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19-2219; Rev 0; 2/02
Thermistor-to-Digital Converter
General Description
The MAX6682 converts an external thermistor's temperature-dependent resistance directly into digital form. The thermistor and an external fixed resistor form a voltage-divider that is driven by the MAX6682's internal voltage reference. The MAX6682 measures the voltage across the external resistor and produces a 10-bit + sign output code dependent on that voltage. The MAX6682 does not linearize the highly nonlinear transfer function of a typical negative temperature coefficient (NTC) thermistor, but it does provide linear output data over limited temperature ranges when used with an external resistor of the correct value. Over the 0 to +50C temperature range, the MAX6682 produces output data that is scaled to 8LSBs/C (for 0.125C resolution), provided that the correct thermistor and external resistor values are used. Other temperature ranges can be easily accommodated, but do not necessarily yield data scaled to an even number of LSBs per degree. The 3-wire SPITM-compatible interface can be readily connected to a variety of microcontrollers. The MAX6682 is a read-only device, simplifying use in systems where only temperature data is required. Power-management circuitry reduces the average thermistor current, minimizing self-heating. Between conversions, supply current is reduced to 21A (typ). The internal voltage reference is shut down between measurements. The MAX6682 is available in a small, 8-pin MAX package and is specified over the -55C to +125C temperature range.
Features
o Converts Thermistor Temperature to Digital Data o Low Average Thermistor Current Minimizes SelfHeating Errors o Low Supply Current, 21A (typ) Including 10k Thermistor Current o Internal Voltage Reference Isolates Thermistor from Power-Supply Noise o 10-Bit Resolution o Accommodates Any Thermistor Temperature Range o Output Data Scaled for Direct Temperature Readings from 0C to +50C o Simple SPI-Compatible Interface o Small, 8-Pin MAX Package
MAX6682
Ordering Information
PART MAX6682MUA TEMP RANGE -55C to +125C PIN-PACKAGE 8 MAX
Typical Operating Circuit
3.3V
Applications
HVAC Medical Devices Battery Packs/Chargers Home Appliances
R+ REXT
0.1F VCC
MAX6682
RMC68HCXX CS SCLK GND SO I/O SCLK MISO
Pin Configuration appears at end of data sheet.
THERMISTOR
SPI is a trademark of Motorola, Inc.
________________________________________________________________ Maxim Integrated Products
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Thermistor-to-Digital Converter MAX6682
ABSOLUTE MAXIMUM RATINGS
Supply Voltage (VCC to GND) .................................-0.3V to +6V SO, SCK, CS, R-, R+ to GND ....................-0.3V to (VCC + 0.3V) R+ Current ........................................................................20mA R- Current ...........................................................................1mA SCK, CS, SO Current .........................................-1mA to +50mA ESD Protection (Human Body Model) .............................2000V Continuous Power Dissipation (TA = +70C) 8-Pin MAX (derate 4.1mW/C above +70C) ............ 328mW Operating Temperature Range (TMIN to TMAX) ...............................................-55C to +125C Storage Temperature Range .............................-65C to +150C Junction Temperature .....................................................+150C
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 = 3V to 5.5V, TA = -55C to +125C, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25C.) (Note 1)
PARAMETER Supply Voltage ADC Total Unadjusted Error ADC Conversion Time R- Input Impedance R- Leakage Current Conversion Rate Reference Voltage Output Reference Load Regulation Reference Supply Regulation Conversion Supply Current Average Supply Current Standby Current Idle Current SERIAL INTERFACE Input Low Voltage Input High Voltage Input Leakage Current Output High Voltage Output Low Voltage VIL VIH ILEAK VOH VOL VIN = GND or VCC ISOURCE = 1.6mA ISINK = 1.6mA VCC 0.4 0.4 0.8 x VCC 1 0.2 x VCC V V A V V IC IA IS IID During conversion, no load 0.5 conversions/s, no load CS low, SCK inactive CS high, analog circuits off VREF ILOAD = 1mA 0 < ILOAD < 2mA 1.10 0 0.7 220 17 3 10 300 29 7 17 SYMBOL VCC TUE tCONV ZIN 1 1 0.5 1.22 1.40 0.1 50 DOUT = 768.935 x (VREXT/VR+) - 134.0923; VIN > 0.1VREF CONDITIONS MIN 3.0 -3 64 TYP MAX 5.5 +3 80 UNITS V LSB ms M nA Hz V %/mA mV/V A A A A
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Thermistor-to-Digital Converter
TIMING CHARACTERISTICS
(VCC = 3V to 5.5V, TA = TMIN to TMAX, unless otherwise noted. Typical values are specified at VCC = 3.3V and TA = +25C.) (Note 2)
PARAMETER Serial Clock Frequency SCK Pulse High Width SCK Pulse Low Width CS Fall to SCK Rise CS Fall to Output Data Valid SCK Fall to Output Data Valid CS Rise to Output High-Z SCK Fall to Output High-Z CS Pulse Width SYMBOL fSCL tCH tCL tCSS tDV tDO tTR tHIZ tCSW CL = 10pF CL = 10pF CL = 10pF CL = 10pF 75 CONDITIONS MIN TYP MAX 5 50 50 35 35 35 25 35 UNITS MHz ns ns ns ns ns ns ns ns
MAX6682
SERIAL INTERFACE TIMING (Figures 5 and 6)
Note 1: All specifications are 100% tested at TA = +25C. Specification limits over temperature are guaranteed by design, not production tested. Note 2: Guaranteed by design.
