DS1720
ECON-Digital Thermometer and
Thermostat
www.maxim-ic.com
FEATURES
Requires no external components
Supply voltage range covers from 2.7V to
5.5V
Measures temperatures from –55°C to
+125°C in 0.5°C increments. Fahrenheit
equivalent is –67°F to +257°F in 0.9°F
increments
Temperature is read as a 9–bit value
Converts temperature to digital word in 1
second (max)
Thermostatic settings are user–definable and
non–volatile
Data is read from/written via a 3–wire serial
interface (CLK, DQ,
RST
)
Applications include thermostatic controls,
industrial systems, consumer products,
thermometers, or any thermally sensitive
system
8–pin SOIC (208 mil) package
PIN ASSIGNMENT
DQ
CLK/CONV
RST
GND
1
2
3
4
8
7
6
5
V
DD
T
HIGH
T
LOW
T
COM
DS1720S 8-Pin SOIC (208-mil)
PIN DESCRIPTION
DQ
CLK/
CONV
– 3–Wire Input/Output
– 3–Wire Clock Input and
Stand–alone
Convert Input
– 3–Wire Reset Input
– Ground
– High Temperature Trigger
– Low Temperature Trigger
– High/Low Combination Trigger
– Power Supply Voltage (3V-5V)
RST
GND
T
HIGH
T
LOW
T
COM
V
DD
DESCRIPTION
The DS1720 Digital Thermometer and Thermostat provides 9–bit temperature readings which indicate
the temperature of the device. With three thermal alarm out-puts, the DS1720 can also act as a thermostat.
T
HIGH
is driven high if the DS1720’s temperature is greater than or equal to a user–defined temperature
T
H
. T
LOW
is driven high if the DS1720’s temperature is less than or equal to a user–defined temperature
T
L
. T
COM
is driven high when the temperature exceeds T
H
and stays high until the temperature falls below
that of TL.
User–defined temperature settings are stored in non–volatile memory, so parts can be programmed prior
to insertion in a system, as well as used in stand–alone applications without a CPU. Temperature settings
and temperature readings are all communicated to/from the DS1720 over a simple 3–wire interface.
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020106
DS1720
ORDER INFORMATION
ORDERING
NUMBER
DS1720S+
DS1720S+T&R
PACKAGE
MARKING
DS1720 (see note)
DS1720 (see note)
DESCRIPTION
DS1720 in Lead-Free 208mil 8-pin SO
DS1720 in Lead-Free 208mil 8-pin SO, 2500 Piece
Tape-and-Reel
DS1720S
DS1720
DS1720 in 208mil 8-pin SO
DS1720S/T&R
DS1720
DS1720 in 208mil 8-pin SO, 2500 Piece Tape-and-Reel
Note: A “+” symbol will also be marked on the package near the Pin 1 indicator.
OPERATION–MEASURING TEMPERATURE
A block diagram of the DS1720 is shown in Figure 1. The DS1720 measures temperatures through the
use of an on–board proprietary temperature measurement technique. A block diagram of the temperature
measurement circuitry is shown in Figure 2.
The DS1720 measures temperature by counting the number of clock cycles that an oscillator with a low
temperature coefficient goes through during a gate period determined by a high temperature coefficient
oscillator. The counter is preset with a base count that corresponds to –55°C. If the counter reaches zero
before the gate period is over, the temperature register, which is also preset to the –55°C value, is
incremented, indicating that the temperature is higher than –55°C. At the same time, the counter is then
preset with a value determined by the slope accumulator circuitry. This circuitry is needed to compensate
for the parabolic behavior of the oscillators over temperature. The counter is then clocked again until it
reaches zero. If the gate period is still not finished, then this process repeats.
The slope accumulator is used to compensate for the nonlinear behavior of the oscillators over
temperature, yielding a high resolution temperature measurement. This is done by changing the number
of counts necessary for the counter to go through for each incremental degree in temperature. To obtain
the desired resolution, therefore, both the value of the counter and the number of counts per degree C (the
value of the slope accumulator) at a given temperature must be known.
