Digital Temperature Sensors Improve System Performance, Reliability

Digital Temperature Sensors Improve System Performance, Reliability

By Ashok Bindra, Electronic Products

Whether it is a CPU board, battery charging circuitry, process control, environmental regulation, or medical system, overheating is a major concern as it affects system performance, reliability, safety, and product liability. Consequently, sensing and monitoring temperature in these products is becoming critical. In fact, it is a requirement in many of these applications. However, depending on the system’s need for accuracy, precision or ease of use, the sensor choice can vary from an IC package to a thermistor, a thin-film or any other technology.

Hence, when it comes to precision over a wide range of temperature, sensors ICs look attractive. And, lately they have made significant progress to surpass other technologies such as thermocouples, thermistors and resistive thermal devices (RTDs). Even though sensor ICs deliver both digital and analog outputs, the digital versions are gaining momentum due to programmability, pre-configuration and communication over a serial bus.

To address the needs of consumer, industrial, computer and medical systems, Atmel Corp. (www.atmel.com) has readied a new family of high-precision digital temperature sensor ICs that come with integrated nonvolatile registers (NVR) and serial EEPROM memory. The family is labeled AT30TS75x and there are five new members. While AT30TS75 is simply an enhanced version of industry standard LM75, AT30TS750 offers NVR registers, as shown in Fig.1. In addition to NVR registers, AT30TSE752/4/8 come with integrated 2-, 4- or 8-kbits of serial EEPROM.

Fig.1: Atmel’s new digital temperature sensor ICs come with on-chip nonvolatile registers (NVRs) and serial EEPROM memory to retain user configured settings across power cycles.

Let us first see what is the significance of integrated NVR registers. In fact, there are many benefits. Besides allowing user-configured or pre-defined power-up defaults, the NVR registers can be programmed for permanent device configuration to prevent erroneous misconfiguration. As a result, the registers reduce or eliminate dependency on host MCU for configuration, allowing the device to run completely self-contained. For example, explains Richard De Caro, Atmel’s strategic marketing director, battery manufacturers can have their batteries pre-programmed to shut off charging if the battery gets too hot without having to depend on a host MCU in the application to program those temperature limits or to shut off charging.

The value-added NVM registers allow user configuration settings to be permanently retained across power cycles to simplify the system design, reduce processor startup code, and improve system reliability. In addition, the AT30TS75x includes programmable high and low temperature alarms, user-selectable temperature resolution up to 12 bits, and an I2C/SMBus compatible serial interface for communication.

Likewise, integrated serial EEPROM provides an ability to store critical temperature data locally on the device. Plus, it allows the user to store configuration and parametric data, including user-programmable data. Besides minimizing data tampering, on-chip storage also enables the user to log temperature extremes that force system shutdowns or failures. In addition, integrated EEPROM lowers system component count for lower bill of material (BOM) cost, cuts printed circuit board (PCB) space and increases product flexibility.

According to De Caro, “Previous generation temperature sensors had to be set up by the microcontroller each time the system boots. With the new family of digital temperature sensors, a system can automatically power up in the right pre-configured state, freeing the host microcontroller from the burden of supporting temperature sensing capability. By eliminating this extra step, system designers can reduce their overall time-to-market, use less power, and improve system reliability.”

In essence, the AT30TS750 utilizes a band-gap type temperature sensor built on a CMOS substrate. An internal sigma-delta analog-to-digital converter (ADC) is used to convert the temperature readings into a digital value with a selectable resolution of 9 to 12 bits. The measured temperature is calibrated in degrees Celsius. However, if the

designer wishes to deal in degrees Fahrenheit, a lookup table or conversion routine is necessary for that application. The result of the digitized temperature measurements are stored in the AT30TS75x’s internal temperature register, which are readable at any time through the device’s serial interface.

Fig.2: Unlike a traditional thermistor circuit, which requires an external ADC converter and many other discrete external components to complete the temperature sensing solution (a), the digital temperature sensor AT30TS75x is factory calibrated and requires no external components to complete the solution (b).

In the normal operating mode, the device performs continuous temperature measurements and updates the contents of the temperature register after each analog-to-digital conversion. When the temperature measurement exceeds the user-defined temperature and fault count limits stored in the high and low temperature limit registers, a dedicated alarm output is activated. The AT30TS75x converts temperatures from -40 C to +125 °C to a digital word of 9 to 12 bits (selectable) and provides a typical accuracy of ±0.5 °C over the operating temperature range of 0 °C to +85 °C. Typical rated accuracy is ±1.0°C over -20 °C to +105 °C temperature and ±2.0 °C over the -40°C to +125 °C temperature range.

Unlike a thermistor circuit shown in Fig.2(a), which requires an external ADC converter and many other discrete external components to complete the temperature sensing circuit, the AT30TS75x is factory calibrated and requires no external components to complete the solution, as shown in Fig.2(b). In short, the digital temperature sensor simplifies design, reduces burden on host CPU, cuts time-to-market, and lowers overall system cost.

Plus, to reduce current consumption and save power, the AT30TS75x features a shutdown mode that turns off all internal circuitry except for the internal power-on reset and serial interface circuits. In addition, the device features a power saving one-shot mode that allows the device to make a temperature measurement and update the temperature register and then return to shutdown mode.

Typical current consumption in the shutdown mode is 0.1 µA, while active current during a temperature measurement is 45 µA. The AT30TS75x family comes in three different package types. These include 8-lead SOIC, 8-pin MSOP, and 8-pad ultra-thin DFN (UDFN).

To allow users to experiment and develop temperature sensing solution for a variety of applications, a starter kit is available for all Atmel digital temperature sensor devices. Labeled AT30TK175STK, the starter kit utilizes the AT30TK175 daughterboard that interfaces to the AT88Microbase board (included in the kit) to provide communication to a PC via a USB interface. Since this kit is modular, it also enables the AT30TK175 daughterboard to connect directly to an STK series AVR development platform to easily add temperature monitoring functionality to such applications. However, to download the firmware for the kit, the user has to first register online with Atmel.