Typical Operating Characteristics
(VCC = 5V, thermistor = 10k nominal, REXT = 7680, TA = +25C, unless otherwise noted.)
TEMPERATURE ERROR vs. POWER-SUPPLY NOISE FREQUENCY
MAX6682 toc01
AVERAGE SUPPLY CURRENT vs. CLOCK FREQUENCY
MAX6682 toc02
AVERAGE SUPPLY CURRENT vs. SUPPLY VOLTAGE
MAX6682 toc03
1.0 0.9 TEMPERATURE ERROR (C) 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 20 VIN = SQUARE WAVE APPLIED TO VCC WITH NO VCC BYPASS CAPACITOR VIN = 250mVP-P
100 90 SUPPLY CURRENT (A) 80 70 60 50 40
SCK IS DRIVEN RAIL-TO-RAIL(R)
100 90 SUPPLY CURRENT (A) 80 70 60 50 40 30
25
1k
10k
100k
1M
10M
3.0
3.5
4.0
4.5
5.0
5.5
FREQUENCY (MHz)
SCK FREQUENCY (Hz)
SUPPLY VOLTAGE (V)
Rail-to-Rail is a registered trademark of Nippon Motorola, Ltd.
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Thermistor-to-Digital Converter MAX6682
Pin Description
PIN 1 2 3 4 5 6 7 8 NAME I.C. R+ RGND CS SO SCK VCC FUNCTION Internally Connected. Connect to GND or leave unconnected. Reference Voltage Output. External resistor positive input. External Resistor Negative Input. Connect R- to the junction of the external resistor and the thermistor. Ground. Ground connection for MAX6682 and ground return for external thermistor. Chip Select. Drive CS low to enable the serial interface. Serial Data Output Serial Clock Input Positive Supply. Bypass VCC to GND with a 0.1F capacitor.
Detailed Description
The MAX6682 is a sophisticated interface circuit that energizes a low-cost thermistor and converts its temperature-dependent resistance to 10-bit digital data. The MAX6682 powers the thermistor only when a measurement is being made; the power dissipated in the thermistor is minimized. This virtually eliminates selfheating, a major component of thermistor error. The simple serial interface is compatible with common microcontrollers.
Temperature Conversion
The MAX6682 converts the voltage drop across the resistor REXT to a digital output using an internal 10-bit ADC. By measuring the voltage across REXT, the output code is directly related to temperature when using an NTC thermistor. Although the relationship between a thermistor's resistance and its temperature is very nonlinear, the voltage across REXT is reasonably linear over a limited temperature range, provided that REXT is chosen properly. For example, over a +10C to +40C range, the relationship between the voltage across REXT and temperature is linear to within approximately 0.2C. Wider temperature ranges result in larger errors. The digital output is available as a 10-bit + sign word. The relationship between the 11-bit digital word and the voltage across REXT (normalized to VR+) is given by: VREXT - 0.174387 x 8 V R+ DOUT = 0.010404 where VREXT/VR+ is the voltage across REXT normalized to the value of VR+.
