AP3441/L

Document number: DS39754 Rev. 3 - 2

10 of 13

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AP3441/L

Application Information (continued)

Input Capacitor

The input capacitor reduces the surge current drawn from the input supply and the switching noise from the device. The input capacitor has to

sustain the ripple current produced during the on time on the upper MOSFET. It must hence have a low ESR to minimize the losses.

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

input capacitor which has RMs rating that is greater than half of the maximum load current.

Due to large di/dt through the input capacitors, electrolytic or ceramics should be used. If a tantalum must be used, it must be surge protected.

Otherwise, capacitor failure could occur. For most applications, a 22µF ceramic capacitor is sufficient.

Output Capacitor

The output capacitor keeps the output voltage ripple small, ensures feedback loop stability and reduces the overshoot of the output voltage. The

output capacitor is a basic component for the fast response of the power supply. In fact, during load transient, for the first few microseconds it

supplies the current to the load. The converter recognizes the load transient and sets the duty cycle to maximum, but the current slope is limited

by the inductor value.

ESR of the output capacitor dominates the output voltage ripple. The amount of ripple can be calculated from the equation below:

ESR

*

ΔI

V

INDUCTOR

CAPACITOR

OUT



An output capacitor with ample capacitance and low ESR is the best option. For most applications, 2x22µF ceramic capacitor will be sufficient.

2

OUT

2

OUT

2

INDUCTOR

OUT

O

V

)

V

V

(

Δ

)

2

ΔI

L(I

C









Where

ΔV is the maximum output voltage overshoot.

Inductor Selection

The inductor is used to supply smooth current to output when it is driven by a switching voltage. Its value is determined based on the operating

frequency, load current, ripple current, and duty cycle. For most application, the value of the inductor will fall in the range of 1.0

mH to 2.2mH.

Choose an inductor that has small DC resistance, has enough current rating and is hard to cause magnetic saturation.

Vo

L(

mH)

1.0V

1.0µH

1.2V

1.0µH

1.5V

1.5 to 2.2µH

1.8V

1.5 to 2.2µH

2.5V

2.2µH

2.8V

2.2µH

3.3V

2.2µH

Table 2. Recommended Inductor Selection Table