APW7077 datasheet(13/21 Pages) ANPEC

13 / 21 page

Rev. A.4 - Sep, 2005

APW7077/A

www.anpec.com.tw

Function Description (Cont.)

OUT

−

In higher output voltage or small output current

application, the step–up DC–DC controller operated in

discontinuous conduction mode almost. For a step-up

converter in a DCM, the duty cycle D is given by

External components values can be calculated from

these equations, however, the optimized value should

obtained through experimental results.

Critical Inductance Value

The minimum value of inductor to maintain continuous

conduction mode can be determined by the following

equation.

Ratio

fsw

D(1

OUT

−

≥

A system can be designed to operate in continuous

mode for load currents above a certain level usually

20 to 50% (Ratio define as 0.2~0.5) of full load at

minimum input voltage. When I

O smaller than (IO*Ratio),

the controller system will into DCM.

∆I

L is the ripple current flowing through the inductor,

which affects the output voltage ripple and core losses.

Based on 20%(Ratio=0.2) current ripple, V

OUT=5V,

O=1A and VIN =1.8V system, the inductance value is

calculated as 6.9uH and a 6.8uH inductor is used.

The inductor current ripple has an expression

fsw

∆

The maximum DC input current can be calculated as

(min)

(max)

OUT

The inductor peak current can be calculated as

Ipk

OUT

∆

NOTES:

D – On–time duty cycle

IL – Average inductor current

IPK – Peak inductor current

IO – Desired dc output current

VIN – Nominal operating dc input voltage

VOUT – Desired dc output voltage

ESR – Equivalent series resistance of the output

capacitor

Inductor Selection

APW7077/A series are designed to work well with a

6.8 to 12uH inductors in most applications 10uH is a

sufficiently low v alue to allow the use of a small

surface mount coil, but large enough to maintain low

ripple. Lower inductance values supply higher output

current, but also increase the ripple and reduce

efficiency. Higher inductor values reduce ripple and

improve efficiency, but also limit output current. The

inductor should have small DCR, usually less than

Ω, to minimize loss. It is necessary to choose an

inductor with a saturation current greater than the peak

current which the inductor will encounter in the

application.

The inductor ripple current is important for a few

reasons. One reason is because the peak switch

current will be the average inductor current (I

L) plus

∆IL.

As a side note, discontinuous operation occurs when

the inductor current falls to zero during a switching

cycle, or

∆IL is greater than the average inductor

current. Therefore, continuous conduction mode occurs









−

⋅

OUT

LOAD

given by

Step–up Converter Operating Mode (Cont.)

APW7077 Datasheet(PDF) 13 Page - Anpec Electronics Coropration

APW7077 Datasheet(HTML) 13 Page - Anpec Electronics Coropration

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

Datasheet Download

Link URL