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DS1775R/TR5
SOT23-5 Digital Thermometer and Thermostat
FEATURES 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
PIN DESCRIPTIONGND Ground
SCL 2–Wire Serial ClockSDA 2–Wire Serial Data Input/Output
VDDPower Supply Voltage
O.S. Thermostat Output Signal
ORDERING INFORMATION Part Number AddressPart Number Address
* "R" denotes SOT 23-5 Package
DESCRIPTIONThe 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 resolutionfrom 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 auser–defined temperature TOS. 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
SOT23-5 Digital ThermometerSDA
GND
O.S.
DS1775
As a digital thermometer, the DS1775 is software compatible with the DS75 2–Wire Thermal Watchdog.
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
OVERVIEWA 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. Asconversions 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. Inapplications 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 toterminate an alarm condition.
The user can also program over–temperature (TOS) and under–temperature (THYST) setpoints for
thermostatic operation. The power–up state of TOS is 80°C and that for THYST is 75°C. The result of each
temperature conversion is compared with the TOS and THYST setpoints. The DS1775 offers two modes for
DS1775
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.
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 TemperatureThe 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 resultis 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.
DS1775
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 thetemperature register. Table 2 describes the exact relationship of output data to measured temperature. The
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 2MSB
MSb(UNIT = °C)LSbLSB
OPERATION–Thermostat ControlIn 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 hightemperature trip register (TOS) 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 (THYST). In this way, any amount of hysteresis may be
obtained. The DS1775 powers up in the comparator mode with TOS=80°C and THYST=75°C and can beused 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 TOS. The fault can only be cleared by either setting the DS1775 in a
shutdown mode or by reading any register (temperature, configuration, TOS, or THYST) on the device.
Following a clear, a subsequent fault can only occur if consecutive conversions fall below THYST. Thisinterrupt/clear process is thus cyclical (TOS, clear, THYST, clear, TOS, clear, THYST, clear, ...). Only the first
of multiple consecutive TOS violations will activate O.S., even if each fault is separated by a clearing
function. The same situation applies to multiple consecutive THYST events.
DS1775
O.S. OUTPUT TRANSFER FUNCTION Figure 2Regardless 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–ProgrammingThere are three areas of interest in programming the DS1775: the configuration register, the TOS register,
and the THYST register. All programming is done via the 2–wire interface by setting the pointer to theappropriate location. Table 3 illustrates the pointer settings for the four registers of the DS1775.
Pointer Register Structure Table 3The 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
DS1775
The only exception is at power–up, in which case the pointer will always be set to 00h, the temperature
register. The pointer address must always proceed data in writing to a register, regardless of which
address is currently selected. Please refer to the “2–Wire Serial Data Bus” section for details of the 2–
wire bus protocol.
Configuration Register ProgrammingThe configuration register is accessed if the DS1775 pointer is currently set to the 01h location. Writing
to or reading from the register is determined by the R/W bit of the 2–wire control byte (See “2–wire
Serial Data Bus” section). Data is read from or written to the configuration register MSb first. The format
of the register is illustrated below in Figure 3. The effect each bit has on DS1775 functionality is
described below along with the power–up state of the bit. The user has read/write access to all bits in the
configuration register. The entire register is volatile, and thus it will power–up in the default state.
Configuration/Status Register Figure 3MSbLSb
SD = Shutdown bit. If SD is “0”, the DS1775 will continuously perform temperature conversions and
store the last completed result in the thermometer register. If SD is changed to “1”, the conversion in
progress will be completed and stored; then the device will revert to a low–power standby mode. The
O.S. output will be cleared if the device is in the interrupt mode and remain unchanged in the comparator
mode. The 2–wire port remains active. The power–up default state is “0” (continuous conversion mode).
TM = Thermostat mode. If TM=“0”, the DS1775 is in the comparator mode. TM=“1” sets the device to
the interrupt mode. See “OPERATION–Thermostat Control” section for a description of the difference
between the two modes. The power–up default state of the TM bit is “0” (comparator mode).
POL = O.S. Polarity Bit. If POL = “1”, the active state of the O.S. output will be high. A “0” stored in
this location sets the thermostat output to an active low state. The user has read/write access to the POLbit, and the power–up default state is “0” (active low).
F0, F1 = O.S. Fault Tolerance bits. The fault tolerance defines the number of consecutive conversionsreturning a temperature beyond limits is required to set the O.S. output in an active state. This may be
necessary to add margin in noisy environments. Table 4 below defines the four settings. The DS1775 willpower up with F0=F1=“0”, such that a single occurrence will trigger a fault.
Fault Tolerance Configuration Table 4
R0, R1 = Thermometer resolution bits. Table 5 defines the resolution of the digital thermometer, basedon the settings of these two bits. There is a direct trade-off between resolution and conversion time, asdepicted in the AC Electrical Characteristics. The default state is R0="0" and R1="0" (9–bit conversions).
DS1775
Thermometer Resolution Configuration Table 5
Thermostat Setpoints ProgrammingThe thermostat registers (TOS and THYST) can be programmed or read via the 2–wire interface. TOS is
accessed by setting the DS1775 data pointer to the 03h location, and to the 02h location for THYST.
The format of the TOS and THYST registers is identical to that of the Thermometer register; that is, 12–bits complement representation of the temperature in °C. The user can program the number of bits (9, 10,
11, or 12) for each TOS and THYST that corresponds to the thermometer resolution mode chosen. For
example, if the 9–bit mode is chosen the 3 least significant bits of TOS and THYST will be ignored by the
thermostat comparator. The format for both TOS and THYST is shown in Table 6. The power–up default
for TOS is 80°C and for THYST is 75°C.
Thermostat Setpoint (TOS/THYST) Format Table 6MSB
MSb(UNIT = °C)LSbLSB
TEMPERATURE/DATA RELATIONSHIPSIf the user does not wish to take advantage of the thermostat capabilities of the DS1775, the 24 bits can beused for general storage of system data that need not be maintained following a power loss.
2–WIRE SERIAL DATA BUSThe DS1775 supports a bi–directional 2-wire bus and data transmission protocol. A device that sends data
onto the bus is defined as a transmitter, and a device receiving data as a receiver. The device that controls
the message is called a “master”. The devices that are controlled by the master are “slaves”. The bus mustbe controlled by a master device which generates the serial clock (SCL), controls the bus access, and
generates the START and STOP conditions. The DS1775 operates as a slave on the two–wire bus.
