Laird

<p>In this example the video output section of a MP3 player is analyzed, which includes the video DAC and video buffer/filter circuitry. The power supply constraints for this example are a single 3-V at 150 mA. In the concept design stage, two options were proposed — Option 1 — drive the video outputs directly from the video DAC using internal drivers. The video DAC chosen could drive the video outputs adequately. However the power consumption of the DAC's video drivers was a little high, but using the on board video drivers would save board space. Option 1 schematic is shown in Figure 1. Option 2 was to use external buffers to lower the operating current. While this option added more circuitry and required more area, the current saved would be significant.</p>

As it can be seen from the figures 7 and 8, the switching transitions are quite fast, about 15ns for the MOSFET turn off and 25ns for the turn on. There is a significant discrepancy between the estimated turn on transition time (65nS) and the actual one; this is due to the lower driver source current and higher Qgd used in the calculations. Current waveform (green trace) shows that the actual driver current is about 260mA, at least before the gate threshold voltage is reached.

C330C300JDG5TATR_Datasheet PDF

MOSFETs can turn on unintentionally when rapidly rising temperatures in extreme environments—such as under-hood automotive electronics applications—cause the threshold voltage to approach zero. Increasing the threshold voltage leads to an undesirable increase in on-resistance, however, so most available solutions utilize a negative voltage driver to prevent unwanted turn-on. This driver also adds size, cost, and complexity to the final circuit design.

Recently a new family of n-channel MOSFETs was introduced that offsets the higher on-resistance with high-density technology. This new solution eliminates the negative gate voltage, resulting in smaller circuits, and lower costs critical for automotive electronics use.

This MOSFET technology, available in 40V and 60V versions, combines a 3.4V threshold voltage—high enough to prevent the MOSFET from turning on unintentionally in high temperatures—with on-resistance as low as 2.7 mΩ. These devices can be used in high-temperature, high-current applications with inductive loads, such as high-side switches, motor drives, and 12V boardnets.

C330C300JDG5TATR_Datasheet PDF

For most of their history, the key figure of merit for power MOSFETs has been device on-resistance (rDS(on) ). In recent years however, advances in power MOSFET technology have reduced on-resistance per area of silicon to such low levels that device manufacturers must now look elsewhere for ways to improve their products.

This is especially true in the case where high-density trench technologies are used to create devices with low drain-to-source voltages. In devices such as these, where the breakdown voltage VDS is less than 60V, the device rDS(on) is dominated by channel resistance, which high transistor cell densities have brought to almost negligible levels. At breakdown voltages above 60V, however, epitaxial (EPI) resistance (surface effects) dominate device rDS(on) . A low area-specific rDS(on) is needed to allow the threshold voltage to go up without drastically increasing the rDS(on) rating of the device, because an increase of threshold voltage results in an increase in on-resistance if nothing else is changed.

C330C300JDG5TATR_Datasheet PDF

The threshold voltage is also affected by die temperature: it goes down when the die temperature goes up. The exact relationship between these two variables is a function of specific materials and processes, and the effect can be compensated for by processes with a low temperature gradient. At best, device manufacturers have been able to achieve values of -1.4V/100°C.

Design Challenges The need to eliminate the possibility of spontaneous MOSFET turn-on as a result of the threshold voltage being pushed too low results in a dilemma for designers. Usually it is an objective to keep the MOSFET drive circuitry as simple and inexpensive as possible, and this dictates the choice of devices that can switch from 0 to 5V (or higher, depending on the rDS(on) rating of the device used) rather than switching from 5V to a negative value (to keep the MOSFET securely off). As long as there is no inductive load, and provided only low levels of current need to be switched, this solution is perfectly adequate.

Audio report (January 2004):

Gas Gauging: Art or Science?Garry Elder, Systems Manager, Texas Instruments

As battery gas gauge” integrated circuit (IC) technology for portable applications has evolved over the past 10 years, the level of sophistication continues to rapidly increase, and calculating battery performance or accuracy remains a highly complex task. The ever-increasing variety of cell types makes it difficult to apply a single scientific formula to determine an accurate state of charge (SOC) in a wide range of portable applications ” from cell phones to notebook computers. Accurate calculation of SOC and run-time data in portable Li-Ion batteries traditionally measures usage stimuli, charge/discharge activity and temperature factors in relation to voltage levels, coulomb change and number of cycles. This gas gauge methodology requires interpretation of how the cells react between the initiation of actions and the corresponding results of those actions to provide SOC and runtime data. In addition, other methods might analyze data by modeling the battery. However, each method has differing elements and resolutions. Each element introduces possible errors into the equation, and the inaccurate resolution ultimately affects the battery's potential long-term maximum performance.

Typical models used in today's gas gauge ICs consist of rate, temperature, Δcoulombs and voltage factors to produce a mAh delta value from the last ΔSOC calculation or an actual SOC. The key issue, in addition to the model's accuracy, is that the model is very inflexible, if flexible at all, once the battery leaves the production floor. The modeling technique, although based on scientific principles and analytical data, is still an art form” when applied to consumer portable applications. A battery's initial SOC setting, which was first determined from the original model's pre-use” properties, and the battery's actual SOC after a few months of regular use can be quite different. As a result, consumer battery pack designers attempt to implement more sophisticated static model techniques and analytical data acquisition systems, while increasing power consumption, solution cost and development cost as highlighted in Figure 1.

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As long as you are aware of the tradeoffs inherent in some filtering types, it makes sense to use DSP filtering to improve the accuracy and resolution of today's real-time oscilloscopes.

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