AP72250

Document number: DS44135 Rev. 1 - 2

22 of 26

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AP72250

Application Information (continued)

13

Input Capacitor

The input capacitor reduces both the surge current drawn from the input supply as well as the switching noise from the device.

While a 10μF input

capacitor is sufficient for most applications, larger values may be used to reduce input ripple without limitations. It is recommended to use at least

X5R or X7R ceramic input capacitors.

14

Output Capacitor

The output capacitor must sustain the ripple current produced during the off-time of the switching period. It must have a low ESR to minimize power

dissipation due to the RMS output current.

The RMS current rating of the output capacitor is a critical parameter and must be higher than the RMS output current. As a rule of thumb, select an

output capacitor with a RMS current rating greater than 2.5A.

Due to large dI/dt through the output capacitor, electrolytic, or ceramic capacitors with low ESR should be used. If using a tantalum capacitor, it must

be surge protected or else capacitor failure could occur. For most applications, a total capacitance of 2 x 22µF using ceramic capacitors is sufficient.

The output capacitor keeps the output voltage ripple small, ensures feedback loop stability, and reduces both the overshoots and undershoots of

the output voltage during load transients. During the first few microseconds of an increasing load transient, the converter recognizes the change

from steady-state and enters maximum duty cycle to supply more current to the load. However, the inductor limits the change in increasing current

depending on its inductance. Therefore, the output capacitor supplies the difference in current to the load during this time. Likewise, during the first

few microseconds of a decreasing load transient, the converter recognizes the change from steady-state and sets the on-time to minimum to reduce

the current supplied to the load. However, the inductor limits the change in decreasing current as well. Therefore, the output capacitor absorbs the

excess current from the inductor during this time.

The ESR of the output capacitor dominates the output voltage ripple. The amount of output voltage ripple can be calculated by:

������������������������������������������������������������ = ������������

������������������������ ∙ (������������������)

Eq. 7

Where:



ESR is the equivalent series resistance of the output capacitor

An output capacitor with large capacitance and low ESR is the best option. For most applications, using 2 x 22µF to 5 x 22µF, X5R or X7R

ceramic capacitors are sufficient.