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080602
FEATURES
Temperature measurements require no
external components
Measures temperatures from 55C to
+125
C. Fahrenheit equivalent is 67F to
257
F
Thermometer accuracy is 2.0C
Thermometer resolution is configurable from
9 to 12 bits (0.5
C to 0.0625C resolution)
Thermostat settings are user definable
Data is read from/written to via a 2wire
serial interface
Wide power supply range (2.7V 5.5V)
Software compatible with DS75 2Wire
Thermal Watchdog in thermometer mode
Spaceconscious SOT235 package with
low thermal time constant
PIN ASSIGNMENT
PIN DESCRIPTION
GND Ground
SCL
2Wire Serial Clock
SDA
2Wire Serial Data Input/Output
V
DD
Power Supply Voltage
O.S.
Thermostat Output Signal
ORDERING INFORMATION
Part Number
Address
Part Number Address
DS1775R/TRL
000
DS1775R/TR4
100
DS1775R/TR1
001
DS1775R/TR5
101
DS1775R/TR2
010
DS1775R/TR6
110
DS1775R/TR3
011
DS1775R/TR7
111
* "R" denotes SOT 23-5 Package
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 2wire serial interface. No additional components are
required; the device is truly a "temperaturetodigital" 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 opendrain thermal alarm output, O.S., becomes active when the temperature of the device exceeds a
userdefined 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 2Wire Thermal Watchdog.
DS1775
SOT23-5 Digital Thermometer
and Thermostat
PRODUCT PREVIEW
www.maxim-ic.com
DS1775R
SOT23-5
3
2
1
5
4
SDA
V
DD
SCL
GND
O.S.
DS1775
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The DS1775 is assembled in a compact SOT235 package allowing for lowcost thermal
monitoring/control in spaceconstrained 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 thermallysensitive system.
DETAILED PIN DESCRIPTION
Table 1
PIN
SYMBOL DESCRIPTION
PIN 1
SCL
Clock input/output pin for 2-wire serial communication port. This input
should be tied to GND for standalone thermostat operation.
PIN 2
GND
Ground pin.
PIN 3
O.S.
Thermostat output Open-drain output becomes active when temperature
exceeds T
OS
. Device configuration defines means to clear over-temperature
state.
PIN 4
V
DD
Supply Voltage 2.7V 5.5V input power pin.
PIN 5
SDA
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. Analogtodigital converter
3. 2wire interface electronics
4. Data registers
5. Thermostat comparator
The factorycalibrated temperature sensor requires no external components. Upon powerup, 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 powersensitive 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 lowpower 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 overtemperature (T
OS
) and undertemperature (T
HYST
) setpoints for
thermostatic operation. The powerup 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.
DS1775
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is programmable. The powerup 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 2wire interface, and all communication is MSb
first.
DS1775 FUNCTIONAL BLOCK DIAGRAM
Figure 1
OPERATIONMeasuring Temperature
The core of DS1775 functionality is its directtodigital temperature sensor. The DS1775 measures
temperature through the use of an onchip temperature measurement technique with an operating range
from 55
C to +125C. Temperature conversions are initiated upon powerup, 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, powerup default). The DS1775 powerup default has the sensor automatically performing
9bit conversions continuously. Details on how to change the settings after powerup are contained in the
"OPERATIONProgramming" section.
The resolution of the temperature conversion is configurable (9, 10, 11, or 12 bits), with 9bit readings
the default state. This equates to a temperature resolution of 0.5
C, 0.25C, 0.125C, or 0.0625C.
Following each conversion, thermal data is stored in the thermometer register in two's complement
format; the information can be retrieved over the 2wire interface with the device pointer set to the
temperature register. Table 2 describes the exact relationship of output data to measured temperature. The
DS1775
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table assumes the DS1775 is configured for 12bit resolution; if the device is configured in a lower
resolution mode, those bits will contain zeros. The data is transmitted serially over the 2wire 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
2
6
2
5
2
4
2
3
2
2
2
1
2
0
MSB
MSb
(UNIT =
C)
LSb
2
-1
2
-2
2
-3
2
-4
0
0
0
0
LSB
TEMP
DIGITAL OUTPUT
(Binary)
DIGITAL OUTPUT (Hex)
+125
C
0111 1101 0000 0000
7D00h
+25.0625
C
0000 1010 0010 0000
1910h
+10.125
C
0000 1010 0010 0000
0A20h
+0.5
C
0000 0000 1000 0000
0080h
+0
C
0000 0000 0000 0000
0000h
-0.5
C
1111 1111 1000 0000
FF80h
-10.125
C
1111 0101 1110 0000
F5E0h
-25.0625
C
1110 0110 1111 0000
E6F0h
-55
C
1100 1001 0000 0000
C900h
OPERATIONThermostat 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 2wire 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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O.S. OUTPUT TRANSFER FUNCTION
Figure 2
Regardless of the mode chosen, the O.S. output is opendrain and the active state is set in the
configuration register. The powerup default is active low. Refer to the "OPERATIONProgramming"
section for instructions in adjusting the thermostat setpoints, thermostat mode, and O.S. active state.
OPERATIONProgramming
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 2wire 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
ACTIVE REGISTER
00h
Temperature (default)
01h
Configuration
02h
T
HYST
03h
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 powerup, in which case the pointer will always be set to 00h, the temperature
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