AP61300/AP61302

Document number: DS43404 Rev. 3 - 2

17 of 21

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AP61300/AP61302

Application Information (continued)

11

Input Capacitor

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

must sustain the ripple current produced during the on-time of Q1. It must have a low ESR to minimize power dissipation due to the RMS input

current.

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

input capacitor with a RMS current rating greater than half of the maximum load current.

Due to large dI/dt through the input 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. Using a ceramic capacitor of 10µF or greater is sufficient for most applications.

12

Output Capacitor

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 sets the off-time to minimum to supply more current to the load. However, the inductor limits the change to 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 increases the off-time 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 effective output capacitance, COUT, requirements can be calculated from the equations below.

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

������������������������������������������������������������ = ∆������������ ∙ (������������������ +

������

������ ∙ ������������������ ∙ ������������������������

)

Eq. 8

Output capacitors with large capacitance and low ESR are the best option. For most applications, a total capacitance of 22µF using ceramic

capacitors is sufficient. To meet the load transient requirements, the calculated COUT should satisfy the following inequality:

������������������������ > ������������������ (

������ ∙ ������������������������������������

������

∆������������������������������������������������������������ ∙ ������������������������

,

������ ∙ ������������������������������������

������

∆������������������������������������������������������������������ ∙ (������������������ − ������������������������)

)

Eq. 9

Where:



ITrans is the load transient



∆VOvershoot is the maximum output overshoot voltage



∆VUndershoot is the maximum output undershoot voltage