2 Implementation

The bq27426 BAT pin is designed not only for 2.45V to 4.50V directly voltage measurement with internal voltage divider enabled, but also it's the internal LDO regulator input pin. It's required to connect an 1-μF capacitor to achieve 1-mV accuracy of voltage-measurement on the bq27426 BAT pin. This voltage measurement accuracy is required by the Impedance Track™ algorithm to have less than 1% state-of-charge error.

If an external resistive voltage divider is directly connected to the BAT pin, the external voltage divider network interferes with the internal voltage divider network and causes the voltage divider ratio to change so that the voltage measurement is not accurate. To prevent the interference, a unity-gain operational amplifier is used to buffer the output of the resistive voltage divider and provide a BAT voltage that is equal to the external voltage divider output. The micro-power operational amplifier generally has much higher input impedance than the internal voltage divider and reduces the voltage measurement error due to loading of the external voltage divider output.

The tolerance of the external voltage divider resistors has a direct impact on the voltage measurement error. A resistor with 0.1% tolerance is required. Considering the temperature range of the application, a temperature coefficient of 50 ppm/°C, or less, is also needed. The resistors must be of the same kind and from the same manufacturer.

The bq27426 gas gauge has typical current consumption of 50 μA under normal operation. The peak current at start up can go up to 8 mA for a short period of time. To ensure the bq27426 starts up properly, the 1-μF capacitor on the BAT pin is necessary, and so the operational amplifier shall have the capability to drive a high-capacitance load. With consideration of size, cost, and power consumption, TI's TS321IDBV is selected for this application.