Microwave Journal

ADI's Triple Output, Synchronous DC/DC Controller Operates to 2.25 MHz in Automotive Start/Stop Systems

January 29, 2018

Analog Devices Inc. (ADI) announces the Power by Linear™LTC7815, a high frequency (up to 2.25 MHz) triple output (buck, buck, boost), synchronous DC/DC controller that maintains all output voltages in regulation during automotive cold crank conditions. A 12 V automotive battery can droop to less than 4 V during engine restart or cold crank, causing reset of infotainment systems and other electronics that operate from 5 V and higher. The high efficiency synchronous boost converter feeds the two step-down converters, avoiding output voltage dropout when the car battery voltage droops, a useful feature in automotive start/stop systems that shut off the engine at idle to save fuel. Alternatively, the buck controllers can be powered from the input for a general-purpose triple output controller.

The LTC7815 operates from an input voltage of 4.5 to 38 V during start-up and maintains operation down to 2.5 V after start-up. The synchronous boost converter can produce output voltages up to 60 V and can run with the synchronous switch fully ON at 100 percent duty cycle to pass through the input voltage when it is high to reduce noise and maximize efficiency. The two step-down converters can produce output voltages from 0.8 to 24 V with the entire system achieving efficiency as high as 95 percent. Its low 45 ns minimum on-time enables high step-down conversions while switching at 2 MHz, avoiding critical noise-sensitive frequency bands such as AM radio and allows for smaller external components.

The LTC7815 can be configured for Burst Mode® operation, reducing quiescent current to 28 μA per channel (38 μA for all three on) while regulating the output voltage at no load, a useful feature for preserving battery run times in always-on systems. The powerful 1.1 Ω onboard all N-channel MOSFET gate drivers minimize switching losses and provide output current of more than 10 amps per channel, limited only by external components. Furthermore, the output current for each converter is sensed by monitoring the voltage drop across the inductor (DCR) or by using a separate sense resistor. The LTC7815's constant frequency current mode architecture enables a selectable frequency from 320 kHz to 2.25 MHz or it can be synchronized to an external clock over the same range.

Additional features include an onboard LDO for IC power and gate drive, output voltage tracking or adjustable soft-start, a power good signal and an external VCC input. The reference voltage accuracy is ±1 percent over a –40°C to 125°C operating temperature range.

The LTC7815 is available in a 38-lead 5 mm x 7 mm QFN package. Two temperature grades are available, with operation from –40 to 125°C for the extended and industrial grades and a high temperature automotive range of –40°C to 150°C.