APW7143 datasheet(13/19 Pages) ANPEC

13 / 19 page

Rev. A.1 - Apr., 2008

APW7143

www.anpec.com.tw

Application Information

(V)

)

0.8

VOUT

⋅

Suggested R2 is in the range from 1K to 20k

Ω. For

portable applications, a 10k

Ω resistor is suggested for

R2. To prevent stray pickup, locate resistors R1 and R2

close to APW7143.

Input Capacitor Selection

Each time, when the P-channel power MOSFET (Q1) turns

on, small ceramic capacitors for high frequency decoupling

and bulk capacitors are required to supply the surge current.

The small ceramic capacitors have to be placed physi-

cally close to the VIN and between the VIN and the anode

of the Schottky diode (D1).

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 (I

RMS)

of the bulk input capacitor is calculated as the following

equation:

(A)

OUT

RMS

⋅

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

tantalum capacitors can be used, but caution must be

exercised with regard to the capacitor surge current

rating.

OUT

ESR

OUT

COUT

VIN

Figure 1 Converter Waveforms

OUT

T=1/F

OSC

COUT

OUT

Output Capacitor Selection

An output capacitor is required to filter the output and

supply the load transient current. The filtering requirements

are the functions of the switching frequency and the ripple

current (

∆I). The output ripple is the sum of the voltages,

having phase shift, across the ESR and the ideal out-

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

calculated as the following equations:

OUT

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

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

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

OSC

OUT

∆

(V)

ESR

∆

where V

D is the forward voltage drop of the diode.

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

calculated as the following equation:

Setting Output Voltage

The regulated output voltage is determined by:

(V)

OUT

OSC

COUT

⋅

∆

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

For the applications, using bulk capacitors, the

∆V

COUT

is much smaller than the V

ESR an d c an b e i g n o r e d .

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

∆V

OUT ) is

shown below:

(V)

ESR

VOUT

⋅

∆

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

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