ADT7302
Rev. B | Page 9 of 16
THEORY OF OPERATION
The ADT7302 is a 13-bit digital temperature sensor with a 14
th
bit that acts as a sign bit. The part houses an on-chip temperature
sensor, a 13-bit ADC, a reference circuit, and serial interface
logic functions in SOT-23 and MSOP packages. The ADC
section consists of a conventional successive approximation
converter based around a capacitor DAC. The parts can run on
a 2.7 V to 5.25 V power supply.
The on-chip temperature sensor allows an accurate measurement
of the ambient device temperature to be made. The specified
measurement range of the ADT7302 is 40癈 to +125癈. The
structural integrity of the device can start to deteriorate when
continuously operated at absolute maximum voltage and
temperature specifications.
CONVERTER DETAILS
The conversion clock for the part is internally generated. No
external clock is required except when reading from and writing
to the serial port. In normal mode, an internal clock oscillator
runs an automatic conversion sequence. During this automatic
conversion sequence, a conversion is initiated every 1.5 second.
At this time, the part powers up its analog circuitry and performs
a temperature conversion. This temperature conversion typically
takes 1.2 ms, after which the analog circuitry of the part auto-
matically shuts down. The analog circuitry powers up again when
the 1.5 second timer times out and the next conversion begins.
Since the serial interface circuitry never shuts down, the result
of the most recent temperature conversion is always available in
the serial output register.
The ADT7302 can be placed into shutdown mode via the
control register. This means that the on-chip oscillator is
shut down and no further conversions are initiated until the
ADT7302 is taken out of shutdown mode. The ADT7302 can
be taken out of shutdown mode by writing all zeros into the
control register. The conversion result from the last conversion
prior to shutdown can still be read from the ADT7302 even
when it is in shutdown mode.
In normal conversion mode, the internal clock oscillator is reset
after every read or write operation. This causes the device to
start a temperature conversion, the result of which is typically
available 1.2 ms later. Similarly, when the part is taken out of
shutdown mode, the internal clock oscillator is started and a
conversion is initiated. The conversion result is available 1.2 ms
later, typically. Every result is stored in a buffer register and is
only loaded into the temperature value register at the first
falling SCLK edge of every serial port activity. Serial port
activity does not interfere with the conversion process and
every conversion completes its process even during a read
operation. A conversion has to be completed before a read
occurs, otherwise its result does not get loaded into the
temperature value register and instead goes into the buffer
register. A new conversion is triggered at the end of each serial
port activity except when a conversion is already in progress.
TEMPERATURE VALUE REGISTER
The temperature value register is a 14-bit read-only register that
stores the temperature reading from the ADC in 13-bit twos
complement format plus a sign bit. The MSB (DB13) is the sign
bit. The ADC can theoretically measure a 255癈 temperature
span. The internal temperature sensor is guaranteed to a low
value limit of 40癈 and a high limit of +125癈. The temperature
data format is shown in Table 5, which also shows the
temperature measurement range of the device (40癈 to
+125癈). A typical performance curve is shown in Figure 11.
Table 5. Temperature Data Format
Temperature
Digital Output DB13&DB0
40癈
11, 1011 0000 0000
30癈
11, 1100 0100 0000
25癈
11, 1100 1110 0000
10癈
11, 1110 1100 0000
0.03125癈
11, 1111 1111 1111
0癈
00, 0000 0000 0000
+0.03125癈
00, 0000 0000 0001
+10癈
00, 0001 0100 0000
+25癈
00, 0011 0010 0000
+50癈
00, 0110 0100 0000
+75癈
00, 1001 0110 0000
+100癈
00, 1100 1000 0000
+125癈
00, 1111 1010 0000
Temperature Conversion Equations
Positive Temperature = ADC Code(d)/32
Negative Temperature = (ADC Code(d)
1
16384)/32
Negative Temperature = (ADC Code(d)
2
8192)/32
1
ADC Code uses all 14 bits of the data byte, including the sign bit.
2
DB13 (the sign bit) is removed from the ADC code.
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