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AUTOMOTIVE ELECTrONICS Follow-us on EETimes Europe contact@absopulse.ch www.absopulse.ch High-Performance, high reliable Power Supplies 15 W to 12 kW+ adapted to your challenging projects l AC/DC Power Supplies l Rectifiers and Battery Chargers l DC/DC Converters l Sine-Wave Inverters l Phase and Frequency Converters l AC- Input, DC-output UPS systems l Open Frame l Wall mount l 19” Cassettes l DC-input dimmable back-light inverters for LCD displays l Complete Power Systems in 19" and 23" Racks ABSOPULSE Marketing Europe GmbH PoBox 1501 8620 Wetzikon CH Tel. +41 78 896 50 49 Fax +41 44 944 38 44 Write us: contact@absopulse.ch See us: www.absopulse.ch Fig. 2: The LTC3676 startup sequence. The LTC3676 PWM switching frequency is specifically trimmed to 2.25MHz with a guaranteed range of 1.7MHz to 2.7MHz. Its internal regulators can also be set to a forced continuous PWM operating mode to prevent operation in pulse skip or burst-mode even at light loads. This not only keeps the frequency fixed but also further reduces voltage ripple on the DC-DC output capacitors. Suppressing radiated and conducted emissions Since there are four switching regulators onboard the LTC3676, each has an associated reactive device (inductor) to be concerned about. One possible solution is to shield the LTC3676 area to prevent EMI from being emitted. Besides being expensive and heavy, this does not solve the problem of contamination by any wires that might be connected to the power supply area. It is better to use source suppression and antenna elimination. Source suppression necessitates good layout/component selection (and internal IC design) to prevent the generation of radio frequency energy. It is often necessary to use shielded inductors and to place those inductors further away from the LTC3676 than the output capacitors. This is because the AC currents circulate from the LTC3676 through the inductor, through the output capacitor, to ground and back to the LTC3676. It is clear that wide traces, preferably area fill, should be used to connect the ground of the output capacitors to the ground of the LTC3676 and to the ground of the PVIN input decoupling capacitors also. The LTC3676 also provides some tools for source suppression. Its DC-DC converters specifically include a dv/dt control feature which slows down the switching edge rates to reduce radiated emissions. Since the buck regulators are synchronous, both the rise and the fall time are both controlled. A slow edge rate of about 3ns rise/fall, was selected to both pass emission requirements and still limit switching losses, which helps to optimize power converter efficiency. Each of the 4 buck switching regulators in the LTC3676 default to this 3ns edge rate control mode, but can also be individually set via I2C, to a faster 1ns rate, to improve efficiency, if limited edge rate and emissions control is not required. In addition to switching time control, the LTC3676 offers some other EMI suppression tools. The frequency of the buck regulators can be changed from 2.25MHz to 1.12MHz. Also, to minimize the input ripple, which can end up radiating through the power input wiring, the buck regulators can be staggered 26 Electronic Engineering Times Europe November 2013 www.electronics-eetimes.com


EETE NOV 2013
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