DS1775
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 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.
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.
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
SCL
GND
O.S.
1
2
3
4
V
DD
5
SDA
SOT23
PIN DESCRIPTION
GND
SCL
SDA
V
DD
O.S.
APPLICATIONS
•
•
•
•
Personal Computers/Servers/Workstations
Cell Phones
Office Equipment
Any Thermally-Sensitive System
Ground
2-Wire Serial Clock
2-Wire Serial Data Input/Output
Power-Supply Voltage
Thermostat Output Signal
Ordering Information appears at end of data sheet
.
1 of 14
19-6687; Rev 5/13
DS1775
Table 1. Detailed Pin Description
PIN
NAME
FUNCTION
Clock Input/Output for 2-Wire Serial Communication Port. This input should
1
SCL
be tied to GND for stand-alone thermostat operation.
2
GND
Ground
Thermostat Output. Open-drain output becomes active when temperature
3
O.S.
exceeds T
OS
. Device configuration defines means to clear overtemperature
state.
4
V
DD
Supply Voltage 2.7V to 5.5V Input Power Pin
Data Input/Output for 2-Wire Serial Communication Port. In the stand-alone
5
SDA
thermostat mode, this input selects hysteresis.
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 (T
OS
) and undertemperature (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 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.
DETAILED DESCRIPTION
2 of 14
DS1775
Figure 1. Block Diagram
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.
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.
3 of 14
OPERATION
Measuring Temperature
DS1775
Table 2. Temperature/Data Relationships
S
MSb
2
-1
2
-2
2
-3
2
6
2
5
2
4
2
-4
2
3
0
2
2
0
2
1
0
2
0
LSb
0
LSB
MSB
(UNIT =
°C)
TEMPERATURE
(°C)
+125
+25.0625
+10.125
+0.5
0
-0.5
-10.125
-25.0625
-55
DIGITAL OUTPUT
(BINARY)
0111 1101 0000 0000
0000 1010 0010 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
F5E0h
E6F0h
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 (T
OS
) 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
HYST
). In this way, any amount of hysteresis may be
obtained. The DS1775 powers up in the comparator mode with T
OS
= +80°C and T
HYST
= +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 T
OS
. The fault can only be cleared by either setting the DS1775 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, etc.). Only the
first of multiple consecutive T
OS
violations activates O.S., even if each fault is separated by a clearing
function. The same situation applies to multiple consecutive T
HYST
events.
4 of 14
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.
There are three areas of interest in programming the DS1775: the configuration register, the 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 DS1775.
Table 3. Pointer Register Structure
POINTER ACTIVE REGISTER
00h
Temperature (default)
01h
Configuration
02h
T
HYST
03h
T
OS
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
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 is always set to 00h, the
5 of 14
Programming
DSP 与 ARM 处理器
京公网安备 11010802033920号
Copyright © 2005-2026 EEWORLD.com.cn, Inc. All rights reserved
电子工程世界
