RT9270

Preliminary

10

DS9270-09 March 2007

www.richtek.com

Application Information

The design procedure of Boost converter can start from

the maximum input current, which is related about

and the maximum power which internal switch can stand.

It can be derived from maximum output power, minimum

input voltage and the efficiency of Boost converter. Once

the maximum input current is calculated, the inductor

value can be determined and the other components as

well.

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%,

the maximum peak inductor current is :

The minimum inductance value is derived from the

following equation :

Depending on the application, the recommended inductor

value is between 2.2uH and 10uH.

Diode Selection

To achieve high efficiency, Schottky diode is good choice

for low forward drop voltage and fast switching time. The

output diode rating should be able to handle the maximum

output voltage, average power dissipation and the

pulsating diode peak current.

Input Capacitor Selection

For better input bypassing, low-ESR ceramic capacitors

are recommended for performance. A 10uF input capacitor

is sufficient for most applications. For a lower output power

requirement application, this value can be decreased.

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.

×

×

×

PK

IN(MAX)

RIPPLE

IN(MAX)

OUT(MAX)

OUT

IN(MIN)

1

I

= I

+ I

= 1.2 I

2

IV

= 1.2 [

]

η ×V

××

××

×

2

IN(MIN)

OUT

IN(MIN)

2

OUT(MAX)

OUT

OSC

η V[V

- V

]

L=

0.4 I

V

f

(V)

(V)

(Hz)

L1

(uH)

C2

(uF)

R3

(k )

C3

(pF)

C4

(pF)

(mA)

3.3

12

670k

10(TDK SLF6028)

33 tantalum

36

1200

33

250

3.3

12

670k

4.7(TDK SLF6028)

33 tantalum

47

1000

22

250

3.3

12

1.3M

10(TDK SLF6028)

33 tantalum

39

1000

33

210

3.3

12

1.3M

4.7(TDK SLF6028)

33 tantalum

51

820

22

210

3.3

7.2

670k

4.7(TDK SLF6028)

47 tantalum

22

820

56

490

3.3

7.2

1.3M

4.7(TDK SLF6028)

47 tantalum

32

1000

56

450

Ω