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19-2304; Rev 0; 1/02
Four-Channel Thermistor Temperature-to-PulseWidth Converter
General Description
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 10A (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 MAX package and is specified from -55C to +125C temperature range. o Simple Single-Wire Interface o Measures Up to Four Thermistor Temperatures o Low-Average Thermistor Current Minimizes SelfHeating Errors o Internal Voltage Reference Isolates Thermistor from Power-Supply Noise o Accommodates Any Thermistor Temperature Range
Features
MAX6691
Ordering Information
PART MAX6691MUB TEMP RANGE -55C to +125C PIN-PACKAGE 10 MAX
Applications
HVAC Home Appliances Medical Devices
Typical Application Circuit
VCC
Pin Configuration
TOP VIEW
TOP VIEW
T1 T2 T1 T2 T3 T4 T3 T4 R1 2 3 4 5 REXT 10 9 VCC I/O N.C. GND R+ 10k
T1 1
MICROCONTROLLER
10 VCC 9 I/O N.C. GND R+
T2 T3 T4 R-
2 3 4 5
MAX6691
8 7 6
MAX6691
8 7 6
MAX
________________________________________________________________ Maxim Integrated Products
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Four-Channel Thermistor Temperature-to-PulseWidth Converter MAX6691
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 = +70C) 10-Pin MAX (derate 5.6mW/C above +70C) ........444.4mW Operating Temperature Range .........................-55C to +125C Junction Temperature ......................................................+150C Storage Temperature Range .............................-65C to +150C Lead Temperature (soldering, 10s) .................................+300C
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 = -55C to +125C, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25C.) (Note1)
PARAMETER THIGH/TLOW Accuracy Supply Voltage Range Supply Current Sleep-Mode Supply Current Input Leakage Current Reference Voltage Output Reference Load Regulation Reference Supply Rejection Logic Input Low Voltage Logic Input High Voltage VIL VIH 0.7 VCC SYMBOL VREXT VCC ICC ISTANDBY ILEAKAGE VREF IREF = 1mA, TA = +25C 0 < IREF < 2mA 1.19 1.24 0.1 0.2 0.3 VCC
CONDITIONS TA = +25C, VCC = 3.3V TA = TMIN to TMAX
MIN
TYP
MAX 0.5 1.0
UNITS % FS V A A A V % % V V
3.0 During conversion, no load 300 3.5
5.5 600 10 1.0 1.32 0.2
TIMING CHARACTERISTICS
(VCC = 3.0V to 5.5V, TA = -55C to +125C, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25C.) (Figure 1) (Note1)
PARAMETER Glitch Immunity on I/O Input Conversion Time Nominal Pulse Width Start Pulse Width Data Ready Pulse Width Error Pulse Width Rise Time Fall Time tCONV tLOW tSTART tREADY tERROR tRISE tFALL CL = 15pF, RL = 10k CL = 15pF, RL = 10k 86 4.0 5 103 103 122 122 600 600 188 188 SYMBOL CONDITIONS MIN TYP 500 102 4.9 156 7.5 MAX UNITS ns ms ms s s s ns ns
Note 1: Specification limits over temperature are guaranteed by design, not production tested.
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Four-Channel Thermistor Temperature-to-PulseWidth Converter MAX6691
__________________________________________Typical Operating Characteristics
(VCC = 5V, REXT = 7.5k, RTH = 12.5k, TA = +25C, unless otherwise noted.)
