DC/DC Controller For Automotive Start/Stop Systems

Linear Technology has introduced the LTC3859, a triple output (buck, buck, boost), low quiescent current synchronous DC/DC controller for automotive/in-vehicle applications. The LTC3859 maintains all output voltages in regulation during automotive cold-crank conditions. A 12V car battery can droop to less than 4V during engine restart or cold crank, causing reset of infotainment systems and other electronics that operate from 5V and higher.

The high-efficiency synchronous boost converter feeds the two step-down converters, avoiding output voltage dropout when the car battery 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 LTC3859 operates from an input voltage of 4.5V 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 at 0 per cent duty cycle (synchronous switch ON) to pass through the input voltage when required to maximise efficiency. The two-step converters can produce output voltages from 0.8V to 24V with the entire system achieving efficiency as high as 95 per cent. In addition, the LTC3859 can be configured for burst-mode operation, which reduces quiescent current to less than 55uA per channel (80uA for all three on) in sleep mode, a useful feature in preserving battery run times.

The 1.1ohm onboard all N-channel gate drivers minimise Mosfet switching losses and allow an output current of more than 10A per channel, only limited 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 LTC3859's constant frequency current-mode architecture allows for a selectable frequency from 50kHz to 900kHz or it can be synchronised to an external clock with its internal phased-lock loop (PLL) from 75kHz to 850kHz.

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 per cent over a -40C to 125C operating temperature range. The LTC3859 is available in the 38-lead SSOP or a 38-pin 5 x 7mm QFN package. The LTC3859E version operates from -40C to 85C, while the LTC3859I industrial-grade version operates from -40C to 125C.

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