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3F27N-Rev.P001

White LED Set-up Converter

EC4501

Function Description

Dimming Control

There are three different types of dimming control circuits:

1. Using a DC Voltage

For some applications, the preferred method of brightness

control is a variable DC voltage to adjust the LED current.

The dimming control using a DC voltage is shown in Figure

3. As the DC voltage increases, the voltage drop on R2

increases and the voltage drop on R1 decreases. Thus, the

LED current decreases. The selection of R2 and R3 will

make the current from the variable DC source much smaller

than the LED current and much larger than the FB pin bias

current.

2. Using a PWM Signal to EN Pin

With the PWM signal applied to the EN pin, the EC4501 is

turned on or off by the PWM signal. The LEDs operate at either

zero or full current. The average LED current increases

proportionally with the duty cycle of the PWM signal. A 0%

duty cycle corresponds to zero LED current. A 100% duty cycle

corresponds to full current. The frequency range of the PWM

signal should be up to 100KHz

3. Using a Filtered PWM Signal

The filtered PWM signal can be considered as an adjustable

DC voltage. It can be used to replace the variable DC voltage

source in dimming control.

B

Board Layout Consideration

paid to the PCB board layout and component placement. To

maximize efficiency, switch rise and fall times are made as

short as possible. To prevent electromagnetic interference

(EMI) problems, proper layout of the high frequency

switching path is essential. The voltage signal of the SW pin

has sharp rise and fall edges. Minimize the length and area of

all traces connected to the SW pin and always use a ground

plane under the switching regulator to minimize interplane

coupling. In addition, the ground connection for the feedback

resistor R1 should be tied directly to the GND pin and not

noise‐ free connection.