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DS1775R/T&R-DS1775R+U-DS1775R1/T&R-DS1775R1+T&R
Digital Thermometer and Thermostat in SOT23
GENERAL DESCRIPTION The DS1775 digital thermometer and thermostat provides temperature readings that indicate the device’s temperature. 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 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 customize the method which clears the fault condition.
As a digital thermometer, the DS1775 is software compatible with the DS75 2-wire thermal watchdog. The DS1775 is assembled in a compact 5-pin SOT23 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 • Personal Computers/Servers/Workstations • Cell Phones • Office Equipment • Any Thermally-Sensitive System
FEATURES • Temperature Measurements Require No External Components • Measures Temperatures from -55°C to +125°C (-67°F to +257°F) • ±2.0°C Thermometer Accuracy • Thermometer Resolution is Configurable from 9 Bits to 12 Bits (0.5°C to 0.0625°C Resolution) • User-Definable Thermostat Settings • Data is Read From/Written to Through a 2-Wire Serial Interface • 2.7V to 5.5V Wide Power-Supply Range • Software Compatible with DS75 2-Wire Thermal Watchdog in Thermometer Mode • Space-Conscious 5-Pin SOT23 Package with Low Thermal Time Constant
PIN CONFIGURATION PIN DESCRIPTION GND Ground SCL 2-Wire Serial Clock SDA 2-Wire Serial Data Input/Output VDD Power-Supply Voltage O.S. Thermostat Output Signal Ordering Information appears at end of data sheet.
DS1775 Digital Thermometer and Thermostat in SOT23
SOT23 5
SDA
VDD
SCL
GND
O.S.
DS1775
Table 1. Detailed Pin Description PIN NAME FUNCTION SCL Clock Input/Output for 2-Wire Serial Communication Port. This input should be tied to GND for stand-alone thermostat operation. 2 GND Ground O.S. Thermostat Output. Open-drain output becomes active when temperature exceeds TOS. Device configuration defines means to clear overtemperature state. 4 VDD Supply Voltage 2.7V to 5.5V Input Power Pin SDA Data Input/Output for 2-Wire Serial Communication Port. In the stand-alone thermostat mode, this input selects hysteresis.
DETAILED DESCRIPTION Figure 1 shows a block diagram of the DS1775. 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 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 overtemperature (TOS) and undertemperature (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 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 becomes active only after the measured temperature exceeds the respective trip-point 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
Figure 1. Block Diagram 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 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 table assumes the DS1775 is configured for 12-bit resolution; if the device is configured in a lower resolution mode, those bits contain zeros. The data is transmitted serially over the 2-wire serial interface, MSb first.
DS1775
Table 2. Temperature/Data Relationships 26 25 24 23 22 21 20 MSB
MSb (UNIT = °C) LSb -1 2-2 2-3 2-4 0 0 0 0 LSB
TEMPERATURE (°C) DIGITAL OUTPUT (BINARY) DIGITAL OUTPUT (HEX) +125 0111 1101 0000 0000 7D00h +25.0625 0000 1010 0010 0000 1910h +10.125 0000 1010 0010 0000 0A20h +0.5 0000 0000 1000 0000 0080h 0 0000 0000 0000 0000 0000h -0.5 1111 1111 1000 0000 FF80h -10.125 1111 0101 1110 0000 F5E0h -25.0625 1110 0110 1111 0000 E6F0h -55 1100 1001 0000 0000 C900h
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 (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 be used as a stand-alone thermostat (no 2-wire interface required) with those setpoints. In the interrupt mode, the O.S. output first becomes 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. This interrupt/clear process is thus cyclical (TOS, clear, THYST, clear, TOS, clear, THYST, clear, etc.). Only the first of multiple consecutive TOS violations activates O.S., even if each fault is separated by a clearing function. The same situation applies to multiple consecutive THYST events.
DS1775
Figure 2. O.S. Output Transfer Function 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. See the Programming section for instructions in adjusting the thermostat setpoints, thermostat mode, and O.S. active state.
Programming There 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 the appropriate location. Table 3 illustrates the pointer settings for the four registers of the DS1775.
Table 3. Pointer Register Structure POINTER ACTIVE REGISTER 00h Temperature (default) 01h Configuration 02h THYST 03h TOS The DS1775 powers 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
DS1775
temperature register. The pointer address must always precede data in writing to a register, regardless of which address is currently selected. See the 2-Wire Serial Data Bus section for details of the 2-wire bus protocol.
Configuration Register Programming The 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 the 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 in Table 4. 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 powers up in the default state.
Table 4. Configuration/Status Register 0 R1 R0 F1 F0 POL TM SD MSb LSb
SD = Shutdown bit. If SD is 0, the DS1775 continuously performs temperature conversions and stores the last completed result in the thermometer register. If SD is changed to 1, the conversion in progress is completed and stored; then the device reverts to a low-power standby mode. The O.S. output is cleared if the device is in the interrupt mode and remains 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 the 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 is high. A 0 stored in this location sets the thermostat output to an active-low state. The user has read/write access to the POL bit, 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 conversions returning 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 5 defines the four settings. The DS1775 powers up with F0 = F1 = 0, such that a single occurrence triggers a fault.
Table 5. Fault Tolerance Configuration
F1 F0 CONSECUTIVE CONVERSIONS BEYOND LIMITS TO GENERATE FAULT 0 0 1 0 1 2 1 0 4 1 1 6
DS1775
R0, R1 = Thermometer resolution bits. Table 6 defines the resolution of the digital thermometer, based on the settings of these two bits. There is a direct trade-off between resolution and conversion time, as shown in the AC Electrical Characteristics. The default state is R0 = 0 and R1 = 0 (9-bit conversions). Table 6. Thermometer Resolution Configuration
R1 R0 THERMOMETER RESOLUTION (BITS) MAX CONVERSION TIME (SECONDS) 0 0 9 0.1875 0 1 10 0.375 1 0 11 0.75 1 1 12 1.5 Thermostat Setpoints Programming The 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-bit 2’s 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 three least significant bits of TOS and THYST are ignored by the thermostat comparator. Table 7 shows the format for both TOS and THYST. The power-up default for TOS is +80°C and for THYST is +75°C. Table 7. Thermostat Setpoint (TOS/THYST) Format 26 25 24 23 22 21 20 MSB
MSb (UNIT = °C) LSb -1 2-2 2-3 2-4 0 0 0 0 LSB TEMPERATURE (°C) DIGITAL OUTPUT (BINARY) DIGITAL OUTPUT (HEX) +80 0101 0000 0000 0000 5000h +75 0100 1011 0000 0000 4B00h +10.125 0000 1010 0010 0000 0A20h +0.5 0000 0000 1000 0000 0080h 0 0000 0000 0000 0000 0000h -0.5 1111 1111 1000 0000 FF80h -10.125 1111 0101 1110 0000 F5E0h -25.0625 1110 0110 1111 0000 E6F0h -55 1100 1001 0000 0000 C900h If the user does not wish to take advantage of the thermostat capabilities of the DS1775, the 24 bits can be used for general storage of system data that need not be maintained following a power loss.
