ADT7302

Rev. B | Page 9 of 16

THEORY OF OPERATION

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°C to +125°C. 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°C temperature

span. The internal temperature sensor is guaranteed to a low

value limit of −40°C and a high limit of +125°C. The temperature

data format is shown in Table 5, which also shows the

temperature measurement range of the device (−40°C to

+125°C). A typical performance curve is shown in Figure 11.

Table 5. Temperature Data Format

Temperature

Digital Output DB13…DB0

−40°C

11, 1011 0000 0000

−30°C

11, 1100 0100 0000

−25°C

11, 1100 1110 0000

−10°C

11, 1110 1100 0000

−0.03125°C

11, 1111 1111 1111

0°C

00, 0000 0000 0000

+0.03125°C

00, 0000 0000 0001

+10°C

00, 0001 0100 0000

+25°C

00, 0011 0010 0000

+50°C

00, 0110 0100 0000

+75°C

00, 1001 0110 0000

+100°C

00, 1100 1000 0000

+125°C

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