The FM75 is a temperature sensor chip, Delta-Sigma analog-to-digital converter and aSMBus compatible serial digital interface. ±2°C typical accuracy over temperature range 40°C to 40°C 125°C to ±1°C 9 to 12-bit resolution over temperature range 0°C to 100 °C (default is 9). Thermal alarm output, out-of-range signal (OS) supports interrupt and comparator modes. The OS is active if a user programmable trip temperature is exceeded.

Basic operation

The FM75 temperature sensing circuit continuously generates an analog voltage proportional to the device temperature. Periodically, the FM75 converts the analog voltage to a two's complement digital value into the temperature register. The FM75 has an SMBus compatible digital serial interface that allows access to temperature data to register at any time. Additionally, the serial interface provides access to all other FM75 registers for custom device operation. The FM75 temperature-to-digital conversion can have a choice of 9, 10, 11 or 12 bit resolution, providing 0.5°C, 0.25°C, 0.125°C and 0.0625°C temperature resolution, respectively. At power-up, the default conversion resolution is 9 bits. The conversion resolution is determined by the R0 and R1 bits in the configuration register. Table 1 gives an example of the relationship between output digital data and external temperature. The 9-bit, 10-bit, 11-bit and 12-bit columns in the table below represent the rightmost bits in the output data stream and can contain temperature information for each conversion precision. Since the output digital data is in two's complement format, the most important part of the temperature is the "sign" bit. If the sign bit is zero, the temperature is positive; if the sign bit is one, the temperature is negative. The FM75 features a shutdown mode that reduces operating current to 150nA. This mode is controlled by the SD bit in the configuration register.

Power-on default conditions

The FM75 starts up in the following default states:

Thermostat Mode: Comparator Mode

OS polarity: active low

Fault Tolerance: 1 fault (i.e. F0 = 0 and F1 = 0 configuration registers)

TOS: 80°C

THYST: 75°C

Register pointer: 00 (temperature register)

Conversion Resolution: 9 bits (i.e. R0 = 0 and R1 = 0 in configuration registers)

After power-up, these conditions can be reprogrammed through the serial interface. Operation section of the FM75 programming instructions.

Thermal alarm function

The FM75 thermal alarm function provides a programmable thermostat function that enables the FM75 to function properly as a stand-alone thermostat without using a serial interface. The overrun signal (OS) output is an alarm output. This signal is an open-drain output, and when at is powered on, this pin is configured to be active low. OS polarity is controlled by the POL bits in the configuration register. The programmable upper trip point temperature for thermal alarms is stored in the TOS register. The programmable hysteresis temperature (ie, lower trip point) is stored in the THYST register. Thermal Alarm has two modes of operation: Comparator Mode and Interrupt Mode. On power-up, the default is comparator mode. The alarm mode is controlled by the CMP/INTR bits in the configuration register. Fault Tolerance For Comparator and Interrupt modes, the alarm "Fault Tolerance" setting acts in determining when the OS output is active. Fault tolerance refers to the number of consecutive times an error condition must be detected before the user is notified. Higher fault tolerance settings can help eliminate the noise in the system that causes false alarms. Alarm tolerance is controlled by bits F0 and F1 in the configuration register. These bits can be used to set the fault tolerance to 1, 2, 4 or 6 as shown in the table below. At power-up, these bits all default to 0 (fault tolerance = 1).

Compare mode

In comparator mode, each time a temperature-to-digital (T-to-D) temperature conversion occurs, the new digit compares the temperature to the value stored in TOS and THYST registers. The OS output is activated if the tolerant number of consecutive temperature measurements is greater than the value stored in the TOS register. For example, if bits F1 and F0 are equal to "10" (Fault Tolerance = 4), four consecutive temperature measurements must exceed TOS to activate the OS output. Once the OS output is active, it will remain active until the first measured temperature drops below the temperature stored in the THYST register. Operating the alarm in comparator mode, fault tolerance = 2 as shown below.

interrupt mode

In interrupt mode, the OS output first becomes active after a fault-tolerant number of consecutive temperature measurements exceeds the value stored in the TOS register (similar to comparator mode). Once the OS is active, it can only be read by the user from either clearing the FM75 registers (temperature, configuration, TOS or THYST) or putting the FM75 in shutdown mode (i.e. by setting the shutdown bit in the configuration register to '1' ). After clearing, only the OS output can be activated the next time the continuous temperature measurement value is lower than the stored value in THYST according to the fault tolerance number. Once activated, the OS output can only be read by the user or deactivated by closing. In interrupt mode, the activation/clearing cycle of the OS is as follows: Temp > TOS, Clear, Temp TOS, clear etc. Operation fault tolerance = 2 Interrupt mode alarms as shown above.