APW7337 datasheet(13/20 Pages) ANPEC

13 / 20 page

Rev. A.2 - Jul., 2013

APW7337

www.anpec.com.tw

Application Information

Setting Output Voltage

Inductor Capacitor Selection

where D is the duty cycle of the power MOSFET.

For a through hole design, several electrolytic capacitors

may be needed. For surface mount designs, solid tanta-

lum capacitors can be used, but caution must be exer-

cised with regard to the capacitor surge current rating.

)

(

)

(

OUT

RMS

⋅

−

)

(

ESR

VOUT

⋅

∆

OUT

OSC

COUT

∆

The regulated output voltage is determined by:

Use small ceramic capacitors for high frequency

decoupling and bulk capacitors to supply the surge cur-

rent needed each time the N-channel power MOSFET

(Q1) turns on. Place the small ceramic capacitors physi-

cally close to the VIN and between the VIN and GND.

The important parameters for the bulk input capacitor are

the voltage rating and the RMS current rating. For reliable

operation, select the bulk capacitor with voltage and

current ratings above the maximum input voltage and

largest RMS current required by the circuit. The capacitor

voltage rating should be at least 1.25 times greater than

the maximum input voltage and a voltage rating of 1.5

times is a conservative guideline. The RMS current (IRMS)

of the bulk input capacitor is calculated as the following

equation:

An output capacitor is required to filter the output and sup-

ply the load transient current. The filtering requirements

are the function of the switching frequency and the ripple

current (DI). The output ripple is the sum of the voltages,

having phase shift, across the ESR and the ideal output

capacitor. The peak-to-peak voltage of the ESR is calcu-

ated as the following equations:

)

(

)

(

VOUT

1 ⋅

To prevent stray pickup, please locate resistors R1 and

R2 close to APW7337.

ESR

)

(

ESR

OSC

OUT

∆

−

∆

The peak- to-peak voltage of the ideal output capacitor is

calculated as the following equations:

For the applications using bulk capacitors, the

∆V

COUT is

much smaller than the V

ESR and can be ignored. Therefore,

the AC peak-to-peak output voltage(

∆V

OUT) is shown below:

Output Capacitor Selection

For the applications using bulk capacitors, the V

ESR is

much smaller than the

∆V

COUT an d c an b e i g no r e d .

Therefore, the AC peak-to-peak output voltage(

∆V

OUT) is to

∆V

COUT.

Figure 1. Converter Waveforms

OUT

T=1/F

OSC

COUT

OUT

........... (1)

........... (2)

........... (3)

........... (4)

........... (5)

OUT

ESR

OUT

COUT

VIN

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