during regenerative braking charges the 48V battery, which in turn reduces fuel consumption and CO2
emissions.
Figure 2 is a block diagram showing how the ISG, LTC3871 along with the 12 volt and 48 volt batteries are incorporated into an internal combustion engine vehicle.
Buck & 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 optimise transient response. When the buck mode is selected, the corresponding error amplifier is enabled, and ITHLOW
Figure 3: LTC3871 four-phase demo board picture
allows both batteries to supply current to the same load if required. Most of the early 48V/12V dual battery DC/DC converter designs use different power components to step-up and step-down the voltage. However, the recently released LTC3871 bi-directional DC/DC controller from the Power by Linear Group uses the same external power components for the step-up conversion as it does for stepping down the voltage.
A single bidirectional IC solution The LTC3871 is a 100V/30V bidirectional two phase synchronous buck or boost controller which provides bi-directional DC/DC control and battery charging between the 12V and 48V board nets. It operates in buck mode from the 48V bus to the 12V bus or in boost mode from 12V to 48V. Either mode is configured on demand via an applied control signal. Up to 12 phases can be paralleled and clocked out-of-phase to minimise input and output filtering requirements for high current applications (up to 250A). Its advanced current-mode architecture provides excellent current matching between phases when paralleled. Up to 5kW can be supplied in buck mode or in boost mode with a 12-phase design. When starting the car, or when additional power is required, the LTC3871 allows both batteries to supply energy simultaneously by converting energy from one board net to the other. Up to 97 per cent efficiency can be achieved and the on-chip current programming loop regulates the maximum current that can be delivered to the load in either direction. Four control loops, two for current and two for voltage, enable control of voltage and current on either the 48V or 12V board nets. The LTC3871 operates at a user selectable fixed frequency between 60kHz and 475kHz, and can be synchronised to an external clock over the same range. The user can select from continuous operation or pulse skipping during light loads. Additional features include overload and short-circuit protection, independent loop compensation for buck and boost modes, EXTVcc for increased efficiency, ±1 per cent output voltage regulation accuracy over temperature, along with undervoltage and overvoltage lockout. The LTC3871 has been qualified to meet AEC-Q100 specifications and was designed for
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diagnostic coverage in ISO26262 systems. The LTC3871 is available in a thermally enhanced 48-lead LQFP package. Three temperature grades are available, with operation from –40°C to 125°C for the extended and industrial grades and a high temp automotive range of –40°C to 150°C. Figure 1 shows its typical applications schematic. The P-Channel MOSFET shown at the top of the schematic is for over current and short circuit protection.
controls the peak inductor current. The other error amplifier being disabled. In boost mode, ITHHIGH disabled.
is enabled while ITHLOW
VLOW
falls below 85 per cent 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.
In a typical boost controller, the voltage is
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 synchronise 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
) down without a blocking diode or MOSFET to block the current. The LTC3871 uses an external low RDS(ON) P-channel MOSFET for input short-circuit protection when VHIGH
is
shorted to ground. In normal operation, the P-channel MOSFET is always on, with its gate-source voltage clamped to 15V maximum. When the UVHIGH pin voltage goes below its 1.2V threshold, the FAULT pin goes low 125µs later. At this point, the PGATE pin turns off the external P-channel MOSFET.
Conclusion The LTC3871 brings a new level of performance, control and simplification to 48V/12V dual battery DC/DC automotive systems by allowing the same external power components to be use for step-down and step-up purposes. It operates on demand in
synchronous diode or the body diode of the synchronous MOSFET conducts current from the input to the output. As a result, an output (VHIGH input (VLOW
) short will drag the
Figure 4: LTC3871 buck & boost efficiency curves with a 4-phase design
Integrated start generator (ISG) The electronically controlled ISG replaces both the conventional starter and alternator with a single electric
device.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
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 utilising one or two LTC3871 devices. 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 per cent peak efficiencies.
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
buck mode from the 48V bus to the 12V bus or in boost mode from 12V to 48V. Up to 12 phases can be paralleled for high power applications and when starting the car or when additional power is required, the LTC3871 allows both batteries to supply energy simultaneously to the same load. The additional 48V battery running a portion of a vehicle’s electrical system will play a central role in increasing available energy, while reducing wiring harness weight and losses. This additional energy capacity paves the way for new technologies, enabling cars to be safer and more efficient, all while lowering its CO2
emissions.
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Components in Electronics November 2018 9
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