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1.5-V analog temp sensors tout industrial temp range

Santa Clara, Calif. — National Semiconductor Corp. today proclaimed the industry's first 1.5-V analog temperature sensors with user-selectable gains, which increase thermal management performance in low-voltage systems.

The LM94021 and LM94022 have four user-selectable gains and monitor temperature from -50°C to 150°C. These temperature sensors' wide range of operation, flexibility and low-power performance make them well-suited for low-voltage, battery-powered systems such as cellular phones, PDAs, MP3 players and digital cameras.

The LM94021 and LM94022 are precision analog-output CMOS temperature sensors that operate from a supply voltage between 1.5 V and 5.5 V. They deliver an output voltage that is inversely proportional to measured temperature, for higher sensitivity at elevated temperatures.

Users can select one of four programmable gain options: -5.5 mV per °C, -8.2 mV per °C, -10.9 mV per °C or -13.6 mV per °C.

The LM94021 and LM94022 feature low quiescent currents of 9 μA and 5.4 μA, respectively. These sensors come in a SC70 package and are footprint-compatible with National's industry-standard LM20 temperature sensor.

The LM94021 and LM94022 are priced at 40 cents each in 1,000-unit quantities. Click here for the LM94021 data sheet. Click here for the LM94022 data sheet.

National Semiconductor Corp. , 1-800-272-9959, www.National.com.

Output Voltage vs. Temperature Chart
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Output Voltage vs. Temperature Chart

It's a combination of features that makes this temperature sensor duo special. The low operating voltage, low quiescent current, ±1.5° of accuracy, a very wide industrial temperature range, and tiny package are characteristics that set these devices apart from the others.

Users of portable electronics devices like MP3 players and cell phones are demanding extended battery life. The way to accomplish this task is to drop the overall operating voltage of the system, said Andrew Jue, director of product marketing for National's Data Conversion Division. “We are currently seeing a trend with baseband processors that are being used in cell phones. They are moving down from 3.3 V and 2.6 V, to 2.4 V and even 1.8 V,” Jue said.

National's latest temp sensors are part of an existing family of analog temp sensors. “These devices are a follow up to our very popular LM20 temp sensor, which is about six years old,” Jue said. The LM20 also operates at low voltages (down to 2.4 V).

Jue believes that the LM20 offered the lowest operating voltage in the market prior to these latest temp sensors. “These devices support the current migration to lower voltages in portable electronics that we are seeing today,” he said.

The main difference between the two newest sensors — the LM94021 and LM94022 is that one has a push-pull output (LM94022), and the other actually uses a current source to drive the output (LM94021).

The LM94021 output gives the converter excellent supply noise rejection, Jue said. The LM94022 output has a strong output source current, which makes it ideal for driving A/D converter inputs without requiring a buffer. Both sensors' outputs are stable while driving up to 1,100 pF of load capacitance (like a filter capacitor on an A/D converter input) — without any external components.

User selectable gain options are another important aspect of these sensors. Typically, these types of temp sensors don't have flexible gains, Jue said. “We put them in there because it helps the user get the best performance in the system. It provides flexibility,” he said.

Depending on the application, the designer can choose the setting that is most beneficial. When the system is very cold, typically a very expensive A/D converter with a wide sensitivity level is needed to measure the temperature accurately, Jue said. However, with user programmable gain options, the designer can program the slope from cold operation to hot operation with an inexpensive A/D converter, which is usually embedded in the microprocessor, to get the same level of accuracy.

While multiple gain options aren't unique to analog ICs like operational amplifiers, they are when you combine them in temperature sensors, especially at 1.8 V, Jue said.

Usually, temperature sensors are specified over the commercial temperature range (0°C to 70°C). Customers want portable products like cell phones that are capable of working in various climates like the ones you'd find in Alaska and the Sahara Desert, Jue said.

Consequently, temperature requirements are changing. In addition to climate changes, these products need to be more robust to accommodate rough handling — like falling and dropping, Jue said. Therefore, National guarantees that these devices will work accurately across this entire temp range, he said.

In addition, new high-performance features in portable applications such as MP3 capabilities, camera functions, etc. generate heat inside these products. This makes it essential that the temperature be monitored over a wide range with the best possible accuracy and sensitivity, Jue said.

The LM20 works over a wide temp range as well (-55°C to +130 degrees C). “That was a fairly wide range for a temperature sensor and now we've extended the range by another 20 degrees,” Jue said.

There is a price premium associated with these new devices. Customers who value lower voltages, a wider temperature range and some of the other above mentioned features, should expect to pay 10 cents more for these devices, he said.

The older LM20 is priced at 30 cents each, versus 40 cents each for the LM94021 and LM94022 in 1,000-piece quantities.

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