www.belling.com.cn

Ver1.0

5/14

BL8530

Before discussion，we define

Vout

Vin

Vout

D

−

≡

.

（1）Inductor Selection

Above all, we should define the minimum value of the inductor that can ensure the boost

DC-DC to operate in the continuous current-mode condition.

f

R

D

D

L

L

2

)

1

(

min

2

−

≥

The above expression is got under conditions of continuous current mode, neglect Schottky

diode’s voltage, ESR of both inductor and capacitor. The actual value is greater that it. If inductor’s

value is less than Lmin，the efficiency of DC-DC converter will drop greatly, and the DC-DC circuit will

not be stable.

Secondly, consider the ripple of the output voltage,

Lf

Vin

D

I

•

=

∆

Lf

DVin

R

D

Vin

ax

L

2

)

1

(

Im

2

+

−

=

If inductor value is too small, the current ripple through it will be great. Then the current through

diode and power switch will be great. Because the power switch on chip is not ideal switch, the

energy of switch will improve. The efficiency will fall.

Thirdly，in general, smaller inductor values supply more output current while larger values start

up with lower input voltage and acquire high efficiency.

An inductor value of 3uH to 1mH works well in most applications. If DC-DC converter delivers

large output current (for example: output current is great than 50mA), large inductor value is

recommended in order to improve efficiency. If DC-DC must output very large current at low input

supply voltage, small inductor value is recommended.

load resistor，then the energy can be calculated by following expression ：

2

)

1

(

D

R

r

load

L

−

≈

∆

η

Consider all above，inductor value of 47uH、ESR<0.5Ω is recommended in most applications.

Large value is recommended in high efficiency applications and smaller value is recommended

（2）Capacitor Selection

Ignore ESR of capacitor，the ripple of output voltage is:

Cf

R

D

Vout

Vout

r

load

=

∆

=

So large value capacitor is needed to reduce ripple. But too large capacitor value will slow

down system reaction and cost will improve. So 100uF capacitor is recommended. Larger

capacitor value will be used in large output current system. If output current is small (<10mA),

small value is needed.

Consider ESR of capacitor，ripple will increase：

Vout

R

ax

r

r

ESR

•

+

=

Im

'

When current is large, ripple caused by ESR will be main factor. It may be greater than 100mV。

The ESR will affects efficiency and increase energy loss. So low-ESR capacitor (for example:

tantalum capacitor) is recommend or connect two or more filter capacitors in parallel.