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COVER STORY ADVERTISEMENT FEAT


found in electric and hybrid vehicles. The LT8708 operates between two batteries


one of the batteries fail. This device can and prevents systemshutdown should


also be used in 48V/12V and 48V/24V


The LT8708 uses a single inductor and dual battery systems.


from2.8V to 80V while producing an operates over an input voltage range


delivering up to several kilowatts of output voltage from1.3V to 80V,


phases. It simplifies bidirectional power external components and number of power depending on the choice of


conversion in battery/capacitor backup systems that need regulation of VOUT, VIN,


and/or IOUT, IIN, both in the forward or


independent forms of regulation allow it reverse direction. This device’s six


The LT8708-1 is used in parallel with to be used in numerous applications.


The LT8708-1 always operates as a slave the LT8708 to add power and phases.


to the master LT8708, can be clocked out-of-phase, and has the capability to deliver as much power as the master. Up to 12 slaves can be connected to a single


power and current capabilities of the master, proportionally increasing the


monitored and limited for the input and Forward and reverse current can be system.


output sides of the converter. All four current limits (forward input, reverse


output) can be set independently using input, forward output, and reverse


four resistors. In combination with the


configured to process power fromV direction (DIR) pin, the chip can be


V OUT or fromVOUT to V IN to


automotive, solar, telecom, and battery- IN, ideal for


The LT8708 is available in a 5mm× powered systems.


operation from–40°C to 125°C for the temperature grades are available, with 8mm, 40-lead QFN package. Three


high temperature automotive range of extended and industrial grades and a


-40°C to 150°C. Figure 1 shows a simplified LT8708 block diagram.


COMPLETE SOLUTION The block diagramin Figure 2 shows the


circuit for dual battery redundancy in an other parts required to complete the


LT8708 works with two LT8708-1 parts automotive application. As shown, the


can deliver up to 60A in either direction. to forma 3-phase solution design that


to and exceeding 12 phases. The AD8417 added for higher power applications up Additional LT8708-1 devices can be


that senses the current flowing into and is a bidirectional current sense amplifier


LTC7001 high-side NMOS static switch current exceeds a preset value, the out of the batteries. And when this


Amplifier Name


EA1 EA2 EA3 EA4 EA5 EA6


/ ELECTRONICS


Pin Name IMON_INN


IMON_ON FBIN


FBOUT


IMON_INP IMON_OP


Definition Negative IIN


Negative IOUT V


V


INVoltage OUTVotalge Positive IIN Positive IOUT


Figure 1:


An LT8708 simplified bidirectional dual 12V battery application schematic


ATURE


combined output of six internal error amplifiers, EA1 to EA6. These amplifiers can be used to limit o respective voltages or in Table 1.


voltage commands the most positive inductor current and, thus, commands


the most power flow fromVIN to VOUT. The minimumVC voltage commands the most


negative inductor current and, thus, commands the most power flow from V


OUT to V


In a simple example IN.


of VOUT regulation,


the FBOUT pin receives the VOUT voltage feedback signal, which is compared to


the internal reference voltage using EA4. Low VOUT voltage raises VC and, thus,


more current flows into V higher VOUT reduces VC


, thus, reducing the OUT. Conversely,


current into VOUT or even drawing current and power fromVOUT.


As previously mentioned, the LT8708 also provides bidirecti regulation capabilities


driver opens the back-to-back MOSFETs to isolate either battery fromthe circuit. The LTC6810-2 the Li-Ion battery


the battery cells with a total


measurement error of less than 1.8mV. Connecting multiple LTC6810-2 devices in parallel to the host processor will create additional redundancy for monitoring other voltages within the circuit. The LTC6810-2 has an isoSPI interface for high speed, RF immune, long distance communications, and it supports bidirectional operation. The device also includes passive balancing with PWM duty cycle control for each cell and the ability to performredundant cell measurements.


CONTROL OVERVIEW


The LT8708 provides an output voltage that can be above, below, or equal to the input voltage. It also provides


bidirectional current monitoring and regulation capabilities at both the input and the output. The ADI proprietary control architecture employs an inductor current-sensing resistor in buck, boost, or buck-boost regions of operation. The inductor current is controlled by the voltage on the VC pin, which is the


Figure 2:


. It accurately measures monitors and controls


diagram for a complete redundancy block A dual battery


solution reverse directions (EA6 and EA2,


respectively). The VIN current can also be regulated or limited in the forward


direction and reverse directions (EA5 and EA1, respectively).


In a common application, VOUT might be regulated using EA4, while the remaining


error amplifiers are m excessive input or out


put current, or an onitoring for


input undervoltage condition. In other applications, such as a battery backup


system, a battery connected to VOUT might be charged with constant current


(EA6) to a maximumvoltage (EA4) and can also be reversed, at times, to supply power back to VIN using the other error amplifiers to regulate VIN and limit the maximumcurrent. See the LT8708 data sheet for additional information on this subj


bject. Table 1: Ta


Error amplifiers (EA1 to EA6)


The LT8708-1 brings a new level of performance, control, and simplification to same voltage dual-battery dc-to-dc automotive systems. Whether using it for energy transfer between two power sources for redundancy, or for backup power in mission critical applications, the LT8708 allows users the ability to operate fromtwo batteries or


supercapacitors that have the same


automotive systems e voltage. This capabilit


be safer and more effi automotive electronic pave the way for new


Analog Devices Ltd. www.analog.com


e: uksales@linear.com ELECTRONICS | MARCH 2019 11


s, enabling cars to advancements in ngineers to help y allows


cient.


at both the input onal current


and the output. The VOUT current can be regulated or limited in the forward and


currents as shown r regulate their


The VC voltage typically has a min-max range of about 1.2V. The maximumVC


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