48V/12V Dual Battery Automotive Systems Require Bi-Directional DC/DC Controllers : Page 6 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.


Multiphase Operation

Multiple LTC3871s can be daisy chained to run out of phase to provide more output current without increasing input and output voltage ripple. The SYNC pin allows the LTC3871 to synchronize to the CLKOUT signal of another LTC3871. The CLKOUT signal can be connected to the SYNC pin of the following LTC3871 stage to line up both the frequency as well as the phase of the entire system. A total of 12 phases can be daisy chained to run simultaneously out-of-phase with respect to each other.

The LTC3871 demonstration circuit DC2348A shown in figure 3 can be configured with two or four phases utilizing one or two LTC3871 devices. The photo below shows the four phase version and when operating in buck mode, this demo circuit has an input voltage range of 30V to 75V and produces a 12V output at up to 60A. When operating in boost mode, the input voltage is from 10V to 13V and produces a 48V at up to 10A.

Figure 3 – LTC3871 Four Phase Demo Board Picture


The LTC3871 efficiency curves in Figure 4 are representative of a four phase demo board design using two LTC3871 devices. The buck mode curve steps the 48V down to 12V at up to 60A, while the boost curve steps up the 12V to 48V at up to 10A. Both with 97% peak efficiencies.

Efficiency Buck Mode

Efficiency Boost Mode

Figure 4 a/b – LTC3871 Buck and Boost Efficiency Curves with a 4-phase Design

Over Current Protection

In buck mode, the LTC3871 includes current fold-back protection to limit power dissipation in an over current condition or when the VLOW is shorted to ground. If the VLOW falls below 85% of its nominal output level, then the maximum sense voltage is progressively lowered from its maximum programmed value to one-third of the maximum value. Foldback current limiting is enabled during soft-start. Under short-circuit conditions with very low duty cycles, the LTC3871 will begin cycle skipping in order to limit the short-circuit current.

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