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DS1720
DS1720 FUNCTIONAL BLOCK DIAGRAM
Figure 1
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DS1720
TEMPERATURE MEASURING CIRCUITRY
Figure 2
This calculation is done inside the DS1720 to provide 0.5°C resolution. The temperature reading is
provided in a 9–bit, two’s complement reading by issuing a READ TEMPERATURE command. Table 1
describes the exact relationship of output data to measured temperature. The data is transmitted serially
through the 3–wire serial interface, LSB first. The DS1720 can measure temperature over the range of
–55°C to +125°C in 0.5°C increments. For Fahrenheit usage, a lookup table or conversion factor must be
used.
TEMPERATURE/DATA RELATIONSHIPS Table 1
TEMP
+85°C
+25°C
+½°C
+0°C
-½°C
-25°C
DIGITAL OUTPUT
(Binary)
0 10101010
0 00110010
0 00000001
0 00000000
1 11111111
1 11001110
DIGITAL OUTPUT
(Hex)
00AA
0032h
0001h
0000h
01FFh
01CEh
Since data is transmitted over the 3–wire bus LSB first, temperature data can be written to/read from the
DS1720 as either a 9–bit word (taking
RST
low after the 9
th
(MSB) bit), or as two transfers of 8–bit
words, with the most significant 7 bits being ignored or set to zero, as illustrated in Table 1. After the
MSB, the DS1720 will output 0s.
Note that temperature is represented in the DS1720 in terms of a ½°C LSB, yielding the following 9–bit
format:
MSB
X
X
LSB
1
1
T = -25°C
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X
X
X
X
X
1
0
0
1
1
1
0
DS1720
Higher resolutions may be obtained by reading the temperature, and truncating the 0.5°C bit (the LSB)
from the read value. This value is TEMP_READ. The value left in the counter may then be read by
issuing a READ COUNTER command. This value is the count remaining (COUNT_REMAIN) after the
gate period has ceased. The value of the slope accumulator may be read (using the READ SLOPE
command), yielding the number of counts per degree C (COUNT_PER_C) at that temperature. The actual
temperature may be then be calculated by the user using the following:
TEMPERATURE = TEMP_READ – 0.25
+
(COUNT_PER_C– COUNT_REMAIN)
COUNT_PER_C
DETAILED PIN DESCRIPTION Table 2
PIN
1
2
SYMBOL
DQ
CLK/
CONV
DESCRIPTION
Data Input/Output pin
for 3–wire communication port.
Clock input pin
for 3–wire communication port. When the DS1720 is used in
a stand–alone application with no 3–wire port, this pin can be used as a
convert pin. Temperature conversion will begin on the falling edge of
CONV
.
Reset input pin
for 3–wire communication port.
Ground pin.
High/Low Combination Trigger.
Goes high when temperature exceeds T
H
;
will reset to low when temperature falls below T
L
.
Low Temperature Trigger.
Goes high when temperature falls below T
L
.
High Temperature Trigger.
Goes high when temperature exceeds T
H
.
Supply Voltage.
2.7V – 5.5V input power pin.
3
4
5
6
7
8
RST
GND
T
COM
T
LOW
T
HIGH
V
DD
OPERATION–THERMOSTAT CONTROLS
Three thermally triggered outputs, T
HIGH
, T
LOW
, and T
COM
, are provided to allow the DS1720 to be used
as a thermostat, as shown in Figure 3. When the DS1720’s temperature meets or exceeds the value stored
in the high temperature trip register, the output T
HIGH
becomes active (high) and remains active until the
DS1720’s measured temperature becomes less than the stored value in the high temperature register, TH.
The T
HIGH
output can be used to indicate that a high temperature tolerance boundary has been met or
exceeded, or as part of a closed loop system can be used to activate a cooling system and to deactivate it
when the system temperature returns to tolerance.
The T
LOW
output functions similarly to the T
HIGH
output. When the DS1720’s measured temperature
equals or falls below the value stored in the low temperature register, the T
LOW
output becomes active.
T
LOW
remains active until the DS1720’s temperature becomes greater than the value stored in the low
temperature register, T
L
. The T
LOW
output can be used to indicate that a low temperature tolerance
boundary has been met or exceeded, or as part of a closed loop system, can be used to activate a heating
system and to deactivate it when the system temperature returns to tolerance.
The T
COM
output goes high when the measured temperature meets or exceeds T
H
, and will stay high until
the temperature equals or falls below T
L
. In this way, any amount of hysteresis can be obtained.
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