Technology Edge

July 2009 Edition

The Technology Edge is a monthly online technical publication that features articles written by National Semiconductor's Engineers. These articles discuss National products, applications and techniques for solving your analog and mixed-signal design challenges, as well as other pertinent technical information.

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AN-1984: Improving System Performance with a Semiconductor Temperature Sensor and Switch

Overview: As electronic systems continue to include more features and higher performance in smaller packages, system heating issues are increasingly becoming a crucial design consideration. System overheating can reduce performance, damage components, or even be a safety concern. Two parameters often monitored to track and mitigate these system heating issues are continuous temperature measurement and over-temperature alarm signaling.

Continuous temperature measurement enables the processor to detect increasing or decreasing temperature trends and take compensating action based on the measured temperature. For example, a Power Amplifier (PA) can exhibit an increase in gain as it is subjected to increased temperatures. The increased gain causes the PA to use more power; this causes it to generate more heat which, in turn, causes more power and heat. This trend is called thermal runaway. In an application such as a wireless sensor network, excessive gain can cause the battery to be drained sooner than desired. By monitoring the temperature, the processor can adjust the gain of the amplifier, keeping power dissipation right where the designer wants it to be.

AN-1982: Small, Wide Input Voltage Range LM2842 Keeps LEDs Cool

Overview: The world seems to be poised to make the leap to LED lighting. Incandescent has been deemed too inefficient, which it is, and fluorescent lighting often does not achieve the lifetime it claims. Fluorescent also has another set of baggage, dangerous chemicals. So the logical choice that most people believe can take the crown is HB (high brightness) LEDs. HBLEDs are becoming more attractive as the manufacturers of such devices are making great strides in increasing the lumens per watt that they can produce which improves the efficiency. It will not be long before LEDs produce significantly more lumens per watt than current fluorescent lamps.

Two of the main initial areas of focus for replacement with LED lighting are low voltage down lighting and track lighting. These would include replacements for standard sizes such as MR16, PAR20, PAR25, PAR30, and PAR38. National already off ers a variety of solutions for driving multiple LEDs beautifully for such applications including the LM342x and LM340x series of LED drivers. But regardless of how the LEDs are driven they have one issue: They produce heat. In these small enclosures a heatsink with free fl owing air is often not enough. This forces the designer of the fixture into a situation where some forced air is required. The most practical solutions for a fixture of such size are either a DC fan or a more elegant and rugged solution provided by the Nuventix, Inc. Synjet®. A fan is generally a more cost effective solution and can run off either 5VDC or 12VDC rail but it can be difficult to fit into these enclosures and has a lifetime measuring in the tens of thousands of hours at best. On the other hand the Synjet is designed specifically to fit into each of these enclosures, runs off of 5VDC, and has a lifespan on the order of 100k hours.