Advertisement

Article

Silicon thermistor; the little temperature sensor that could

Andigilog, a fabless analog and mixed-signal semiconductor company, today announced the SiMISTOR family, the industry's first cost-effective, linear and highly accurate silicon thermistors. SiMISTOR products, manufactured using mature CMOS processes, can replace traditional thermistors in high-volume cell phones, PCs and peripherals, PDAs, and consumer device applications. The new SiMISTOR family also readily displaces thermistors in traditional industrial applications, such as thermostats and portable medical devices.

The new SiMISTOR silicon thermistors measure temperature to 1 Celsius and provide a linear voltage output proportional to temperature, making the SiMISTOR family more accurate than traditional thermistors. The architecture of the SiMISTOR family provides for improved bill-of-material costs by reducing the complexity of the system required to measure temperature.

“Andigilog's new SiMISTOR family of silicon thermistors now provides engineers with the traditional cost advantages of thermistors, while providing the performance enhancements of silicon temperature sensors,” said E.G. Lee, of Samsung Electronic s cell phone division. “The decreasing size and increasing processing speeds required of today's computing devices are straining the current capabilities of temperature subsystems. A silicon thermistor, such as the SiMISTOR product line with its improved accuracy and linearity, can improve the system's performance by reducing the guardband. It also helps decrease system costs in PCs, cell phones and other devices,” noted Susie Inouye, senior research analyst, Databeans. “Manufacturers, and ultimately users, will benefit from this new approach to thermal management.”

SiMISTOR Product Details
The linearity and accuracy of the SiMISTOR family products eliminate expensive analog-to-digital converters, look-up tables and complex processing equatio ns that traditional thermistors need to get an accurate temperature measurement. In addition, SiMISTOR sensors are calibrated during the manufacturing process, eliminating lot-to-lot variations found in traditional thermistors.

SiMISTOR family product features include:
???Accuracy of 1° C at 25 ° C
???Operating Range: -40° C to +125° C
??Non-linearity < 0.8°C ??Single supply voltage ??Calibrated during manufacturing ??Extremely linear output ramp ??Slope and offset choices to optimize system accuracy over desired range ??Low operating current of less than 35 µA (aSM120) or less than 14 µA (aSM121, aSM122) ??Low self-heating: 0.2°C maximum in still air ??Tiny, leadless quad packages are smaller than packaging for conventional silicon sensors and thermistors

The SiMISTOR family includes the aSM120, aSM121 and the aSM122 sensors. The aSM120 sensor operates with either a traditional series resistor or a current source. It has an operating voltage range of +2.7V to +6V. The aSM120 sensor offset voltage at 0 °C is +2V with an output temperature slope of 10mV/°C. The aSM121 and aSM122 sensors do not require the traditional series resistor for the output voltage. They have a specific pin for output temperature. These parts also have an operating voltage range of +2.7V to +6V. The aSM121 sensor has an offset of 500mV at 0 °C with an output temperature slope of 10mV/°C, while the aSM122 has an offset voltage of 600mV at 0 °C with an output temperature slope of 15mV/°C.

In many computing and consumer applications, space is at a premium. Andigilog's SiMISTOR products offer the smallest packaging available for a silicon thermal sensing device. The family comes in quad flat no-lead (QFN) plastic packages with a maximum of 0.8 mm (height), with the aSM120 sensor packaged in a two-lead version (1.3mm x 0.9mm) and the aSM121 and aSM122 sensors packaged in three-lead versions (1.2mm x 1.5mm).

The aSM120, aSM121 and aSM122 products are sampling now, with volume production beginning Q2 2004. The pricing for the aSM120, in the two-lead QFN package, is $0.49 per unit U.S. in 1K unit quantities. The pricing for the aSM121 and aSM122, in the three-lead QFN package, is $0.35 per unit U.S. in 1K unit quantities. An evaluation kit is available now, free of charge.

For more information, to order samples and evaluation kits, or to get information on local product representatives, go to www.andigilog.com or call +602-940-6200.

Andigilog managed to marry silicon with a thermistor to arrive at a new product they call a SiMISTOR. This trio of products provides +/-1°C accuracy at ambient, have an operating range from -40 to +125°C and are available in two styles of packaging — maybe the smallest in the industry. The company offers a two-pin version and a three-pin version of the product.

The extreme accuracy provided is no small matter to designers. It is important for helping cell phones maintain lock in the PLL and for preventing smart batteries from exploding. It's also important for lap top computers because as they get smaller there is more heat in a smaller space, which can cause the processor to run slower or fail more quickly. So accuracy becomes more important to control these failures.

Andigilog's SiMISTOR can be compared to the thermistor and the IC-based temperature sensor. However, the thermistor market is much larger than the temperature sensor IC market, so the company is targeting the thermistor market. Thermistors are currently used because they are inexpensive, have a two-wire resistive measurement, have a high output and are relatively fast. However, it is a nonlinear output and not very accurate, so resistance drops quickly as the temperature rises, and it requires system calibration and very accurate ADCs. If you have a low-precision ADC, (horizontal bars) then you will have small changes in temperature (vertical bars) at the low end of the temperature range but large swings at the high end of the temperature range. That means you need to have a very accurate ADC to resolve the temperature to 1 degree C. Silicon sensors, however, are linear so the same low accuracy ADC will give the same performance at low and high temperatures.

Better accuracy isn't the only piece of the puzzle needed to win the minds of engineers; it has to fit in their current designs. That's why Andigilog decided to offer a two-pin version and a three-pin version of the SiMISTOR. Good move. The two-pin versions are also available in different slopes for different applications. For example, at ambient the parts are accurate to 1°C; from -10° to +100°C the parts are typically within +/-2°C; and at the outside range of -40 to +125°C the parts accurate to +/-3°C. Most of the parts by other manufacturers offer the +/-3°C at ambient temperatures.

Some examples of SiMISTOR applications include the cell phone baseband, the display driver, the power amp, the transmit function. Typically, cell phone manufacturers have been using thermistors in these applications because they are inexpensive and are smaller than even the SC-70 packaged analog temperature sensors, but they aren't very accurate. So how do manufacturers deal with thermistors that aren't quite accurate enough? They have to use higher resolution ADCs, and that also means they need to do system calibration, which means they need to store the offset coefficient for the non-linearity in a look-up table or they need to interpolate to come up with the temperature. This adds expense. For example, in a current topology if the design is using a thermistor then it needs a pull-up resistor in series and that will cause it to get rid of extra power and it also requires a more accurate ADC with a look up table, and finally software to support it.

Andigilog suggests using a cheaper 8-bit ADC or use the one built into the baseband and use an EPROM for the look-up tables. The software can be used for communications instead of being focused on the temperature. This method also doesn't require that the system be calibrated as tightly because of inaccuracies in the temperature.

This redesign would eliminate a resistor and thermistor and replace it with the aSM121 or aSM122 and then run the input into the ADC as opposed to the thermistor. So it's a minor change in your design.

The aSM120 is a two-pin device and calibrated to +/-1°C, and it requires a pull-up resistor. However, what's novel with this part is that it is powered by the output pin. The aSM121 and 122 are more typical temperature sensors using a power supply pin and an output pin.

Finally, the prices of these SiMISTORs compare very favorably to the thermistor products. When you look at the BOM of the two the SiMISTOR runs in the middle of the pack of prices.

Product Family

0 comments on “Silicon thermistor; the little temperature sensor that could

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.