Triple output synchronous controller meets cold crank conditions
All output voltages are maintained in regulation during automotive cold crank conditions, using the Power by Linear LTC7815, triple output, synchronous DC/DC controllerbuck, buck boost, synchronous DC/DC controller from Analog Devices operates at a high frequency (up to 2.25MHz).
A 12V automotive battery can droop to less than 4V during engine restart or cold crank, causing the reset of infotainment systems and other electronics that operate from five volts and higher, explains Analog Devices.
The high efficiency synchronous boost converter feeds the two step-down converters, avoiding output voltage dropout when the car battery voltage droops. Analog Devices points out that this is 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 38V during start-up and maintains operation down to 2.5V after start-up. The synchronous boost converter can produce output voltages up to 60V and can run with the synchronous switch fully ON at 100 per cent duty cycle to pass through the input voltage when it is high to reduce noise and maximise efficiency. The two step-down converters can produce output voltages from 0.8 to 24V with the system achieving efficiency as high as 95 per cent. Its low 45ns minimum on-time enables high step-down conversions while switching at 2MHz, 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-microA per channel (38-microA for all three on) while regulating the output voltage at no load. This, notes Analog Devices, is a useful feature for preserving battery run times in always-on systems. The powerful 1.1-Ohm onboard all n-channel MOSFET gate drivers minimise switching losses and provide output current of more than 10A per channel, limited only by external components. 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 320kHz to 2.25MHz or it can be synchronised 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 ± one per cent over a -40 to +125 degree C operating temperature range.
The LTC7815 is available in a 38-lead 5.0 x 7.0mm QFN package. Two temperature grades are available, with operation from -40 to +125 degree C for the extended and industrial grades and a high temperature automotive range of -40 to +150 degree C.
