Multi-output power management ICs in infotainment systems: Page 5 of 8

October 30, 2013 //By Steve Knoth, Jeff Marvin, Linear Technology Corporation
Multi-output power management ICs in infotainment systems
With automotive infotainment systems being increasingly incorporating highly integrated and very powerful microprocessors and FPGAs, power management for these infotainment systems is growing more complex. A solution that helps to keep the designs simple is multi-output power management chips.
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 & Conducted Emissions

Since there are 4 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

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