AP2953AS8PR datasheet(5/9 Pages) CHIPOWN

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AP2953A

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V2.0

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Application Information

Component Selection

Setting the Output Voltage

The output voltage is set using a resistive voltage divider

from the output voltage to FB (see Typical Application

circuit on page 1). The voltage divider divides the output

voltage down by the ratio:

Where VFB is the feedback voltage and VOUT is the

output voltage.

Thus the output voltage is:

R2 can be as high as 100kΩ, but a typical value is 10kΩ.

Using the typical value for R2, R1 is determined by:

For example, for a 3.3V output voltage, R2 is 10kΩ, and R1

is 26.1kΩ. Table 1 lists recommended resistance values of R1

and R2 for standard output voltages.

Inductor

The inductor is required to supply constant current to the

output load while being driven by the switched input voltage.

A larger value inductor will result in less ripple current that

will result in lower output ripple voltage. However, the larger

value inductor will have a larger physical size, higher series

resistance, and/or lower saturation current. A good rule for

determining the inductance to use is to allow the peak-to-peak

ripple current in the inductor to be approximately 30% of the

maximum switch current limit. Also, make sure that the peak

inductor current is below the maximum switch current limit.

The inductance value can be calculated by:

Where VOUT is the output voltage, VIN is the input

voltage, fS is the switching frequency, and ΔIL is the peak-

to-peak inductor ripple current.

Choose an inductor that will not saturate under the maximum

inductor peak current. The peak inductor current can be

calculated by:

Where ILOAD is the load current.

The choice of which style inductor to use mainly depends on

the price vs. size requirements and any EMI requirements.

Optional Schottky Diode

During the transition between high-side switch and low-side

switch, the body diode of the lowside power MOSFET

conducts the inductor current. The forward voltage of this

body diode is high. An optional Schottky diode may be

paralleled between the SW pin and GND pin to improve

overall efficiency. Table 2 lists example Schottky diodes and

their Manufacturers.

Input Capacitor

The input current to the step-down converter is discontinuous,

therefore a capacitor is required to supply the AC current to the

step-down converter while maintaining the DC input voltage.

Use low ESR capacitors for the best performance. Ceramic

capacitors are preferred, but tantalum or low-ESR electrolytic

capacitors may also suffice. Choose X5R or X7R dielectrics

when using ceramic capacitors. Since the input capacitor

absorbs the input switching current it requires an adequate

ripple current rating. The RMS current in the input capacitor

can be estimated by:

The worst-case condition occurs at VIN = 2VOUT, where

ICIN = ILOAD/2. For simplification, choose the input

capacitor whose RMS current rating greater than half of the

maximum load current. The input capacitor can be electrolytic,

tantalum or ceramic. When using electrolytic or tantalum

capacitors, a small, high quality ceramic capacitor, i.e. 0.1μF,

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