Dallas Semiconductor DS75X, DS75U, DS75S Datasheet

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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

nine (default) to 12 bits (0.5°C to 0.0625°C
resolution)

 9–bit readout mode features a max

conversion time of 150 ms.

 Thermostatic settings are user definable.
 Data is read from/written via a 2–wire serial

interface. (open drain I/O lines). 3–bit
addressability

 Wide power supply range (2.7V – 5.5V).
 Applications include personal computers,

cellular telephones, office equipment, or any
thermally sensitive system.

 Pin/software compatible to LM75CIM–x

Thermal Watchdog in 9–bit (default) mode.

 8–pin 150 mil SOIC package.

PIN ASSIGNMENT

PIN DESCRIPTION

SDA – 2–Wire Serial Data Input/Output
SCL – 2–Wire Serial Clock
GND – Ground
O.S. – Thermostat Output Signal
A0 – Chip Address Input
A1 – Chip Address Input
A2 – Chip Address Input
V

DD

– Power Supply Voltage

DESCRIPTION

The DS75 2–wire thermal watchdog provides 9–bit temperature readings which indicate the temperature
of the device. Thermostat settings and temperature readings are all communicated to/from the DS75 over
a simple 2–wire serial interface. No additional components are required; the device is truly a
“temperature–to–digital” converter.

The DS75 has three address bits that allow a user to multidrop up to eight sensors along the 2–wire bus,
greatly simplifying the bussing of distributed temperature sensing networks.

The open–drain thermal alarm output, O.S., becomes active when the temperature of the d evice 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.

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.

DS75

2–Wire Thermal Watchdog

www.dalsemi.com

6

3

1

2

4

8

7

5

SD

A

O.S.

GND

V

DD

A

0

A

1

A

2

SCL

DS75S

8-PIN SOIC (150 MIL)

DS75

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Applications for the DS75 include personal computers/servers, cellular telephones, of fice equipment, or
any microprocessor–based thermally–sensitive system.

DETAILED PIN DESCRIPTION Table 1

PIN SYMBOL DESCRIPTION

1SDAData input/output pin for 2–wire serial communication port.
2SCLClock input/output pin for 2–wire serial communication port.
3 O.S. Thermostat output Becomes active when temperature exceeds TOS. Device

configuration defines means to clear over–temperature state.

4 GND

Ground pin.

5A

2

Address input pin.

6A

1

Address input pin.

7A

0

Address input pin.

8VDDSupply Voltage 2.7V – 5.5V input power pin.

OVERVIEW

A block diagram of the DS75 is shown in Figure 1. The DS75 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 DS75

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 DS75 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

HYST

) setpoints for

thermostatic operation. The power–up state of TOS is 80°C and that for T

HYST

is 75°C. The result of each

temperature conversion is compared with the TOS and T

HYST

setpoints. The DS75 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 ex ceeds the respe ctive 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 DS75 is in the comparator mode with a single fault
generating an active O.S. Digital data is written to/read from the DS75 via a 2–wire interface, and all
communication is MSb first. Multipoint sensing is possible with the DS75 by uniquely setting the 3–bit
address of up to 8 parts on the 2–wire bus.

DS75

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DS75 FUNCTIONAL BLOCK DIAGRAM Figure 1

OPERATION–Measuring Temperature

The core of DS75 functionality is its direct–to–digital temperature sensor. The DS75 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 continuousl y unless the user interv enes
by altering the configuration register to put the DS75 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 DS75 power–up default has the sensor automatically performing
9–bit conversions continuously. Details on how to change the settings after pow-er 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 tempe rature. T he
table assumes the DS75 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.

DS75

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Temperature/Data Relationships Table 2

S262

5

2

4

2

3

2

2

2

1

2

0

MSB

MSb (UNIT = °C) LSb

2

–1

2

–2

2

–3

2

–4

0000LSB

TEMPERATURE/DATA RELATIONSHIPS

Table 2 cont’d

TEMP

DIGITAL OUTPUT

(Binary)

DIGITAL

OUTPUT

(Hex)

+125°C

0111 1101 0000 0000 7D00h

+25.0625°C

0001 1001 0001 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 E5E0h

–25.0625°C

1110 0110 1111 0000 E6F0h

–55°C

1100 1001 0000 0000 C900h

OPERATION–Thermostat Control

In its comparator operating mode, the DS75 functions as a thermostat with programmable hysteresis, as
shown in Figure 2. When the DS75’s temperatur e meets or exceeds t he value stored in the hi gh temper a­ture trip register (T

OS

) a consecutive number of times defined by the configuration register, the output
becomes active, and will stay active until the temperature falls below the temperature stored in the low
temperature trigger register (T

HYST

) the first time. In this way, any amount of hysteresis may be obtained.

The DS75 powers up in the comparator mode with T

OS

=80°C and T

HYST

=75°C, and the device 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 DS75 in a

shutdown mode or by reading any register (temperature, configuration, TOS, 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 (TOS, clear, T

HYST

, clear, TOS, 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.