Power Electronics

Triple Output, Buck/Buck/Boost Synchronous DC/DC Controller

Linear Technology Corporation introduces the LTC3859, a triple output (buck, buck, boost), low quiescent current synchronous DC/DC controller that maintains all output voltages in regulation during automotive cold crank conditions. A 12V car battery can droop to less than 4 volts during engine restart or cold crank, causing reset of infotainment systems and other electronics that operate from 5 volts 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% duty cycle (synchronous switch ON) to pass through the input voltage when required to maximize efficiency. The two step-down converters can produce output voltages from 0.8V to 24V with the entire system achieving efficiency as high as 95%. In addition, the LTC3859 can be configured for Burst Mode® operation, which reduces quiescent current to less than 55µA per channel (80µA for all three on) in sleep mode, a useful feature in preserving battery run times. The powerful 1.1Ohm onboard all N-channel gate drivers minimize MOSFET switching losses and allow an output current of more than 10 amps 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 synchronized 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% over a -40ºC to 125ºC operating temperature range

The LTC3859 is available in the 38-lead SSOP or a 38-pin 5mm x 7mm QFN package. The LTC3859E versions operate from -40ºC to 85ºC, with 1000-piece price starting at $4.95 each. The LTC3859I industrial grade version operates from -40ºC to 125ºC, with 1,000-p iece pricing starting at $5.47 each. Both versions are available from stock. For more information, visit www.linear.com.

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