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19-2304; Rev 1; 2/07
Four-Channel Thermistor Temperature-to-Pulse-
Width Converter
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’s 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 10µA (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 µMAX package
and is specified from -55°C to +125°C temperature
range.
♦
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
Features
MAX6691
Ordering Information
PART
MAX6691MUB
TEMP RANGE
-55°C to +125°C
PIN-
PACKAGE
10 µMAX
PKG
CODE
U10-2
Applications
HVAC
Home Appliances
Medical Devices
Typical Application Circuit
V
CC
Pin Configuration
TOP VIEW
TOP VIEW
T1
T2
T1
T2
T3
T4
T3
T4
R-
1
2
3
4
5
R
EXT
10
9
V
CC
I/O
N.C.
GND
R+
10kΩ
T1
1
MICRO-
CONTROLLER
10
V
CC
9
I/O
N.C.
GND
R+
T2
T3
T4
R-
2
3
4
5
MAX6691
8
7
6
MAX6691
8
7
6
µMAX
________________________________________________________________
Maxim Integrated Products
1
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.
Four-Channel Thermistor Temperature-to-Pulse-
Width Converter
MAX6691
ABSOLUTE MAXIMUM RATINGS
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–T4 Current................................................±20mA
ESD Protection (Human Body Model) .............................±2000V
Continuous Power Dissipation (T
A
= +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
(V
CC
= 3.0V to 5.5V, T
A
= -55°C to +125°C, unless otherwise noted. Typical values are specified at V
CC
= 3.3V and T
A
= +25°C.) (Note1)
PARAMETER
T
HIGH
/T
LOW
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
V
IL
V
IH
0.7
✕
V
CC
SYMBOL
V
REXT
V
CC
I
CC
I
STANDBY
I
LEAKAGE
V
REF
I
REF
= 1mA, T
A
= +25°C
0 < I
REF
< 2mA
1.19
1.24
0.1
0.2
0.3
✕
V
CC
During conversion, no load
CONDITIONS
T
A
= +25°C, V
CC
= 3.3V
T
A
= T
MIN
to T
MAX
3.0
300
3.5
MIN
TYP
MAX
0.5
1.0
5.5
600
10
1.0
1.32
0.2
UNITS
% FS
V
µA
µA
µA
V
%
%
V
V
TIMING CHARACTERISTICS
(V
CC
= 3.0V to 5.5V, T
A
= -55°C to +125°C, unless otherwise noted. Typical values are specified at V
CC
= 3.3V and T
A
= +25°C.)
(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
t
CONV
t
LOW
t
START
t
READY
t
ERROR
t
RISE
t
FALL
C
L
= 15pF, R
L
= 10kΩ
C
L
= 15pF, R
L
= 10kΩ
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
µs
µs
µs
ns
ns
Note 1:
Specification limits over temperature are guaranteed by design, not production tested.
2
_______________________________________________________________________________________
Four-Channel Thermistor Temperature-to-Pulse-
Width Converter
MAX6691
__________________________________________Typical Operating Characteristics
(V
CC
= 5V, R
EXT
= 7.5kΩ, R
TH
= 12.5kΩ, T
A
= +25°C, unless otherwise noted.)
SLEEP-MODE SUPPLY CURRENT
vs. SUPPLY VOLTAGE
MAX6691 toc01
T
HIGH
/T
LOW
ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
MAX6691 toc02
T
HIGH
/T
LOW
ERROR
vs. POWER-SUPPLY NOISE FREQUENCY
T
HIGH
/T
LOW
FULL-SCALE ERROR (%)
V
IN
= SQUARE WAVE
APPLIED TO V
CC
WITH
NO VCC BYPASS
CAPACITOR
V
IN
= 250mV
P-P
T
A
= -55°C
T
A
= +25°C
MAX6691 toc03
4.5
SLEEP-MODE SUPPLY CURRENT (µA)
1.0
T
HIGH
/T
LOW
FULL-SCALE ERROR (%)
4.0
0.5
V
IN
= SQUARE WAVE
APPLIED TO V
CC
WITH
NO VCC BYPASS
CAPACITOR
V
IN
= 250mV
P-P
1.0
0.5
3.5
0
V
CC
= 5.0V V
CC
= 3.3V
-0.5
0
T
A
= +85°C
-0.5
T
A
= +125°C
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
R-
R+
GND
N.C.
I/O
V
CC
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 R
EXT
between R- and R+.
Reference Voltage Output. Connect R
EXT
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 10kΩ pullup resistor from I/O pin to V
CC
.
Supply Voltage. Bypass V
CC
to GND with a capacitor of at least 0.1µF.
_______________________________________________________________________________________
3
Four-Channel Thermistor Temperature-to-Pulse-
Width Converter
MAX6691
THERMISTOR 1
DATA
T
HIGH1
T
LOW
THERMISTOR 2
DATA
T
HIGH2
T
LOW
T
LOW
THERMISTOR 3
DATA
THERMISTOR 4
DATA
T
HIGH4
T
LOW
t
CONV
t
START
CONV REQUEST,
PULLED LOW BY
µC
t
READY
DATA READY,
PULLED LOW BY
MAX6691
t
ERROR
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 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 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 (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
125µs. 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:
T
HIGH
V
R
EXT
=
EXT
−
0.0002 =
−
0.0002
T
LOW
V
REF
R
EXT
+R
TH
The relationship between V
EXT
and the temperature of
a thermistor is determined by the values of R
EXT
and
the thermistor’s 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:
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.
4
_______________________________________________________________________________________
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