The AFE serves an important role for the gas gauge two-, three-, or four-cell lithium-ion battery pack gas gauge chipset solution. The AFE provides all the high voltage interface needs and hardware current protection features. It offers an I2C compatible interface to allow the gas gauge to have access to the AFE registers and to configure the AFE's protection features. The AFE also integrates cell balancing control. In many situations, the state-of-charge (SOC) of the individual cells may differ from each other in a multi-cell battery pack, causing voltage difference between cells and cell-imbalance. The AFE incorporates a bypass path for each cell. These bypass paths can be utilized to reduce the charging current into any cell and, thus, allow for an opportunity to balance SOC of the cells during charging. Since the Impedance Track gas gauges can determine the chemical SOC of each cell, a right decision can be made when cell balancing is needed.
Multiple over current protection thresholds with different activated times as shown in Figure 2 make the battery pack protection more robust. The gas gauge has two tiers of charge/discharge over-current protection settings, and the AFE provides a third level of discharge over-current protection. In case of short-circuit conditions when the MOSFETs and the battery can be damaged within seconds, the gas gauge chipset entirely depends on the AFE to autonomously shut off the MOSFETs before such damage occurs.
Figure 2. Multi-level Battery Over Current Protection
While the gas gauge IC and its associated AFE provide over-voltage protection, the sampled nature of the voltage monitoring limits the response time of this protection system. Most applications will require a fast-response, real-time, independent over-voltage monitor that operates in conjunction with the gas gauge and the AFE. It monitors individual cell voltages independent of the gas gauge and AFE, and provides a logic level output which toggles if any of the cells reaches a hard-coded over-voltage limit. The response time of the over voltage protection is determined by the value of an external delay capacitor. In a typical application, the output of the second level protector would trigger a chemical fuse or other fail-safe protection device to permanently disconnect the Li-Ion cell from the system.