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Reducing EMI in automotive environments

Posted: 24 Jan 2014     Print Version  Bookmark and Share

Keywords:electromagnetic interference  EMI  DC/DC  switchmode regulators  LT8614 

Figure 5 shows the switch node at 13.2V input. One can see the extremely low deviation from the ideal square wave of the LT8614, shown in Ch2. All time domain measurements in figures 3, 4 and 5 are done with 500MHz Tektronix P6139A probes with close probe tip shield connection to the PCB GND plane, both on the standard demo boards.

Besides their 42V absolute maximum input voltage ratings for automotive environments, their dropout behaviour is also very important. Often critical 3.3V logic supplies need to be supported through cold crank situations. The LT8614 Silent Switcher regulator maintains the close to ideal behaviour of the LT861x family in this case. Instead of higher under-voltage lockout voltages and maximum duty cycle clamps of alternative parts, the LT8610/11/14 devices operate down to 3.4V and start skipping off cycles as soon as necessary (figure 6). This results in the ideal dropout behaviour as shown in figure 7.

Figure 5: Ch1: LT8610, Ch2: LT8614, both at 13.2V in, 3.3V 2.2A out.

Figure 6: Ch1: LT8610, Ch2: LT8614 switch mode dropout behaviour.

Figure 7: LT8614 dropout behaviour.

The LT8614's minimum on-time of 30ns enables large step-down ratios even at high switching frequencies. As a result, it can supply logic core voltages with a single step-down from inputs up to 42V.

Conclusion
It is well known that EMI considerations for automotive environments require careful attention during the initial design process in order to ensure that they will pass EMI testing once the system is completed. Until now, there was not a sure way to guarantee that this could easily be attained with the right power IC selection. However, this may change with the introduction of LT8614 Silent Switcher regulator, touted to reduce EMI from switching regulators by more than 20dB, while increasing conversion efficiencies with no drawbacks. A 10 time improvement of EMI in the frequency range above 30MHz is attained without compromising minimum on- and off-times or efficiency in the same board area. This is accomplished with no special components or shielding, representing a significant breakthrough in switching regulator design.

About the author
Tony Armstrong, Director of Product Marketing Manager for Linear Technology's power product group, joined the Company in May of 2000. He is responsible for all aspects of the power conversion and management products from conception through obsolescence. Prior to joining Linear, Tony held various positions in marketing, sales and operations at Siliconix Inc., Semtech Corp., Fairchild Semiconductors and Intel Corp. (Europe). He attained a BS (Honours) in Applied Mathematics from the University of Manchester in England in 1981.

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