DC/DC Controller For Automotive Applications

Linear Technology has introduced the LTC3857/-1, an ultra-low quiescent current, two-phase dual-output synchronous step-down DC/DC controller. The LTC3857/-1 is said to draw only 50uA when one output is active and only 80uA when both outputs are enabled. With both outputs shut down, the LTC3857/-1 draws 8uA. This part is said to be ideal for automotive applications where one or both supplies remain active in standby mode.

The LTC3857/-1's input supply range of 4-38V is wide enough to protect against automotive high-voltage transients, continue operation during cold crank and cover a range of battery chemistries. Each output can be set from 0.8-24V at output currents up to 20A, with efficiencies as high as 95 per cent. The LTC3857/-1 is suitable for powering GPS systems, audio systems, CD/DVD players and disk drives. The controller features 1.1ohm on-chip Mosfet gate drivers.

It operates with a selectable fixed frequency between 50kHz and 900kHz, and can be synchronised to an external clock from 75kHz to 850kHz with its phased-locked loop (PLL). The user can select from continuous operation, pulse skipping and low ripple burst-mode operation during light loads, which safely powers pre-biased loads. The LTC3857/-1's two-phase operation reduces input capacitance requirements.

Its current-mode architecture provides easy loop compensation, fast transient response and good line regulation, according to Linear Technology. Both outputs have adjustable soft-start to control the turn-on time and feature a tight +/- 1 per cent reference voltage accuracy over the operating temperature range of -40C to +85C. Output current sensing is accomplished by measuring the voltage drop across the output inductor (DCR) or by using an optional sense resistor.

Current foldback limits Mosfet heat dissipation during overload conditions. There are two versions offered; the LTC3857 is fully featured part with functions that include a clock out, phase modulation, two separate 'power good' signals and adjustable current limit. The LTC3857-1 has one 'power good' signal monitoring both outputs and a 50mV fixed current sense threshold voltage. The LTC3857 is offered in a 32-lead 5 x 5mm QFN package and the LTC3857-1 is housed in a 28-pin SSOP package.


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