SLUSAN9A August 2011 – August 2015
Information in the following applications sections is not part of the TI component specification, and TI does not warrant its accuracy or completeness. TI’s customers are responsible for determining suitability of components for their purposes. Customers should validate and test their design implementation to confirm system functionality.
The bq20z655-R1 is a gas gauge with primary protection support, and that can be used with a 2-series to 4-series Li-Ion/Li Polymer battery pack. To implement and design a comprehensive set of parameters for a specific battery pack, users need the BQEV graphical user-interface tool installed on a PC during development. The firmware installed on the BQEV tool has default values for this product, which are summarized in the bq20z655 Technical Reference Manual (SLUU493). Using the tool, BQEV these default values can be changed to cater to specific application requirements during development once the system parameters, such as fault trigger thresholds for protection, enable/disable of certain features for operation, configuration of cells, chemistry that best matches the cell used, and more are known. This data is referred to as the golden image.
Table 3 shows the default settings for the main parameters. Use the BQEV tool to update the settings to meet the specific application or battery pack configuration requirements.
|Cell configuration||4s1p (4 series with 1 parallel)|
|Design capacity||4400 mAH|
|Device chemistry||0100 (LION)|
|Cell overvoltage at standard temperature||4300 mV|
|Cell undervoltage||2200 mV|
|Cell Shutdown voltage||1750 mV|
|Overcurrent in CHARGE mode||6000 mA|
|Overcurrent in DISCHARGE mode||–6000 mA|
|Short circuit in CHARGE mode||0.1 V/Rsense across SRP, SRN|
|Short circuit in DISCHARGE mode||0.1 V/Rsense across SRP, SRN|
|Safety overvoltage||4500 mV|
|Internal and external temperature sensor||External temperature sensor is used|
|Battery Trip Point (BTP) with active high interrupt||Disabled|
For the Impedance Track™ algorithm to work properly, the exact chemistry of the lithium cells needs to be known and the correct .SENC file needs to be loaded.
If you are using the bqEASY design wizard, it asks you to choose the correct chemistry from a list of manufacturers and model numbers, or test for a compatible chemistry using a 4-point test.
Success of the 4-point test is contingent on an accurate voltage calibration.
The process for updating the .SENC file is outlined in detail in the application report Updating Firmware With The bq20zxx and EVM.
The high-current path begins at the PACK+ terminal of the battery pack. As charge current travels through the pack, it finds its way through protection FETs, a chemical fuse, the lithium-ion cells and cell connections, and the sense resistor, and then returns to the PACK– terminal. In addition, some components are placed across the PACK+ and PACK– terminals to reduce effects from electrostatic discharge.
Select the N-channel charge and discharge FETs for a given application. Most portable battery applications are a good match for the CSD17308Q3. The TI CSD17308Q3 is a 47-A, 30-V device with Rds(on) of 8.2 mΩ when the gate drive voltage is 8 V.
If a precharge FET is used, R1 is calculated to limit the precharge current to the desired rate. Be sure to account for the power dissipation of the series resistor. The precharge current is limited to (VCHARGER – VBAT)/R1 and maximum power dissipation is (Vcharger – Vbat)2/R1.
The gates of all protection FETs are pulled to the source with a high-value resistor between the gate and source to ensure they are turned off if the gate drive is open.
Capacitors C1 and C2 help protect the FETs during an ESD event. Using two devices ensures normal operation if one becomes shorted. To have good ESD protection, the copper trace inductance of the capacitor leads must be designed to be as short and wide as possible. Ensure that the voltage rating of both C1 and C2 are adequate to hold off the applied voltage if one of the capacitors becomes shorted.
The important part to remember about the cell connections is that high current flows through the top and bottom connections; therefore, the voltage sense leads at these points must be made with a Kelvin connection to avoid any errors due to a drop in the high-current copper trace. The location marked 4P in indicates the Kelvin connection of the most positive battery node.
As with the cell connections, the quality of the Kelvin connections at the sense resistor is critical. The sense resistor must have a temperature coefficient no greater than 50 ppm to minimize current measurement drift with temperature. Choose the value of the sense resistor to correspond to the available overcurrent and short circuit ranges of the bq20z655. Select the smallest value possible to minimize the negative voltage generated on the VSS nodes during a short circuit.
A pair of series 0.1-μF ceramic capacitors is placed across the PACK+ and PACK– terminals to help in the mitigation of external electrostatic discharges. The two devices in series ensure continued operation of the pack if one of the capacitors becomes shorted. Optionally, a tranzorb such as the SMBJ2A can be placed across the terminals to further improve ESD immunity.
The System Present signal is used to inform the gas gauge whether the pack is installed into or removed from the system. In the host system, this pin is grounded. The PRES pin of the bq20z655 is occasionally sampled to test for system present. To save power, an internal pullup is provided by the gas gauge during a brief 4-μs sampling pulse once per second. A resistor can be used to pull the signal low and the resistance must be 20 kΩ or lower to insure that the test pulse is lower than the VIL limit. The pullup current source is typically 10 μA to 20 μA.
Because the System Present signal is part of the pack connector interface to the outside world, it must be protected from external electrostatic discharge events. An integrated ESD protection on the PRES device pin reduces the external protection requirement to just R29 for an 8-kV ESD contact rating. However, if it is possible that the System Present signal may short to PACK+, then a resistor, diode combo must be included for high-voltage protection.
The SMBus clock and data pins have integrated high-voltage ESD protection circuits, however, adding a Zener diode and series resistor provides more robust ESD performance.