For this design a circuit was added to limit the system current to 8 A. This is most impactful when the device is being used with a 12-V car adapter and can pull the battery voltage below the input threshold of the system. This circuit includes the INA213B to amplify the current across a 2-mΩ sense resistor. This INA213B has a gain of 50 V/V so an input of 8 A results in an 800-mV output.

Figure 2-4 Current-Limit Circuit

To convert this signal into a usable voltage for the ILIM pin of the BQ25731 devices, an op amp configured as a difference amplifier was used. The voltage on the BQ25731 ILIM pin is converted to charge current based on Equation 5.

where

• IDPM is the target input current limit
• Rac is the 5- or 10-mΩ resistor chosen for the BQ25731

For the difference amplifier when:

where

• $R_1=R_2=R_f=R_g, V_{out}=V_2-V_1$

To correctly set the BQ25731 charge current, set the ILIM pin at 1.0 V with 8 A of system current and 1.8 V with 0-A system current. Limit the charge current for each of the 2 charger devices to between 0 A and 4 A.

Set V2 at 1.8 V in this case. With V2 at 1.8 V, the voltage at the amplifiers positive input is equal to V2/2, which is 0.9 V. This voltage then needs to be created with the system 3.3-V rail and a resistor divider to match the calculated set point. This was implemented with a R7 and R9 as shown in the schematic above.