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Table 1 shows the relationship between the voltage across REXT and the MAX6682's digital output code. It also shows the temperature that would produce the listed value of VREXT when a standard thermistor is used in conjunction with REXT = 7680. The MAX6682 produces output codes scaled to the actual temperature when used with the standard thermistor and REXT = 7680 over the +10C to +40C temperature range. Under these conditions, the nominal accuracy is about 0.2C between +10 and +40C, and about 1.5C from 0C to +50C. In Table 1, the 3LSBs of the output code represent fractional temperatures. The LSB has a value of 0.125C. All table entries assume no errors in the values of REXT or the thermistor resistance. Table 1 also assumes the use of one of the following standard thermistors: Betatherm 10K3A1, Dale 1M1002, or Thermometrics C100Y103J. These thermistors have a nominal resistance of 10k at +25C and very similar temperatureto-resistance functions. They give the results shown in Table 1. Different temperature ranges can be accommodated as well using different values of REXT (see Choosing the External Resistor). The MAX6682 works with thermistors other than the ones listed above, but the transfer functions vary somewhat.
Applications Information
Thermistors and Thermistor Selection
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
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Thermistor-to-Digital Converter MAX6682
Table 1. Temperature vs. Digital Output for Standard Thermistor with REXT = 7680
THERMISTOR TEMPERATURE (C) +60.000 +50.000 +40.000 +30.000 +25.000 +20.000 +10.000 0 -0.725 -2.000 -5.000 VREXT (mV) WITH STANDARD THERMISTOR AND REXT = 7680* 921.6 830.6 720.5 595.4 530.1 464.4 339.7 232.3 225.5 213.6 187.4 DECIMAL VALUE OF DOUT (1LSB = 0.125C) +55.875 +48.625 +40.000 +30.125 +25.000 +19.875 +10.000 +1.500 +1.000 0.125 -2.000 DOUT 001 1011 1111 001 1000 0101 001 0100 0000 000 1111 0001 000 1100 1000 000 1001 1111 000 0101 0000 000 0000 1100 000 0000 1000 000 0000 0001 111 1111 0000
*Assumes VR+ = 1.220V.
NTC thermistor is very nonlinear and can be described by the following approximation: 1 / T = A + BlnR + C(lnR)3 where T is absolute temperature, R is the thermistor's resistance, and A, B, and C are coefficients that vary with manufacturer and material characteristics. The general shape of the curve is shown in Figure 1. The highly nonlinear relationship between temperature and resistance in an NTC thermistor makes it somewhat more difficult to use than a digital-output temperature
sensor IC, for example. However, by connecting the thermistor in series with a properly chosen resistor and using the MAX6682 to measure the voltage across the resistor, a reasonably linear transfer function can be obtained over a limited temperature range. Errors decrease for smaller temperature ranges. Figures 2 and 3 show typical thermistor nonlinearity curves for a standard thermistor in conjunction with series resistors chosen to optimize linearity over two different temperature ranges: +10C to +40C and 0C to +70C.
THERMISTOR RESISTANCE vs. TEMPERATURE
120 100 80 60 40 20 0 -40 -20 0 20 40 60 80 100 120 TEMPERATURE (C) LINEARITY ERROR (C) 3.0 2.5 THERMISTOR RESISTANCE (k) 2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 -2.5 0
THERMISTOR NONLINEARITY vs. TEMPERATURE
10
20
30
40
50
60
70
80
TEMPERATURE (C)
Figure 1. Thermistor Resistance vs. Temperature
Figure 2. Thermistor Nonlinearity vs. Temperature for a Standard Thermistor from 0C to +70C 5
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Thermistor-to-Digital Converter MAX6682
THERMISTOR NONLINEARITY vs. TEMPERATURE
0.25 0.20 0.15 LINEARITY ERROR (C) 0.10 0.05 0 -0.05 -0.10 -0.15 -0.20 -0.25 0 5 10 15 20 25 30 35 40 45 TEMPERATURE (C)
in the middle of the temperature range (+35C for the 0C to +70C range). 3) Find REXT using the equation below: REXT = RMID (RMIN + RMAX ) - 2RMINRMAX RMIN + RMAX - 2RMID
Table 3 shows nominal output data for several temperatures when REXT has been chosen according to the equation above for a temperature range of 0C to +70C. The output data is not conveniently scaled to the actual temperature over this range, but the linearity is better than 2.4C over the 0C to +70C range (Figure 2). The temperature weighting over this range is 0.14925C/LSB.
