RT8509

7

DS8509-01 March 2012

www.richtek.com

©

Copyright 2012 Richtek Technology Corporation. All rights reserved.

is a registered trademark of Richtek Technology Corporation.

where

current. The input peak current can then be obtained by

adding the maximum input current with half of the inductor

ripple current as shown in the following equation :

PEAK

IN(MAX)

I1.2 x I

=

Note that the saturated current of the inductor must be

determined according to the following equation :

2

IN

OUT

IN

2

OUT

OUT(MAX)

OSC

x (V ) x(V

V )

L

0.4 x (V

) xI

x f

η

−

=

performance, a shielded inductor is preferred to avoid EMI

problems.

Diode Selection

Schottky diodes are chosen for their low forward voltage

drop and fast switching speed. When selecting a Schottky

diode, important parameters such as power dissipation,

reverse voltage rating, and pulsating peak current should

all be taken into consideration. A suitable Schottky diode's

reverse voltage rating must be greater than the maximum

output voltage and its average current rating must exceed

the average output current. Last of all, the chosen diode

should have a sufficiently low leakage current level, since

it will increase with temperature.

Output Capacitor Selection

The output ripple voltage is an important index for

estimating chip performance. This portion consists of two

parts. One is the product of the inductor current with the

ESR of the output capacitor, while the other part is formed

by the charging and discharging process of the output

capacitor. As shown in Figure 1,

based on the ideal energy equalization. According to the

definition of Q, the Q value can be calculated as the

following equation :

IN

L

OUT

IN

L

OUT

IN

OUT

OUT1

OUT

OSC

11

1

Q

x

I

I

I

I

I

I

22

2

V

1

x

x

C

x V

Vf

⎡⎤

⎛⎞ ⎛

⎞

=+ Δ −

+

− Δ −

⎜⎟ ⎜

⎟

⎢⎥

⎝⎠ ⎝

⎠

⎣⎦

=Δ

estimate the value of

equation :

OUT

OUT1

OUT

OSC

D x I

V

x C

x f

η

Δ=

where D is the duty cycle and

η is the boost converter

efficiency. Finally, taking ESR into account, the overall

output ripple voltage can be determined by the following

equation :

OUT

OUT

IN

OUT

OSC

D x I

V

I

x ESR

x C

x f

η

Δ=

+

Time

Time

Inductor Current

Output Current

Output Ripple

Voltage (ac)

Δ

Δ

Input Current

Figure 1. The Output Ripple Voltage without the

Contribution of ESR

Input Capacitor Selection

Low ESR ceramic capacitors are recommended for input

capacitor applications. Low ESR will effectively reduce

the input voltage ripple caused by switching operation. A

10

μF capacitor is sufficient for most applications.

Nevertheless, this value can be decreased for lower output

current requirement. Another consideration is the voltage

rating of the input capacitor which must be greater than

the maximum input voltage.

OUT

OUT(MAX)

IN(MAX)

IN

RIPPLE

IN(MAX)

V

x I

I

=

x V

I

= 0.4 x I

η

as an example, the inductor ripple current can be

calculated according to the following equations :