White LED Step-Up Regulator
EC4513
Preliminary
PAGE
7 / 7
DR18EV9005-VER1.0
When choosing an inductor, make sure the inductor can handle the average and peak currents
giving by following formulas (80% efficiency assumed):
where:
•△IL is the peak-to-peak inductor current ripple in Ampere
• L inductance in µH
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 12 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
inductor's peak current, the average current is IO, and RMS 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. When placing 4 LEDs in series, make sure the worst total forward voltage does not exceed 18V.
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 prescreen 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
paths (VIN-L-LX pin-PGND, and VIN-L-D-C2-PGND) should be as short as possible.
The trace connected to pin (CS) is most important. The current sense resister R1 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 principle. Please refer to the EC4513 Application Brief for the
layout.
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