APW7313 datasheet(14/20 Pages) ANPEC

14 / 20 page

Rev. A.6 - Apr., 2013

APW7313

www.anpec.com.tw

Application Information(Cont.)

Output Capacitor Selection (Cont.)

The load transient requirements are the function of the

slew rate (di/dt) and the magnitude of the transient load

urrent. These requirements are generally met with a

mix of capacitors and careful layout. High frequency ca-

pacitors initially supply the transient and slow the current

load rate seen by the bulk capacitors. The bulk filter ca-

pacitor values are generally determined by the ESR

(Effective Series Resistance) and voltage rating require-

ments rather than actual capacitance requirements.

High frequency decoupling capacitors should be placed

as close to the power pins of the load as physically

possible. Be careful not to add inductance in the circuit

board wiring that could cancel the usefulness of these

low inductance components. An aluminum electrolytic

capacitor’s ESR value is related to the case size with lower

ESR available in larger case sizes. However, the Equiva-

lent Series Inductance (ESL) of these capacitors increases

with case size and can reduce the usefulness of the ca-

pacitor to high slew-rate transient loading.

Table1 Capacitor Selection Guide

Vender

Model

Capacitance

(

µF)

Voltage

Rating(V)

Si2e

muRata

GRM31CR61E106K

X5R

1206

muRata

GRM31CR61C226K

X5R

1206

The operating frequency and inductor selection are inter-

related in that higher operating frequencies permit the

use of a smaller inductor for the same amount of inductor

ripple current. However, this is at the expense of efficiency

due to an increase in MOSFET gate charge losses. The

equation (2) shows that the inductance value has a direct

effect on ripple current.

Accepting larger values of ripple current allows the use of

low inductances, but results in higher output voltage ripple

and greater core losses. A reasonable starting point for

setting ripple current is

∆I< 0.4 x I

OUT(max). Please be no-

ticed that the maximum ripple current occurs at the maxi-

mum input voltage. The minimum inductance of the in-

uctor is calculated by using the following equation:

Inductor Value Calculation

where

........... (6)

IN(MAX)

1.2

340000

)

OUT

≤

(H)

408000

)

OUT

≥

Table2 Inductor Selection Guide

Vender

Model

Inductance

(

µH)

DCR

Ω)

Current

Rating(A)

CYNTEC

PCMB063T-100MS

Gausstek

PL94P051M-15U

Gausstek

PL94P051M-10U

3.8

)

(

ESR

VOUT

⋅

∆

OUT

OSC

COUT

∆

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:

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.

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

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

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