PRODUCT PREVIEW
DS1775
SOT23-5 Digital Thermometer
and Thermostat
www.maxim-ic.com
FEATURES
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Temperature measurements require no
external components
Measures temperatures from –55°C to
+125°C. Fahrenheit equivalent is –67°F to
257°F
Thermometer accuracy is
±2.0°C
Thermometer resolution is configurable from
9 to 12 bits (0.5°C to 0.0625°C resolution)
Thermostat settings are user definable
Data is read from/written to via a 2–wire
serial interface
Wide power supply range (2.7V – 5.5V)
Software compatible with DS75 2–Wire
Thermal Watchdog in thermometer mode
Space–conscious SOT23–5 package with
low thermal time constant
PIN ASSIGNMENT
SCL
GND
O.S.
1
2
3
4
V
DD
5
SDA
DS1775R
SOT23-5
PIN DESCRIPTION
GND
SCL
SDA
V
DD
O.S.
Ground
2–Wire Serial Clock
2–Wire Serial Data Input/Output
Power Supply Voltage
Thermostat Output Signal
ORDERING INFORMATION
Part Number
Address
DS1775R/TRL
000
DS1775R/TR1
001
DS1775R/TR2
010
DS1775R/TR3
011
* "R" denotes SOT 23-5 Package
Part Number
DS1775R/TR4
DS1775R/TR5
DS1775R/TR6
DS1775R/TR7
Address
100
101
110
111
DESCRIPTION
The DS1775 SOT23-5 Digital Thermometer and Thermostat provides temperature readings which
indicate the temperature of the device. Thermostat settings and temperature readings are all
communicated to/from the DS1775 over a simple 2–wire serial interface. No additional components are
required; the device is truly a “temperature–to–digital” converter.
For applications that require greater temperature resolution, the user can adjust the readout resolution
from 9 to 12 bits. This is particularly useful in applications where thermal runaway conditions must be
detected quickly.
The open–drain thermal alarm output, O.S., becomes active when the temperature of the device exceeds a
user–defined temperature T
OS
. The number of consecutive faults required to set O.S. active is
configurable by the user. The device can also be configured in the interrupt or comparator mode, to
customize the method which clears the fault condition.
As a digital thermometer, the DS1775 is software compatible with the DS75 2–Wire Thermal Watchdog.
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080602
DS1775
The DS1775 is assembled in a compact SOT23–5 package allowing for low–cost thermal
monitoring/control in space–constrained applications. The low thermal mass allows for time constants
previously only possible with thermistors.
Applications for the DS1775 include personal computers/servers/workstations, cellular telephones, office
equipment, or any thermally–sensitive system.
DETAILED PIN DESCRIPTION
Table 1
PIN
PIN 1
PIN 2
PIN 3
PIN 4
PIN 5
SYMBOL
SCL
GND
O.S.
V
DD
SDA
DESCRIPTION
Clock input/output pin
for 2-wire serial communication port. This input
should be tied to GND for standalone thermostat operation.
Ground pin.
Thermostat output
Open-drain output becomes active when temperature
exceeds T
OS
. Device configuration defines means to clear over-temperature
state.
Supply Voltage
2.7V – 5.5V input power pin.
Data input/output pin
for 2-wire serial communication port. In the standalone
thermostat mode, this input selects hysteresis.
OVERVIEW
A block diagram of the DS1775 is shown in Figure 1. The DS1775 consists of five major components:
1. Precision temperature sensor
2. Analog–to–digital converter
3. 2–wire interface electronics
4. Data registers
5. Thermostat comparator
The factory–calibrated temperature sensor requires no external components. Upon power–up, the DS1775
begins temperature conversions with the default resolution of 9 bits (0.5°C resolution). The host can
periodically read the value in the temperature register, which contains the last completed conversion. As
conversions are performed in the background, reading the temperature register does not affect the
conversion in progress.
In power–sensitive applications the user can put the DS1775 into a shutdown mode, under which the
sensor will complete and store the conversion in progress and revert to a low–power standby state. In
applications where small incremental temperature changes are critical, the user can change the conversion
resolution from 9 bits to 10, 11, or 12. Each additional bit of resolution approximately doubles the
conversion time. This is accomplished by programming the configuration register. The configuration
register defines the conversion state, thermometer resolution/conversion time, active state of the
thermostat output, number of consecutive faults to trigger an alarm condition, and the method to
terminate an alarm condition.
The user can also program over–temperature (T
OS
) and under–temperature (T
HYST
) setpoints for
thermostatic operation. The power–up state of T
OS
is 80°C and that for T
HYST
is 75°C. The result of each
temperature conversion is compared with the T
OS
and T
HYST
setpoints. The DS1775 offers two modes for
temperature control, the comparator mode and the interrupt mode. This allows the user the flexibility to
customize the condition that would generate and clear a fault condition. Regardless of the mode chosen,
the O.S. output will become active only after the measured temperature exceeds the respective trippoint a
consecutive number of times; the number of consecutive conversions beyond the limit to generate an O.S.
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DS1775
is programmable. The power–up state of the DS1775 is in the comparator mode with a single fault
generating an active O.S.
