EL1848
For the same inductance, higher overall efficiency can be
obtained by using lower DCR inductor.
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-
85
EFFICIENCY vs I O
V IN =3.3V FOR
DIFFERENT L
screen the LEDs.
PCB Layout Considerations
83
L=33μH
L=22μH
L=15μH
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
81
L=10μH
ground does not interference with the sensitive signals
connected to Signal Ground. Both grounds should only be
79
77
5
10
15
L=Coilcraft
LPO1704 SERIES
1mm HEIGHT
20 25
30
connected at one point right at the chip. The heavy current
paths (V IN -L-L X pin-PGND, and V IN -L-D-C 2 -PGND) should
be as short as possible.
The trace connected to the CS pin is most important. The
I O (mA)
FIGURE 20. EFFICIENCY OF DIFFERENT INDUCTANCE
(4 LEDs IN A SERIES)
The diode should be Schottky type with minimum reverse
voltage of 20V. The diode's peak current is the same as
inductor's peak current, the average current is I O , and RMS
current is:
current sense resister R 1 should be very close to the pin
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
I DRMS =
I LAVG × I O
principle. Please refer to the EL1848 Application Brief for the
layout.
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. 14V maximum voltage enables
3 LEDs can be placed in series.
10
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