MAXIM MAX6691

19-2304; Rev 0; 1/02
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.
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
PIN-PACKAGE
-55°C to +125°C
10 µMAX
The MAX6691 is available in a 10-pin µMAX package
and is specified from -55°C to +125°C temperature
range.
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
2
T3
3
T4
R-
10 VCC
9
I/O
8
N.C.
4
7
GND
5
6
R+
MAX6691
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
300
3.5
IREF = 1mA, TA = +25°C
1.19
0 < IREF < 2mA
Reference Supply Rejection
VIL
Logic Input High Voltage
VIH
% FS
5.5
V
600
µA
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
Conversion Time
Nominal Pulse Width
TYP
UNITS
ns
tCONV
86
102
156
ms
4.9
7.5
ms
tLOW
4.0
Start Pulse Width
tSTART
5
Data Ready Pulse Width
tREADY
103
122
188
µs
Error Pulse Width
tERROR
103
122
188
µs
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
0.8
THIGH/TLOW
-20
0
20
40
60
80
100 120
TEMPERATURE (°C)
TEMPERATURE (°C)
0.6
0.4
Figure 4. THIGH/TLOW vs. Temperature, REXT = 5110Ω
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:
0.2
0
0
20
40
60
80
100
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.
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 =
RMIN (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
Betatherm
Dale
Thermometrics
PART
10K3A1
1M1002
C100Y103J
WEBSITE
www.betatherm.com/indexna.htm
www.vishay.com/brands/dale/main.html
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
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
© 2002 Maxim Integrated Products
Printed USA
is a registered trademark of Maxim Integrated Products.
MAX6691
Package Information