MAX31725/MAX31726
±0.5°C Local Temperature Sensors
13Maxim Integrated
Data Format
Bit D5 selects the temperature data format for the
temperature, T
OS
, and T
HYST
registers. When D5 is 0
(normal format), the data format is two’s complement with
a range of -128NC to +127.99609375NC.
Set D5 to 1 for extended temperature format. In extended
format, the measured temperature equals the two’s
complement value plus 64NC, thereby extending the upper
temperature data range to 191.99609375NC and allowing
temperatures as high as 150NC to be measured. See
Table 4. Once set, the data format does not update until
the completion of the following temperature conversion.
After setting D5 to 1, new extended temperature data
is guaranteed ready after a time equal to twice the max
conversion time.
TIMEOUT Enable
Set D6 to 1 to disable bus timeout.
Set D6 to 0 to reset the I
2
C-compatible interface when
SDA is low for more than 50ms (nominal).
One-Shot
The ONE-SHOT function helps to reduce average supply
current when continuous conversions are not necessary.
Setting D7 to 1 while the device is in shutdown mode
immediately begins a new temperature conversion. After
the conversion has completed, the device returns to
shutdown mode. D7 returns to 0 when the conversion
completes. Writing 1 to D7 has no effect when the device
is not in shutdown.
Power-On Reset Value
The configuration register always powers up to a known
state, as indicated in Table 2. These default POR values
correspond to the following modes of operation:
• Comparator mode
• OS active low
• 1 fault, fault queue
• Normal data format
• Timeout enabled for MAX31725; timeout disabled for
MAX31726
RESET Input (MAX31726 Only)
The RESET input on the MAX31726 provides a way to
reset the I
2
C bus and all the internal registers to their
initial POR values. To reset, apply a low pulse width
a duration of at least the minimum reset pulse width
(t
RESET
) to the RESET input.
Applications Information
The MAX31725 and MAX31726 measure the temperature
of their own die. The thermal path between the die and
the outside world determines the accuracy of temperature
measurements. External temperature is conducted to the
die primarily through the leads and the exposed pad.
Because of this, the device most easily measures the PCB
temperature. For ambient temperature measurements,
mount the device on a PCB (or a section of the PCB) that
is at ambient temperature. Temperature errors due to
self-heating of the device die are minimal due to the low
supply current.
Digital Noise Issues
Both devices feature an integrated lowpass filter on the
SCL and SDA digital lines to mitigate the effects of bus
noise. Although this filtering makes communication robust
in noisy environments, good layout practices are always
recommended. Minimize noise coupling by keeping
digital traces away from switching power supplies.
Ensure that digital lines containing high-speed data cross
at right angles to the SDA and SCL lines. Excessive noise
coupling into the SDA and SCL lines on the device—
specifically noise with amplitude greater than 400mV
P-P
(typical hysteresis), overshoot greater than 300mV above
+V
DD
, and undershoot more than 300mV below GND—
can prevent successful serial communication. Serial bus
not-acknowledge is the most common symptom, causing
unnecessary traffic on the bus. Care must be taken to
ensure proper termination within a system with long PCB
traces or multiple slaves on the bus. Resistance can be
added in series with the SDA and SCL lines to further
help filter noise and ringing. If it proves to be necessary,
a 5kI resistor should be placed in series with the SCL
line, placed as close as possible to the SCL pin. This 5kI
resistor, with the 5pF to 10pF stray capacitance of the
device provide a 6MHz to 12MHz lowpass filter, which is
sufficient filtering in most cases.
Table 6. Configuration Register Fault
Queue Bits
FAULT QUEUE [1]
BIT D4
FAULT QUEUE [0]
BIT D3
NUMBER OF
FAULTS
0 0 1 (POR state)
0 1 2
1 0 4
1 1 6
