Silence is golden: 65V, 8A synchronous step-down switching regulator

-April 30, 2017

Silence is golden especially with switching regulators in automobiles that are a two-edged sword: they are very efficient, thus extending the battery life in an EV, but their switching noise can interfere with AM Radio reception and safety critical systems with radiated EMI.

Linear Technology Corporation, which was recently acquired Analog Devices, Inc., has announced the LT8645S, an 8A, 65V input capable synchronous step-down switching regulator.

How they reduce EMI/EMC emissions

The designers put a great deal of thought and ingenuity into this IC. The Silent Switcher 2 architecture uses two internal input capacitors as well as internal BST (a drive voltage, higher than the input voltage, to the topside power switch) and INTVCC capacitors to minimize the area of the hot loops. They have combined this with very well controlled switching edges and an internal construction with an integral ground plane, plus the use of copper pillars in lieu of bond wires. The device utilizes internal top and bottom high efficiency power switches with the boost diode, oscillator, control and logic circuitry integrated into a single die.

For applications requiring the lowest noise operation, the IC can be programmed to run in pulse-skipping mode. Its switching frequency is programmable and synchronizable from 200kHz to 2.2MHz. Special design techniques and a new high speed process enable high efficiency over a wide input voltage range.

An ultralow EMI application using Spread Spectrum for lowest noise

Although this improved EMI/EMC performance is not sensitive to board layout, the data sheet even has a low EMI PC board recommended layout which will add an extra layer of noise reduction for designers.

The switcher can easily pass the automotive CISPR25, Class 5 peak EMI limits with a 2MHz switching frequency over its entire load range. Spread spectrum frequency modulation is also available to lower EMI/EMC levels further. Every step has been taken to provide designers with many levels of insurance to minimize noise.

It’s efficient too

The regulator’s synchronous rectification delivers efficiency as high as 94% with a switching frequency of 2MHz. Its 3.4V to 65V input voltage range is ideal for dual cell transportation, 48V automotive and industrial applications. Its internal high efficiency switches can deliver up to 8A of continuous output current to voltages as low as 0.97V. Using Burst Mode operation leads to only 2.5µA of quiescent current, well suited for applications such as automotive/transportation always-on systems, which need to extend operating battery life. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 10mVP-P.

12V input to 5V output efficiency

The IC’s unique design maintains a minimum dropout voltage of only 60mV (at1A) under all conditions, enabling it to excel in scenarios such as automotive cold-crank. In addition, a fast minimum on-time of only 40ns enables 2MHz constant frequency switching from a 24V input to a 2.0V output, enabling designers to optimize efficiency while avoiding critical noise-sensitive frequency bands.

Automotive application

Here is an example of one way to configure the regulator in a step-down architecture for an automotive application with soft start and power good.

Temperature range

An industrial temperature version, the LT8645SIV, is tested and guaranteed to operate from a –40°C to 125°C operating junction temperature.

Package and pricing

The IC has a 32-lead, 4×6mm LQFN package and high switching frequency keeps external inductors and capacitors small, providing a compact, thermally efficient board footprint.

Pricing starts at $5.95 each for 1,000-piece quantities.

For more information, visit the Linear Technology website.


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