HM9284CS6 datasheet(7/9 Pages) HMSEMI

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APPLICATIONS INFORMATION

Adjusting LED Current

Set the maximum LED current using a resistor

from FB to GND. Calculate the resistance as

follows:.

LED

R =

where I

LED is the desired maximum current

through the LEDs.

LED Dimming Control



Using a PWM Signal to EN Pin

Figure 3



Using a DC Voltage

Using a variable DC voltage to adjust the

brightness is a popular method in some

applications. According to the Superposition

Theorem, as the DC voltage increases, the

voltage contributed to V

FB increases and the

voltage drop on R1 decreases, i.e. the LED

current decreases. For example, if the V

DC range

is from 0V to 2.8V, the selection of resistors sets

dimming control of LED current from 20mA to

0mA.The LED current can be calculated by the

following equation:

LED

)

(

−

Figure4



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.

The recommended application circuit is shown in

the Figure 5. In this circuit, the output ripple

depends on the frequency of PWM signal. For

smaller output voltage ripple (<100mV), the

recommended frequency of 2.8V PWM signal

should be above 20kHz. To fix the frequency of

PWM signal and change the duty cycle of PWM

signal can get different output current. Figure 5.

shows the relationship between LED current and

PWM duty cycle.The LED current can be

calculated by the following equation:

RCS

VOUT

7 WLEDs

VDC

For controlling the LED brightness， the HM9284

can pe orm the dimming control by applying a

PWM signal to EN pin. The internal soπstart and

the wide range dimming frequency can eliminate

inrush current and 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 pe orm correctly for preventing

the flicker issue， the suggested PWM frequency

≥20kHz and ≤1MHz.

HM9284 1.2MHz High Efficiency Boosf Whtte Led Oriver

Shenzhen H&M Semiconductor Co.Ltd

http：//www.hmsemi.com

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