MAX6691_07 MAXIM [Maxim Integrated Products], MAX6691_07 Datasheet - Page 4

• Manufacturer Part Number: MAX6691_07
• Description: Four-Channel Thermistor Temperature-to-Pulse-Width Converter
• Manufacturer: MAXIM [Maxim Integrated Products]

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.
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
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 5µs
(Figure 1). When the microcontroller releases the I/O
pin, the MAX6691 applies the reference voltage (V
to the external resistor (R
sequentially to each of the four external thermistors (T1
through T4).
When the measurements are complete (after a period
equal to T
125µs. The I/O pin remains high for a period proportion-
al to the first V
first thermistor). The MAX6691 then pulls the I/O pin low
for a period proportional to V
pulse pairs follow, corresponding to T2 through T4,
after which the I/O pin is released.
The relationship between pulse width, R
mistor resistance (R
Four-Channel Thermistor Temperature-to-Pulse-
Width Converter
Figure 1. Timing Diagram
4
T
T
HIGH
LOW
_______________________________________________________________________________________
PULLED LOW BY C
CONV REQUEST,
CONV
t
START
V
V
REF
EXT
EXT
), the MAX6691 pulls the I/O pin low for
measurement (corresponding to the
TH
Temperature Measurement
t
0
CONV
.0002 =
) can be described as:
Detailed Description
PULLED LOW BY
DATA READY,
EXT
MAX6691
REF
t
READY
R
), which is connected
EXT
. Three more high/low
R
EXT
CC
T
HIGH1
+R
THERMISTOR 1
by the external
TH
EXT
DATA
, and ther-
0 0002
T
.
LOW
REF
)
T
HIGH2
THERMISTOR 2
The relationship between V
a thermistor is determined by the values of R
the thermistor’s characteristics. If the relationship
between R
controller with no on-chip ADC can measure T
T
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
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:
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.
LOW
DATA
HIGH
T
LOW
and accurately determine the temperature at the
Thermistors and Thermistor Selection
) is a short pulse of less than 5% of T
EITHER OPEN OR
THERMISTOR IS
SHORT
t
TH
ERROR
and the temperature is known, a micro-
THERMISTOR 3
Applications Information
1
T
DATA
T
LOW
A B(InR) + (
T
EXT
HIGH4
THERMISTOR 4
and the temperature of
DATA
C InR
T
LOW
)
3
LOW
HIGH
EXT
.
and
and