SLEEP-MODE SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX6691 toc01
THIGH/TLOW ERROR vs. POWER-SUPPLY NOISE FREQUENCY
MAX6691 toc02
THIGH/TLOW ERROR vs. POWER-SUPPLY NOISE FREQUENCY
THIGH/TLOW FULL-SCALE ERROR (%) VIN = SQUARE WAVE APPLIED TO VCC WITH NO VCC BYPASS CAPACITOR VIN = 250mVP-P TA = -55C TA = +25C
MAX6691 toc03
4.5 SLEEP-MODE SUPPLY CURRENT (A)
1.0 THIGH/TLOW FULL-SCALE ERROR (%)
4.0
0.5
VIN = SQUARE WAVE APPLIED TO VCC WITH NO VCC BYPASS CAPACITOR VIN = 250mVP-P
1.0
0.5
3.5
0 VCC = 5.0V VCC = 3.3V -0.5
0 TA = +85C -0.5 TA = +125C
3.0
2.5 3.0 3.5 4.0 4.5 5.0 5.5 SUPPLY VOLTAGE (V)
-1.0 0 5 10 15 20 25 POWER-SUPPLY NOISE FREQUENCY (MHz)
-1.0 0 5 10 15 20 25 POWER-SUPPLY NOISE FREQUENCY (MHz)
Pin Description
PIN 1 2 3 4 5 6 7 8 9 10 NAME T1 T2 T3 T4 RR+ GND N.C. I/O VCC FUNCTION Thermistor 1. Connect to external thermistor 1. Thermistor 2. Connect to external thermistor 2. Thermistor 3. Connect to external thermistor 3. Thermistor 4. Connect to external thermistor 4. External Resistor Low Side. Connect REXT between R- and R+. Reference Voltage Output. Connect REXT between R- and R+. Ground. Ground connection for MAX6691 and ground return for external thermistor(s). No Connection. Do not make a connection to this pin. I/O Connection to Microcontroller. Connect a 10k pullup resistor from I/O pin to VCC. Supply Voltage. Bypass VCC to GND with a capacitor of at least 0.1F.
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Four-Channel Thermistor Temperature-to-PulseWidth Converter MAX6691
THERMISTOR 1 DATA THIGH1 TLOW THERMISTOR 2 DATA THIGH2 TLOW TLOW THERMISTOR 3 DATA THERMISTOR 4 DATA THIGH4 TLOW
tCONV tSTART CONV REQUEST, PULLED LOW BY C 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.
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.
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 10k). To initiate measurement of up to four thermistor temperatures, the external microcontroller pulls the I/O pin low for at least 5s (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 125s. 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
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.
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Four-Channel Thermistor Temperature-to-PulseWidth Converter MAX6691
THERMISTOR RESISTANCE vs. TEMPERATURE
120 100 80 60 40 20 0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
THIGH/TLOW vs. TEMPERATURE FOR BETATHERM 10K3A1 THERMISTOR WITH REXT = 7680
1.0 0.9
THERMISTOR RESISTANCE (k)
0.8 0.7 THIGH/TLOW 0.6 0.5 0.4 0.3 0.2 0.1 0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C)
Figure 2. Thermistor Resistance vs. Temperature
THIGH/TLOW vs. TEMPERATURE FOR BETATHERM 10K3A1 THERMISTOR WITH REXT = 5110
1.2 1.0 0.8 THIGH/TLOW 0.6 0.4 0.2 0 0 20 40 60 80 100 120 140 TEMPERATURE (C)
Figure 4. THIGH/TLOW vs. Temperature, REXT = 5110
NTC thermistors are often described by the resistance at +25C. Therefore, a 10k thermistor has a resistance of 10k at +25C. 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 10k 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 0C to +70C). 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 (+35C for the 0C to +70C range). 3) Find REXT using the equation below: REXT = RMIN (RMIN + RMAX ) - 2RMIN x RMAX RMIN + RMAX - 2RMID
Figure 3. THIGH/TLOW vs. Temperature, REXT = 5110
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.
Power-Supply Considerations
The MAX6691 accuracy is relatively unaffected by power-supply coupled noise. In most applications,
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Four-Channel Thermistor Temperature-to-PulseWidth Converter MAX6691
Table 1. Standard Thermistors
MANUFACTURER Betatherm Dale Thermometrics PART 10K3A1 1M1002 C100Y103J www.betatherm.com/indexna.htm www.vishay.com/brands/dale/main.html www.thermometrics.com WEBSITE
bypass V CC to GND by placing a 0.1F to 1.0F 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 1C. For example, consider a 10k (at +25C) NTC thermistor in series with a 5110 resistor operating +40C 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 +40C of 1.22C. 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 = 0.364W, or only about 0.00036C self-heating error.
Chip Information
TRANSISTOR COUNT: 7621 PROCESS: BiCMOS
Functional Diagram
R+
REFERENCE
R-
VOLTAGE-TO-PWM CONVERTER
I/O
T1 T2 T3 T4
MAX6691
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Four-Channel Thermistor Temperature-to-PulseWidth Converter
Package Information
10LUMAX.EPS
MAX6691
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 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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