RT8035

7

DS8035-05 October 2012

www.richtek.com

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

©

⎥⎦

⎤

⎢⎣

⎡

+

≤

OUT

L

OUT

8fC

1

ESR

ΔI

ΔV

Applications Information

The basic RT8035 application circuit is shown in Typical

Application Circuit. External component selection is

determined by the maximum load current and begins with

the selection of the inductor value and operating frequency

Inductor Selection

For a given input and output voltage, the inductor value

and operating frequency determine the ripple current. The

ripple current

with higher inductance.

Having a lower ripple current reduces the ESR losses in

the output capacitors and the output voltage ripple. Highest

efficiency operation is achieved at low frequency with small

ripple current. This, however, requires a large inductor.

A reasonable starting point for selecting the ripple current

is

below a specified maximum, the inductor value should be

chosen according to the following equation :

Inductor Core Selection

Once the value for L is known, the type of inductor must

be selected. High efficiency converters generally cannot

afford the core loss found in low cost powdered iron cores,

forcing the use of more expensive ferrite or mollypermalloy

cores. Actual core loss is independent of core size for a

fixed inductor value but it is very dependent on the

inductance selected. As the inductance increases, core

losses decrease. Unfortunately, increased inductance

requires more turns of wire and therefore copper losses

will increase.

Ferrite designs have very low core losses and are preferred

at high switching frequencies, so design goals can

concentrate on copper loss and preventing saturation.

Ferrite core material saturates “hard”, which means that

⎥⎦

⎤

⎢⎣

⎡ −

×

⎥⎦

⎤

⎢⎣

⎡

×

=

IN

OUT

OUT

L

V

V

1

L

f

V

ΔI

⎥

⎦

⎤

⎢

⎣

⎡

−

×

⎥⎦

⎤

⎢⎣

⎡

Δ

×

=

IN(MAX)

OUT

L(MAX)

OUT

V

V

1

I

f

V

L

inductance collapses abruptly when the peak design

current is exceeded. This results in an abrupt increase in

inductor ripple current and consequent output voltage ripple.

Do not allow the core to saturate!

Different core materials and shapes will change the size/

current and price/current relationship of an inductor.

Toroid or shielded pot cores in ferrite or permalloy materials

are small and don't radiate energy but generally cost more

than powdered iron core inductors with similar

characteristics. The choice of which style inductor to use

mainly depends on the price vs size requirements and

any radiated field/EMI requirements.

trapezoidal current at the source of the top MOSFET. To

prevent large ripple voltage, a low ESR input capacitor

sized for the maximum RMS current should be used. RMS

current is given by :

1

V

V

V

V

I

I

OUT

IN

IN

OUT

OUT(MAX)

RMS

−

=

commonly used for design because even significant

deviations do not offer much relief. Note that ripple current

ratings from capacitor manufacturers are often based on

only 2000 hours of life which makes it advisable to further

derate the capacitor, or choose a capacitor rated at a higher

temperature than required. Several capacitors may also

be paralleled to meet size or height requirements in the

design.

resistance (ESR) that is required to minimize voltage ripple

and load step transients, as well as the amount of bulk

capacitance that is necessary to ensure that the control

loop is stable. Loop stability can be checked by viewing

the load transient response as described in a later section.

The output ripple,