The BQ2461x has 600-kHz switching frequency to allow the use of small inductor and capacitor values. Inductor saturation current should be higher than the charging current (ICHG) plus half the ripple current (IRIPPLE):
The inductor ripple current depends on input voltage (VIN), duty cycle (D = VOUT/VIN), switching frequency (fs) and inductance (L):
The maximum inductor ripple current happens with D = 0.5 or close to 0.5. For example, the battery charging voltage range is from 9 V to 12.6 V for a 3-cell battery pack. For 20-V adapter voltage, 10-V battery voltage gives the maximum inductor ripple current. Another example is a 4-cell battery, the battery voltage range is from 12 V to 16.8 V, and 12-V battery voltage gives the maximum inductor ripple current.
Usually inductor ripple is designed in the range of (20%–40%) maximum charging current as a trade-off between inductor size and efficiency for a practical design.
The BQ2461x has cycle-by-cycle charge undercurrent protection (UCP) by monitoring the charging-current sensing resistor to prevent negative inductor current. The typical UCP threshold is 5-mV falling edge corresponding to 0.5-A falling edge for a 10-mΩ charging-current sensing resistor.