10

When choosing an inductor, make sure the inductor can

handle the average and peak currents giving by following

formulas (80% efficiency assumed):

where:

•

• L inductance in µH

• FS switching frequency, typical 1MHz

A wide range of inductance (6.8µH - 68µH) can be used for

the converter to function correctly. For the same series of

inductors, the lower inductance has lower DC resistance

(DCR), which has less conducting loss. But the ripple current

is bigger, which generates more RMS current loss. Figure 11

shows the efficiency of the demo board under different

inductance for a specific series of inductor. For optimal

efficiency in an application, it is a good exercise to check

several adjacent inductance values of your preferred series

of inductors.

For the same inductance, higher overall efficiency can be

obtained by using lower DCR inductor.

The diode should be Schottky type with minimum reverse

voltage of 20V. The diode's peak current is the same as

current is:

Ensure the diode's ratings exceed these current

requirements.

White LED Connections

One leg of LEDs connected in series will ensure the

uniformity of the brightness. 18V maximum voltage enables

4 LEDs can be placed in series.

However, placing LEDs into series/parallel connection can

give higher efficiency as shown in the efficiency curves. One

of the ways to ensure the brightness uniformity is to pre-

screen the LEDs.

PCB Layout Considerations

The layout is very important for the converter to function

properly. Power Ground ( ) and Signal Ground ( ) should

be separated to ensure the high pulse current in the power

ground does not interference with the sensitive signals

connected to Signal Ground. Both grounds should only be

connected at one point right at the chip. The heavy current

be as short as possible.

The trace connected to the CS pin is most important. The

When the trace is long, use a small filter capacitor close to

the CS pin.

The heat of the IC is mainly dissipated through the PGND

pin. Maximizing the copper area around the plane is

preferable. In addition, a solid ground plane is always helpful

for the EMI performance.

The demo board is a good example of layout based on the

principle. Please refer to the EL7513 Application Brief for the

layout.

I

LAVG

I

O

V

O

×

0.8

V

IN

×

------------------------

=

I

LPK

I

LAVG

1

2

---

∆I

L

×

+

=

∆I

L

V

IN

V

O

V

IN

–

()

×

LV

O

F

S

×

×

---------------------------------------------

=

DIFFERENT L

L=Coilcraft

LPO1704 SERIES

1mm HEIGHT

85

83

81

79

77

5

1015202530

L=33µH

L=22µH

L=10µH

L=15µH

FIGURE 21. EFFICIENCY OF DIFFERENT INDUCTANCE

(4 LEDs IN A SERIES)

I

DRMS

I

LAVG

I

O

×

=

EL7513