Application
4.2
Cell Undervoltage and Overvoltage Comparators
An independent bank of comparators with their own separate undervoltage and overvoltage limits provides
additional cell voltage monitoring and FAULT line control.
4.3
Temperature and Auxiliary Signal Monitoring
The bq76PL455A-Q1 supports eight temperature and auxiliary channels. The bq76PL455EVM implements
a connector for all of these channels, in order to test remote temperature and auxiliary signal monitoring.
Negative thermal coefficient (NTC) thermistors or external signals can be connected to the
bq76PL455EVM’s 8x2 pin right angle temperature sense J2 - Thermistor connector. Separate
undervoltage and overvoltage limits can be set for each of the eight temperature/auxiliary channels. If
these limits are reached, a fault is flagged.
10-k
Ω
pull up resistors are fitted to support NTC use. Choose a thermistor, depending on the application
and need and change the 10-k
Ω
pull-up resistors, as required.
4.4
Cell Balancing
Based on the cell voltage data, the main system controller can determine when individual cells reach their
maximum limit during charging. Without passive balancing, if an individual cell reaches its maximum
voltage, then stack charging should stop to prevent overcharging and damaging that cell. However, that
leaves other cells in the same series string less than 100% charged if all of the cells are not evenly
matched and do not reach maximum charge at the same time. Premature termination of stack charging
due to an individual cell reaching maximum voltage leaves the stack inefficiently charged to less than its
theoretical maximum. With passive balancing, a switchable external shunt resistor across each cell is used
to discharge cells and limit the voltage of individual cells already at maximum voltage to allow further
charging of the battery pack and raising all the cells to their maximum state of charge. The battery stack is
charged to its true maximum potential with passive balancing.
Each of the 16 battery channels on the evaluation module incorporates an external N-channel MOSFET in
series with a shunt 75-
Ω
, 1-W resistor across the battery cell. A secondary load is also incorporated in the
form of an LED which lights when the MOSFET is on and balancing is active. The 16 N-channel
MOSFETs are controlled by 16 EQ lines from the bq76PL455A-Q1 allowing independent balancing of
each battery as determined from the cell voltage measurements.
To balance an individual cell, the EQ line for the N-Channel MOSFET on the selected channel is driven
high. Once it is turned on, current flows through the shunt resistor and LED circuit to dissipate excess
charge. The standard bq76PL455EVM is supplied with 16 × 75-
Ω
, 1-W (at 25C° ambient) shunt resistors
which give a 56-mA balancing current for a cell at 4.2 V. Make the shunt resistor values smaller or larger
to suit the application requirements. Minimum resistor value and maximum balancing current are limited
only by the 1 W (at 25C°) power rating of the individual 2512 size balancing resistors and resulting PCB
temperature when all 16 balancing resistors are on.
4.5
Embedded Control Logic
The bq76PL455A-Q1 incorporates control logic to manage serial communications with the PC or main
system controller to control the monitoring and balancing functions and to respond back to the PC or main
system controller with the requested data.
The bq76PL455A-Q1 embedded control logic uses both EEPROM and RAM to store control register-
based configuration data. EEPROM maintains configuration data while the bq76PL455A-Q1 is in power
down mode. The configuration data is transferred from EEPROM to RAM when the bq76PL455A-Q1
wakes; RAM data are used during normal operation. Modify the RAM data with a main system controller
or PC GUI.
New register settings written into RAM by the system controller or PC GUI can be transferred to EEPROM
on the bq76PL455A-Q1 IC if required to be automatically loaded on the next power cycle. In a typical,
real-world implementation, the EEPROM is not required to be written frequently, therefore, the
bq76PL455A-Q1 EEPROM is designed with limited endurance write cycles unsuited to the evaluation
module environment.
7
SLUUBA7A – April 2015 – Revised July 2015
bq76PL455EVM and GUI User Guide
Copyright © 2015, Texas Instruments Incorporated