Digital data is written to/read from the DS1775 via a 2–wire interface, and all communication is MSb
first.
DS1775 FUNCTIONAL BLOCK DIAGRAM
Figure 1
OPERATION–Measuring Temperature
The core of DS1775 functionality is its direct–to–digital temperature sensor. The DS1775 measures
temperature through the use of an on–chip temperature measurement technique with an operating range
from –55°C to +125°C. Temperature conversions are initiated upon power–up, and the most recent result
is stored in the thermometer register. Conversions are performed continuously unless the user intervenes
by altering the configuration register to put the DS1775 into a shutdown mode. Regardless of the mode
used, the digital temperature can be retrieved from the temperature register by setting the pointer to that
location (00h, power–up default). The DS1775 power–up default has the sensor automatically performing
9–bit conversions continuously. Details on how to change the settings after power–up are contained in the
“OPERATION–Programming” section.
The resolution of the temperature conversion is configurable (9, 10, 11, or 12 bits), with 9–bit readings
the default state. This equates to a temperature resolution of 0.5°C, 0.25°C, 0.125°C, or 0.0625°C.
Following each conversion, thermal data is stored in the thermometer register in two’s complement
format; the information can be retrieved over the 2–wire interface with the device pointer set to the
temperature register. Table 2 describes the exact relationship of output data to measured temperature. The
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DS1775
table assumes the DS1775 is configured for 12–bit resolution; if the device is configured in a lower
resolution mode, those bits will contain zeros. The data is transmitted serially over the 2–wire serial
interface, MSb first. The MSb of the temperature register contains the “sign” (S) bit, denoting whether the
temperature is positive or negative. For Fahrenheit usage, a lookup table or conversion routine must be
used.
Temperature/Data Relationships
Table 2
S
MSb
2
-1
2
-2
TEMP
+125°C
+25.0625°C
+10.125°C
+0.5°C
+0°C
-0.5°C
-10.125°C
-25.0625°C
-55°C
2
-3
2
6
2
5
2
4
2
-4
2
3
0
2
2
0
2
1
0
2
0
LSb
0
LSB
MSB
(UNIT =
°C)
DIGITAL OUTPUT
(Binary)
0111 1101 0000 0000
0000 1010 0010 0000
0000 1010 0010 0000
0000 0000 1000 0000
0000 0000 0000 0000
1111 1111 1000 0000
1111 0101 1110 0000
1110 0110 1111 0000
1100 1001 0000 0000
DIGITAL OUTPUT (Hex)
7D00h
1910h
0A20h
0080h
0000h
FF80h
F5E0h
E6F0h
C900h
OPERATION–Thermostat Control
In its comparator operating mode, the DS1775 functions as a thermostat with programmable hysteresis, as
shown in Figure 2. When the DS1775’s temperature meets or exceeds the value stored in the high
temperature trip register (T
OS
) a consecutive number of times, as defined by the configuration register, the
output becomes active and stays active until the first time that the temperature falls below the temperature
stored in the low temperature trigger register (T
HYST
). In this way, any amount of hysteresis may be
obtained. The DS1775 powers up in the comparator mode with T
OS
=80°C and T
HYST
=75°C and can be
used as a standalone thermostat (no 2–wire interface required) with those setpoints.
In the interrupt mode, the O.S. output will first become active following the programmed number of
consecutive conversions above T
OS
. The fault can only be cleared by either setting the DS1775 in a
shutdown mode or by reading any register (temperature, configuration, T
OS
, or T
HYST
) on the device.
Following a clear, a subsequent fault can only occur if consecutive conversions fall below T
HYST
. This
interrupt/clear process is thus cyclical (T
OS
, clear, T
HYST
, clear, T
OS
, clear, T
HYST
, clear, ...). Only the first
of multiple consecutive T
OS
violations will activate O.S., even if each fault is separated by a clearing
function. The same situation applies to multiple consecutive T
HYST
events.
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DS1775
O.S. OUTPUT TRANSFER FUNCTION
Figure 2
Regardless of the mode chosen, the O.S. output is open–drain and the active state is set in the
configuration register. The power–up default is active low. Refer to the “OPERATION–Programming”
section for instructions in adjusting the thermostat setpoints, thermostat mode, and O.S. active state.
OPERATION–Programming
There are three areas of interest in programming the DS1775: the configuration register, the T
OS
register,
and the T
HYST
register. All programming is done via the 2–wire interface by setting the pointer to the
appropriate location. Table 3 illustrates the pointer settings for the four registers of the DS1775.
Pointer Register Structure
Table 3
POINTER
00h
01h
02h
03h
ACTIVE REGISTER
Temperature (default)
Configuration
T
HYST
T
OS
The DS1775 will power up with the temperature register selected. If the host wishes to change the data
pointer it simply addresses the DS1775 in the write mode (R/
W
=0), receives an acknowledge, and writes
the 8 bits that correspond to the new desired location. The last pointer location is always maintained so
that consecutive reads from the same register do not require the host to always provide a pointer address.
The only exception is at power–up, in which case the pointer will always be set to 00h, the temperature
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