DS75
2–Wire Thermal Watchdog
www.dalsemi.com
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
SDA
SCL
O.S.
GND
1
2
3
4
8
7
6
5
V
DD
A
0
A
1
A
2
DS75S
8-PIN SOIC (150 MIL)
PIN DESCRIPTION
SDA
SCL
GND
O.S.
A0
A1
A2
V
DD
–
–
–
–
–
–
–
–
2–Wire Serial Data Input/Output
2–Wire Serial Clock
Ground
Thermostat Output Signal
Chip Address Input
Chip Address Input
Chip Address Input
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 device 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.
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DS75
Applications for the DS75 include personal computers/servers, cellular telephones, office equipment, or
any microprocessor–based thermally–sensitive system.
DETAILED PIN DESCRIPTION
Table 1
PIN
1
2
3
4
5
6
7
8
SYMBOL
SDA
SCL
O.S.
GND
A
2
A
1
A
0
V
DD
DESCRIPTION
Data input/output pin
for 2–wire serial communication port.
Clock input/output pin
for 2–wire serial communication port.
Thermostat output
Becomes active when temperature exceeds T
OS
. Device
configuration defines means to clear over–temperature state.
Ground pin.
Address input pin.
Address input pin.
Address input pin.
Supply 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 (T
OS
) and under–temperature (T
HYST
) setpoints for
thermostatic operation. The power–up 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 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 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 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.
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DS75
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 continuously unless the user intervenes
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 temperature. The
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.
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DS75
Temperature/Data Relationships
Table 2
S
MSb
2
–1
2
–2
2
–3
2
6
2
5
2
4
2
3
2
2
2
1
2
0
LSb
0
0
0
LSB
MSB
(UNIT =
°C)
2
–4
0
TEMPERATURE/DATA RELATIONSHIPS
Table 2 cont’d
TEMP
+125°C
+25.0625°C
+10.125°C
+0.5°C
+0°C
–0.5°C
–10.125°C
–25.0625°C
–55°C
DIGITAL OUTPUT
(Binary)
0111 1101 0000 0000
0001 1001 0001 0000
0000 1010 0010 0000
0000 0000 1000 0000
0000 0000 0000 0000
1111 1111 1000 0000
1111 0101 1110 0000
1110 0110 1111 0000
1100 1001 0000 0000
DIGITAL
OUTPUT
(Hex)
7D00h
1910h
0A20h
0080h
0000h
FF80h
E5E0h
E6F0h
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 temperature meets or exceeds the value stored in the high tempera-
ture 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, 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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DS75
O.S. OUTPUT TRANSFER FUNCTION
Figure 2
Regardless of the mode chosen, the O.S. output is open–drain and the active state is set in the configura-
tion 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–Programming
There are three areas of interest in programming the DS75: Configuration register, T
OS
register, and the
T
HYST
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 DS75.
Pointer Register Structure Table 3
POINTER
00h
01h
02h
03h
ACTIVE REGISTER
Temperature (default)
Configuration
T
HYST
T
OS
The DS75 will power up with the temperature register selected. If the host wishes to change the data
pointer, it simply addresses the DS75 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 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.
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