Serial Interface
The Typical Application Circuit shows the MAX6682 interfaced with a microcontroller. In this example, the MAX6682 processes the reading from REXT and transmits the data through an SPI-compatible interface. Force CS low and apply a clock signal at SCK to read the results at SO. Forcing CS low immediately stops any conversion in process. Initiate a new conversion by forcing CS high. Force CS low to output the first bit on the SO pin. A complete read requires 11 clock cycles. Read the 11 output bits on the rising edge of the clock, if the first bit D10 is the sign bit. Bits D10-D0 contain the converted temperature in the order of MSB to LSB. After the 11th clock cycle, SO goes to a high-impedance state. SO remains high impedance until CS is pulsed high and brought back low. Figure 4 is the SO output.
Figure 3. Thermistor Nonlinearity vs. Temperature for a Standard Thermistor from +10C to +40C
10-BIT TEMPERATURE READING Bit 10 MSB (Sign) 9 8 7 6 5 4 3 2 1 0 LSB
Figure 4. SO Output
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 2. When used with one of these thermistors and the recommended series resistor, the MAX6682 provides output data scaled in C over the +10C to +40C temperature range.
Power-Supply Considerations
The MAX6682 accuracy is relatively unaffected by power-supply coupled noise. In most applications, bypass V CC to GND by placing a 0.1F ceramic bypass capacitor close to the supply pin of the devices.
Choosing the External Resistor
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
Thermal Considerations
Self-heating degrades the temperature measurement accuracy of thermistors. The amount of self-heating depends on the power dissipated in the thermistor 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 mW the thermistor dissipates, its temperature rises by 1C. For example, con-
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Thermistor-to-Digital Converter
sider a 10k (at +25C) NTC thermistor in series with a 5110 resistor operating at +40C with a constant 5V bias. If it is one of the standard thermistors in Table 2, its resistance is 5325 at this temperature. The power dissipated in the thermistor is: (5)2(5325) / (5325 + 5110)2 = 1.22mW This thermistor would therefore have a self-heating error at +40C of 1.22C. Because the MAX6682 uses a small reference voltage and energizes the thermistor less than 2% of the time, the self-heating of the thermistor under the same conditions when used with the MAX6682 is only: (1.22)2(5325)(0.02) / (5325 + 5110)2 =1.46W, or only about 0.0015 (self-heating error)
MAX6682
Table 2. Standard Thermistors
MANUFACTURER Betatherm Dale Thermometrics PART 10K3A1 1M1002 WEBSITE www.betatherm.com www.vishay.com/brands/ dale/main.html
C100Y103J www.thermometrics.com
Table 3. Temperature vs. Digital Output for Standard Thermistor with REXT = 5110
THERMISTOR TEMPERATURE (C) +75.000 +70.000 +60.000 +50.000 +40.000 +30.000 +25.000 +20.000 +10.000 0 -5.000 VREXT (mV) WITH STANDARD THERMISTOR AND REXT = 5110* 946.0 908.6 820.6 715.7 597.4 473.5 412.6 354.1 249.2 165.1 131.5 DECIMAL VALUE OF DOUT (USING 1LSB = 0.125C) 57.75 54.875 47.875 39.625 30.25 20.5 15.750 11.125 2.875 -3.750 -6.375 DOUT 001 1100 1110 001 1011 0111 001 0111 1111 001 0011 1101 000 1111 0010 000 1010 0100 000 0111 1110 000 0101 1001 000 0001 0111 111 1110 0010 111 1100 1101
*Assumes VR+ = 1.220V.
tCSS CS
tCH 1 SCK tDV B10 MSB tDO B9 B8 B7 B6 B5 B4 tCL tTR B3 B2 B1 B0 LSB
SO
Figure 5. Serial Interface Timing
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Thermistor-to-Digital Converter MAX6682
tCSW CS
1 SCK tDV SO B10 MSB
2
11 tHIZ
1
2
B9
B2
B1
B0 LSB
B10 MSB
B9
Figure 6. Serial Interface Timing 2
Functional Diagram
VCC
Pin Configuration
TOP VIEW
BANDGAP
I.C. 1 R+ 2
8 7
VCC SCK SO CS
MAX6682
RR+ DIGITAL CONTROL CS SCK SO
3
6 5
GND 4
MAX
R-
ADC
Chip Information
TRANSISTOR COUNT: 4909 PROCESS: BiCMOS
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Thermistor-to-Digital Converter
Package Information
8LUMAXD.EPS
MAX6682
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 _____________________ 9 (c) 2002 Maxim Integrated Products Printed USA is a registered trademark of Maxim Integrated Products.

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