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RT9278

High Efficiency Boost LDO Converter & High Power White LED Driver

Richtek Technology 

RT9278

Preliminary

Application Information

RT9278 integrates a high-efficiency synchronous rectifier

step-up DC-DC converter and a linear regulator controller.

To fully utilize its advantages, peripheral components

should be appropriately selected. The following information

provides basic considerations for component selection.

Inductor Selection

For a better efficiency in high switching frequency

converter, the inductor selection has to use a proper core

material such as ferrite core to reduce the core loss and

choose low ESR wire to reduce copper loss. The most

important point is to prevent the core saturated when

handling the maximum peak current. Using a shielded

inductor can minimize radiated noise in sensitive

applications. The maximum peak inductor current is the

maximum input current plus the half of inductor ripple

current. The calculated peak current has to be smaller

than the current limitation in the electrical characteristics.

A typical setting of the inductor ripple current is 20% to

40% of the maximum input current. If the selection is

40%

IPK

= IIN(MAX)

+

1

2

IRIPPLE

=

1.2

×

I

IN(MAX)

=

1.2

×







(IOUT(MAX) × VOUT

η × VIN(MIN)







The minimum inductance value is derived from the following

equation:

L = η × IIN(MIN)2 × [VOUT - VIN(MIN)]

0.4 × IOUT(MAX) × VOUT2 × fOSC

Depending on the application, the recommended inductor

value is between 2.2µH and 10µH.

Input Capacitor Selection

For better input bypassing, low-ESR ceramic capacitors

are recommended for performance. A 10µF input capacitor

is sufficient for most applications. For a lower output power

requirement application, this value can be decreased.

www.richtek.com

12

Output Capacitor Selection

For lower output voltage ripple, low-ESR ceramic capacitors

are recommended. The tantalum capacitors can be used

as well, but the ESR is bigger than ceramic capacitor. The

output voltage ripple consists of two components: one is

the pulsating output ripple current flows through the ESR,

and the other is the capacitive ripple caused by charging

and discharging.

VRIPPLE = VRIPPLE(ESR) + VRIPPLE(C)

≅

IPEAK

× ESRCOUT

+

IPP

2×C×

f

Output Voltage Setting

Referring to application circuits (Figure 6), the output

voltage of the switching regulator (VOUT) can be set with

below equation :

VOUT

=

(1+

R2 )×

R1

VFB,

VFB = 0.8V (typ.)

Linear Regulator

Linear Regular MOSFETs Selection

The linear controller of RT9278 was designed to drive an

external P-Channel MOSFET. The main consideration of

pass MOSFETs of linear regulator is package selection

for efficient removal of heat. The power dissipation of a

linear regulator is

Plinear = (VIN-VOUT) x IOUT (W)

The criterion for selection of package is the junction

temperature below the maximum desired temperature with

the maximum expected ambient temperature.

Layout Guide

l A full GND plane without gap break.

l VDD to GND noise bypass − Short and wide connection

for the 1µF MLCC capacitor between Pin7 and Pin3.

l VIN to GND noise bypass − Add a capacitor close to L1

inductor, when VIN is not an ideal voltage source.

l Minimized FB node copper area and keep far away

from noise sources.

l The MOSFETs of linear regulator should have wide pad

to dissipate the heat.

DS9278-04 August 2007

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