Datasheet

PowerLAN Communication Link
Safety
Cell Balancing
bq78PL114
SLUS850B – SEPTEMBER 2008 – REVISED APRIL 2009 ................................................................................................................................................ www.ti.com
measured by up to four additional external temperature sensors. With the bq78PL114S12 firmware upgrade, up
to eight more are available in remote bq76PL102 dual-cell battery monitors. Coulomb counting is captured
continuously by a dedicated 18-bit integrating delta-sigma ADC in the bq78PL114. The CPU in the bq78PL114 is
also responsible for system data calculations, black-box forensic data storage, and communicating parameters
via the SMBus interface. The bq78PL114S12 firmware can be downloaded to existing bq78PL114 parts using
the bqWizard GUI or during the manufacturing checkout process.
PowerLAN technology is Texas Instruments ’ patented serial network and protocol designed specifically for
battery management in a multicell system environment. The PowerLAN link is used to initiate and report
measurements of cell voltage and temperature, and control cell balancing. The bq78PL114 serves as the master
controller of the PowerLAN link and can interface to multiple bq76PL102 dual-cell battery monitors, which
measure and balance additional cells. The bq78PL114 monitors the first three or four cells, and bq76PL102s can
be added to monitor more series cells.
The PowerLAN link isolates voltages from adjacent bq76PL102 devices to permit high-voltage stack assembly
without compromising precision and accuracy. The PowerLAN link is expandable to support up to 12 cells in
series. Each bq76PL102 handles voltage and temperature measurements, as well as balancing for two cells. The
PowerLAN link provides high ESD tolerance and high immunity to noise generated by nearby digital circuitry or
switching currents. Each bq76PL102 has both a PowerLAN input and PowerLAN output: Received data is
buffered and retransmitted, permitting high numbers of nodes without loss of signal fidelity. Signals are
capacitor-coupled between nodes, providing dc isolation.
Unique in the battery-management controller market, the bq78PL114 simultaneously measures voltage and
current using independent and highly accurate delta-sigma ADCs. This technique removes virtually all systemic
noise from measurements, which are made during all modes of battery operation: charge, discharge, and rest.
The bq78PL114 also directs all connected bq76PL102 dual-cell battery monitors to measure each cell voltage
simultaneously with the bq78PL114 measurements. Battery impedance and self-discharge characteristics are
thus measured with an unprecedented level of accuracy in real time. The bq78PL114 applies this precise
information to SmartSafety algorithms to detect certain anomalies and conditions which may be indicative of
internal cell faults, before they become serious problems.
The bq78PL114 uses its enhanced measurement system to detect system faults including cell under- and
overvoltage, cell under- and overtemperature, system overvoltage, and system overcurrent. First-level safety
algorithms first attempt to open the MOSFET safety switches. If this fails, second-level safety algorithms activate
the SPROT output, normally used to open a fuse and provide permanent, hard protection for the systems.
External MOSFET control inputs with programmable polarity can also be used to operate the safety MOSFETs
under control of user supplied circuitry. The bq78PL114 continuously monitors these inputs. If any MOSFET fails
to open when commanded; the 2
nd
level safety algorithms also activate the SPROT output. All first- and
second-level safety algorithms have fully programmable time delays to prevent false triggering.
Patented PowerPump cell balancing technology drastically increases the useful life of battery packs by
eliminating the cycle life fade of multi-cell packs due to cell imbalance. PowerPump technology efficiently
transfers charge from cell to cell, rather than simply bleeding off charging energy as heat as is typically done with
resistive-bleed balancing circuits. Balancing is configurable and may be performed during any battery operational
modes: charge, discharge, or rest. Compared to resistive bleed balancing, virtually no energy is lost as heat. The
actual balance current is externally scalable and can range from 10 mA to 1 A (100 mA typical) depending on
component selection and system or cell requirements.
A variety of techniques, such as simple terminal voltage, terminal voltage corrected for impedance and
temperature effects, or state-of-charge balancing, is easily implemented by the bq78PL114. By tracking the
balancing required by individual cells, overall battery safety is enhanced, often allowing early detection of soft
shorts or other cell failures. Balancing is achieved between all cells within the pack as dynamically determined by
the bq78PL114.
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