EUP2518 datasheet(6/8 Pages) EUTECH

Part #	EUP2518

Description	White LED Step-Up Converter In TSOT-23 Package

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Manufacturer	EUTECH [Eutech Microelectronics Inc]

Direct Link	http://www.eutechmicro.com

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EUP2518

DS2518

Ver1.0

Mar. 2008

6

Application Information

LED Current Control

The EUP2518 regulates the LED current by setting the

current sense resistor (RFB) connecting to feedback and

ground. The internal feedback reference voltage is

104mV. The LED current can be set from following

equation easily.

LED

I

104mV

FB

R

=

-------------------------------------(1)

In order to have an accurate LED current, precision

resistors are preferred (1% is recommended). The table

for RFB selection is shown below.

RFB Resistor Value selection

ILED (mA)

RFB (Ω)

5

21

10

10.4

15

6.93

20

5.2

CAP and Inductor Selection

The recommended value of inductor for 2 to 6 WLEDs

applications are 10µH to 47µH. Small size and better

efficiency are the major concerns for portable device. A

22µH inductor with low DCR (Inductor resistance) is

recommended to improve efficiency. To avoid inductor

saturation current rating should be considered. A 1µF

ceramic capacitor is recommended for the input

capacitance in the real system, and a larger capacitor

will get smaller input voltage ripple. A 0.22uF output

ceramic capacitor is sufficient for most applications.

Dimming Control

a. Using a PWM Signal to EN Pin

For controlling the LED brightness, the EUP2518 can

perform the dimming control by applying a PWM

signal to EN pin. The internal soft-start and wide range

dimming frequency from 100Hz to 100KHz can

insignificantly reduce audio noise when dimming. The

average LED current is proportional to the PWM signal

duty cycle. The magnitude of the PWM signal should

be higher than the maximum enable voltage of EN pin,

in order to let the dimming control perform correctly.

Figure 3. PWM Dimming Control Using the EN Pin

b. Using a DC Voltage

Using a variable DC voltage to adjust the brightness is

a popular method in some applications. The dimming

control using a DC voltage circuit is shown in Figure 4.

According to the Superposition Theorem, as the DC

voltage increases, the voltage contributed to VFB

increases and the voltage drop on RFB decreases, i.e.

the LED current decreases. For example, if the VDC

range is from 0V to 2V, the selection of resistors in

Figure 4 sets dimming control of LED current from

20mA to 0mA.

Figure 4. Dimming Control Using a DC Voltage

c. Using a Filtered PWM signal

Another common application is using a filtered PWM

signal as an adjustable DC voltage for LED dimming

control. A filtered PWM signal acts as the DC voltage

to regulate the output current. In this circuit, the output

ripple depends on the frequency of PWM signal. For

smaller

output

voltage

ripple

(<100mV),

the

recommended frequency of 2V PWM signal should be

above 2kHz. To fix the frequency of PWM signal and

change the duty cycle of PWM signal can get different

output current. According to the application circuit of

Figure 5, output current is from 20.5mA to 5.5mA by

adjusting the PWM duty cycle from 10% to 90%.

Figure 5. Filtered PWM Signal for LED Dimming

Control

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