Synchronous step-down switching delivers high efficiency and ultralow emissions

Linear Technology has announced the LT8645S, an 8A, 65V input capable synchronous step-down switching regulator. Its unique Silent Switcher 2 architecture uses two internal input capacitors as well as internal BST and INTVCC capacitors to minimize the area of the hot loops. Combined with very well controlled switching edges and internal construction with an integral ground plane and the use of copper pillars in lieu of bond wires, the device dramatically reduces EMI/EMC emissions. This improved EMI/EMC performance is not sensitive to board layout, simplifying design and reducing risk even when using two layer PC boards. The device 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. The device’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. The internal high efficiency switches can deliver up to 8A of continuous output current to voltages as low as 0.97V. The device’s Burst Mode operation offers only 2.5µA of quiescent current, well suited for applications such as automotive/transportation always-on systems, which need to extend operating battery life. The device’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. Furthermore, a fast minimum on-time of only 40ns enables 2MHz constant frequency switching from a 24V input to a 2V output, enabling designers to optimize efficiency while avoiding critical noise-sensitive frequency bands. The device’s 32-lead, 4mm x 6mm LQFN package and high switching frequency keeps external inductors and capacitors small, providing a compact, thermally efficient footprint. The device utilizes internal top and bottom high efficiency power switches with the necessary boost diode, oscillator, control and logic circuitry integrated into a single die. Low ripple Burst Mode operation maintains high efficiency at low output currents while keeping output ripple below 10mVP-P. For applications requiring the lowest noise operation, the LT8645S 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, and its current mode topology enables fast transient response and excellent loop stability. Other features include internal compensation, a power good flag, output soft-start/tracking and thermal protection.