High Efficiency 1MHz, Step up Regulator

EC4513

Page 13 of 15

3G16N-Rev.P001

Application Information

Connecting More LED Strings

The EC4513 can drive 8 LED strings in parallel and up to 8

LEDs per string (VF<3.5V). Each string must have the same

number of LEDs. In the applications that have the same total

number of LEDs, more strings and less LEDs in series are

more efficiency than less strings and more LEDs in series.

Brightness Control

The method for dimming the LEDs is to apply a PWM voltage

through an RC filter into the FB pin.

The RC filter is used to convert the PWM voltage into an analog

voltage. The values of the R and C depend upon the frequency

of the PWM voltage and the amount of allowable ripple voltage

on FB pin. The LED current is proportional to the PWM duty

cycle. 0 % duty delivers maximum LED current and 100% duty

delivers minimum LED current. The values of R1 and R2 are

calculated by the following equations:

where:

Figure 4. Dimming with the PWM Voltage

Inductor Selection

A larger value of inductor will reduce the peak inductor current,

resulting in smaller input ripple current, higher efficiency and

reducing stress on the internal MOSFET. However,the larger

value of inductor has a large dimension, lower saturation current,

and higher series resistance.

A good rule for determining the inductance is to allow the

peak-to-peak ripple current to be approximately 30% to 50%

of the maximum input current. Calculate the required

inductance value by the equation:

It is necessary to choose an inductor that ensures the inductor

saturation current rating to exceed the peak inductor current for

the application.

To make sure that the peak inductor current is below the

current-limit 2.5A. Calculating the peak inductor current by the

following equation：

where

η is the effiency

Schottky Diode Selection

A fast recovery time and low forward voltage Schottky diode is

necessary for optimum efficiency. Ensure that the

diode’s

average and peak current rating exceed the average output

current and peak inductor current. In addition, the

diode’s

reverse voltage must exceed output voltage.

Capacitor Selection

An input capacitor is required to supply the ripple current to the

inductor and stabilize the input voltage. Larger input capacitor

values and lower ESR provide smaller input voltage ripple and

noise. The typical value for input capacitor is 2.2uF to 10uF.

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