ACE703

White LED PFM Step-Up DC-DC Converter

VER 1.2

7

f

R

D

D

L

L

2

)

1

(

min

2

−

≥

Lf

DVin

R

D

Vin

ax

L

2

)

1

(

Im

2

+

−

=

Lf

Vin

D

I

•

=

Δ

2

)

1

(

D

R

r

load

L

−

≈

Δ

η

Selection of the external components

Thus it can be seen, the inductor and shottky diode affect the conversion efficiency greatly. The inductor

and the capacitor also have great influence on the output voltage ripple of the converter. So it is

necessary to choose a suitable inductor, a capacitor and a right shottky diode, to obtain high efficiency,

low ripple and low noise.

(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.

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,

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. he 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.

The ESR of inductor will affect efficiency greatly. Suppose ESR value of inductor is Rl, Rload is load

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

For example: input 1.5V,output is 3.0V,Rload=20Ω,rL=0.5Ω,The energy loss is 10%.

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.