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PDF MAX6691 Data sheet ( Hoja de datos )
| Número de pieza | MAX6691 | |
| Descripción | Four-Channel Thermistor Temperature-to-Pulse- Width Converter | |
| Fabricantes | Maxim Integrated | |
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19-2304; Rev 0; 1/02
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 (REXT) form a voltage-divider that
is driven by the MAX6691’s internal voltage reference
(VREF). VREF and the voltage across REXT 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.
Applications
HVAC
Home Appliances
Medical Devices
Features
o Simple Single-Wire Interface
o Measures Up to Four Thermistor Temperatures
o Low-Average Thermistor Current Minimizes Self-
Heating Errors
o Internal Voltage Reference Isolates Thermistor
from Power-Supply Noise
o Accommodates Any Thermistor Temperature
Range
PART
MAX6691MUB
Ordering Information
TEMP RANGE
-55°C to +125°C
PIN-PACKAGE
10 µMAX
Typical Application Circuit
TOP VIEW
T1 1
T2 2
T1 T3 3
T2 T4
4
T3 R-
5
T4
VCC
MAX6691
REXT
VCC
10
I/O
9
8 N.C.
7 GND
6 R+
10kΩ
MICRO-
CONTROLLER
Pin Configuration
TOP VIEW
T1 1
T2 2
T3 3
T4 4
R- 5
MAX6691
µMAX
10 VCC
9 I/O
8 N.C.
7 GND
6 R+
________________________________________________________________ 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.
1 page  
Four-Channel Thermistor Temperature-to-Pulse-
Width Converter
THERMISTOR RESISTANCE
vs. TEMPERATURE
120
100
80
60
40
20
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 = 7680Ω
1.0
0.9
0.8
0.7
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 4. THIGH/TLOW vs. Temperature, REXT = 5110Ω
THIGH/TLOW vs. TEMPERATURE FOR BETATHERM
10K3A1 THERMISTOR WITH REXT = 5110Ω
1.2
1.0
0.8
0.6
0.4
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 mea-
sure the voltage across the resistor, a reasonably linear
transfer function can be obtained over a limited temper-
ature 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 tempera-
ture ranges.
NTC thermistors are often described by the resistance
at +25°C. Therefore, a 10kΩ thermistor has a resis-
tance of 10kΩ at +25°C. When choosing a thermistor,
ensure that the thermistor’s minimum resistance (which
occurs at the maximum expected operating tempera-
ture) in series with REXT does not cause the voltage ref-
erence 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 tem-
perature range. RMIN is the minimum thermistor
value (at the maximum temperature) and RMAX is
the maximum thermistor value (at the minimum tem-
perature). 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
5 Page | ||
| Páginas | Total 7 Páginas | |
| PDF Descargar | [ Datasheet MAX6691.PDF ] | |
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