Rainbow Electronics DS1775R-T User Manual

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PRELIMINARY

DS1775

SOT23-5 Digital Thermomete

and Thermostat

FEATURES

PIN ASSIGNMENT

 Temperature measurements require no

external components

SCL

SDA

 Measures temperatures from –55°C to

+125°C. Fahrenheit equivalent is –67°F to 257°F

GND

O.S.

 Thermometer accuracy is ±2.0°C  Thermometer resolution is configurable from

9 to 12 bits (0.5°C to 0.0625°C resolution)

DS1775R

SOT23-5

 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

PIN DESCRIPTION

GND Ground SCL 2–Wire Serial Clock SDA 2–Wire Serial Data Input/Output V

O.S. Thermostat Output Signal

Power Supply Voltage

low thermal time constant

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 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.

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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 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 TOS. Device configuration defines means to clear over-temperature

state. PIN 4 V

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. 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 (TOS) and under–temperature (T thermostatic operation. The power–up state of T temperature conversion is compared with the T

is 80°C and that for T

and T

setpoints. The DS1775 offers two modes for

HYST

is 75°C. The result of each

HYST

) setpoints for

HYST

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

S262

MSb

-1

-2

-3

TEMP DIGITAL OUTPUT

(UNIT = °C)

-4

MSB

LSb

0000 LSB

DIGITAL OUTPUT (Hex)

(Binary)

+125°C

+25.0625°C

+10.125°C

+0.5°C

+0°C

-0.5°C

-10.125°C

-25.0625°C

-55°C

0111 1101 0000 0000 7D00h 0000 1010 0010 0000 1910h 0000 1010 0010 0000 0A20h 0000 0000 1000 0000 0080h 0000 0000 0000 0000 0000h 1111 1111 1000 0000 FF80h 1111 0101 1110 0000 F5E0h 1110 0110 1111 0000 E6F0h 1100 1001 0000 0000 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 (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 (T obtained. The DS1775 powers up in the comparator mode with TOS=80°C and T used as a standalone thermostat (no 2–wire interface required) with those setpoints.

). In this way, any amount of hysteresis may be

HYST

=75°C and can be

HYST

In the interrupt mode, the O.S. output will first become active following the programmed number of consecutive conversions above T shutdown mode or by reading any register (temperature, configuration, T Following a clear, a subsequent fault can only occur if consecutive conversions fall below T interrupt/clear process is thus cyclical (TOS, clear, T of multiple consecutive T

violations will activate O.S., even if each fault is separated by a clearing

function. The same situation applies to multiple consecutive T

. The fault can only be cleared by either setting the DS1775 in a

HYST

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, clear, TOS, clear, T

events.

HYST

, or T

, clear, ...). Only the first

HYST

) on the device.

HYST

. This

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