48V/12V Dual Battery Automotive Systems Require Bi-Directional DC/DC Controllers : Page 5 of 7

August 16, 2017 //By Bruce Haug, Linear Technology
48V/12V Dual Battery Automotive Systems Require Bi-Directional DC/DC Controllers
Fuel economy requirements and ADAS functions have led to the introduction of a 48V supply bus for cars that coexists with the traditional 12V rail. Both systems are interconnected through bi-directional DC/DC controllers. This article describes the requirements and considerations to optimize the interplay.

The ISG has three important features which are the start-stop function, electricity generation and power assistance. The ISG allows the internal combustion engine to turn off its motor to save fuel at stops and instantly re-starts upon pressing of the gas pedal. Normally referred to as a start-stop system, an ISG makes for a smoother transition when starting the engine. Like a conventional alternator, the ISG produces electric power when the vehicle is running. In addition, the ISG can help to decelerate the vehicle by generating electric power (regenerative braking). The electric power generated during regenerative braking charges the 48V battery, which in turn reduces fuel consumption and its resultant CO2 emissions.

Figure 2 shows a block diagram how the ISG, LTC3871 along with the
12 volt and 48 volt batteries are incorporated into an internal combustion engine vehicle.


Buck and Boost Modes

The LTC3871 can be dynamically and seamlessly switched from buck mode to boost mode and vice versa via a simple control signal. There are two separate error amplifiers for VHIGH or VLOW regulation. Having two error amplifiers allows fine tuning of the loop compensation for the buck and boost modes independently to optimize transient response. When the buck mode is selected, the corresponding error amplifier is enabled, and ITHLOW voltage controls the peak inductor current. The other error amplifier being disabled. In boost mode, ITHHIGH is enabled while ITHLOW is disabled. During a buck to boost or a boost to buck transition, the internal soft-start is reset. Resetting soft-start and parking the ITH pin at the zero current level ensures a smooth transition to the newly selected mode.

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