9.2.2.3 Output Capacitor

The output capacitor also should have enough ripple current rating to absorb output switching ripple current. The output capacitor RMS current I_COUT is given as:

Equation 15.

The output capacitor voltage ripple can be calculated in Equation 16:

Equation 16.

At certain input/output voltages and switching frequencies, the voltage ripple can be reduced by increasing the output filter inductor and capacitor values.

The BQ24650 has an internal loop compensator. To achieve good loop stability, the resonant frequency of the output inductor and output capacitor must be designed between 12 kHz and 17 kHz. The preferred ceramic capacitor has a 35 V or higher rating, X7R or X5R.

Ceramic capacitors show a de-bias effect. This effect reduces the effective capacitance when a DC-bias voltage is applied across a ceramic capacitor, as on the output capacitor of a charger. The effect may lead to a significant capacitance drop, especially for high voltages and small capacitor packages. See the manufacturer’s datasheet about performance with a DC bias voltage applied. It may be necessary to choose a higher voltage rating or nominal capacitance value to achieve the required value at the operating point.