http://www.belling.com.cn

- 6-

8/8/2006

BL8501

Thus, with changing the value of R2 and R4, output voltage can be set in the specified range.

In the DC-DC converter, with the load current and external components such as L and C, phase

might be behind 180 degree. In this case, the phase margin of the system will be less and stability

will be worse. To prevent this, phase margin should be secured with proceeding the phase. A pole

is formed with external components L and C3.

pole

F

~ 1/2

3

C

L

A zero (signal back to zero) is formed with R4 and C4.

)

4

4

2

/(

1

~

C

R

F

zero

For example, if L=27

H, C3=47

F, the cut off frequency of the pole is approximately 4.5kHz.

To make the cut off frequency of the pole as much as 4.5kHz, set R4=33

k

and C4=1000pF. If

OUT

V

is set at 5.5V, R2=7.3

k

is appropriate.

R3 prevents feedback of the noise to

FB

V

pin, about 2.7

k

is appropriate value.

External Components

1. Inductor

Select an inductor that peak current does not exceed

max

IL

. If larger current than allowable

current flows, magnetic saturation occurs and makes transform efficiency worse.

When the load current is definite, the smaller value of L, the larger the ripple current.

Provided that the allowable current is large in that case and DC current is small, therefore,

for large output current, efficiency is better than using an inductor with a large value of L and

vice versa.

2. Diode

Use a diode with low

F

Reverse voltage rating should be more than

IN

V

and current rating should be equal or more

than

max

IL

.

3. Capacitors

As for

IN

least 10

F for stable operation.

OUT

C

can reduce ripple of Output Voltage, therefore 47

F or more value of tantalum type

capacitor is recommended.

4. Lx Transistor

Pch Power MOSFET is required for this IC.