω 1.0 120 0.8 0.7 80 THIGH/TLOW THERMISTOR RESISTANCE (kΩ) 0.9 100 60 40 0.6 0.5 0.4 0.3 0.2 20 0.1 0 0 -40 -20 0 20 40 60 80 -40 100 120 Figure 2. Thermistor Resistance vs. Temperature THIGH/TLOW vs. TEMPERATURE FOR BETATHERM 10K3A1 THERMISTOR WITH REXT = 5110Ω 1.2 1.0 THIGH/TLOW 0.8 0.6 0.4 0.2 0 0 20 40 60 80 100 -20 0 20 40 60 80 100 120 TEMPERATURE (°C) TEMPERATURE (°C) 120 140 TEMPERATURE (°C) Figure 3. THIGH/TLOW vs. Temperature, REXT = 5110Ω 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 (REXT) and using the MAX6691 to measure the voltage across the resistor, a reasonably linear transfer function can be obtained over a limited temperature range. Linearity improves for smaller temperature ranges. Figures 3 and 4 show typical THIGH/TLOW curves for a standard thermistor in conjunction with values of REXT chosen to optimize linearity over two series resistors chosen to optimize linearity over two different temperature ranges. Figure 4. THIGH/TLOW vs. Temperature, REXT = 7680Ω NTC thermistors are often described by the resistance at +25°C. Therefore, a 10kΩ thermistor has a resistance of 10kΩ at +25°C. When choosing a thermistor, ensure that the thermistor’s minimum resistance (which occurs at the maximum expected operating temperature) in series with REXT does not cause the voltage reference output current to exceed about 1mA. Some standard 10kΩ thermistors with similar characteristics are listed in Table 1. Choosing REXT Choose REXT to minimize nonlinearity errors from the thermistor: 1) Decide on the temperature range of interest (for example 0°C to +70°C). 2) Find the thermistor values at the limits of the temperature range. R MIN is the minimum thermistor value (at the maximum temperature) and RMAX is the maximum thermistor value (at the minimum temperature). Also find RMID, the thermistor resistance in the middle of the temperature range (+35°C for the 0°C to +70°C range). 3) Find REXT using the equation below: REXT = RMID (RMIN + RMAX ) − 2RMIN × RMAX RMIN + RMAX − 2RMID Power-Supply Considerations The MAX6691 accuracy is relatively unaffected by power-supply coupled noise. In most applications, _______________________________________________________________________________________ 5 MAX6691 Four-Channel Thermistor Temperature-to-PulseWidth Converter 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 bypass V CC to GND by placing a 0.1µF to 1.0µF 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’s package and can vary considerably. A typical thermistor might have a dissipation constant equal to 1mW/°C. For every milliwatt the thermistor dissipates, its temperature rises by 1°C. For example, consider a 10kΩ (at +25°C) NTC thermistor in series with a 5110Ω resistor operating +40°C with a constant 5V bias. If it is one of the standard thermistors previously mentioned, its resistance is 5325Ω at this temperature. The power dissipated in the thermistor is: This thermistor therefore has a self-heating error at +40°C of 1.22°C. 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 = 0.364µW, or only about 0.00036°C self-heating error. Chip Information TRANSISTOR COUNT: 7621 PROCESS: BiCMOS (5V)2 (5325Ω) / (5325Ω + 5110Ω)2 = 1.22mW Functional Diagram R+ R- REFERENCE VOLTAGE-TO-PWM CONVERTER T1 T2 MAX6691 T3 T4 6 _______________________________________________________________________________________ I/O Four-Channel Thermistor Temperature-to-PulseWidth Converter 10LUMAX.EPS e 4X S 10 10 INCHES H Ø0.50±0.1 0.6±0.1 1 1 0.6±0.1 BOTTOM VIEW TOP VIEW D2 MILLIMETERS MAX DIM MIN 0.043 A 0.006 A1 0.002 A2 0.030 0.037 0.120 D1 0.116 0.118 D2 0.114 0.120 E1 0.116 0.118 E2 0.114 0.199 H 0.187 L 0.0157 0.0275 L1 0.037 REF b 0.007 0.0106 e 0.0197 BSC c 0.0035 0.0078 0.0196 REF S α 0° 6° MAX MIN 1.10 0.15 0.05 0.75 0.95 3.05 2.95 2.89 3.00 3.05 2.95 2.89 3.00 4.75 5.05 0.40 0.70 0.940 REF 0.177 0.270 0.500 BSC 0.090 0.200 0.498 REF 0° 6° E2 GAGE PLANE A2 c A b A1 α E1 D1 L L1 FRONT VIEW SIDE VIEW PROPRIETARY INFORMATION TITLE: PACKAGE OUTLINE, 10L uMAX/uSOP APPROVAL DOCUMENT CONTROL NO. 21-0061 REV. 1 1 Revision History Pages changed at Rev 1: 1, 5, 7 Maxim cannot assume responsibility for use of any circuitry other than circuitry entirely embodied in a Maxim product. No circuit 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. MAX6691 Package Information