APW7008_12 datasheet(16/22 Pages) ANPEC

16 / 22 page

Rev. A.4 - Jun., 2009

APW7008

www.anpec.com.tw

Application Information (Cont.)

3. Digital dimming with external NMOS transistors

The third method for dimming the LEDs is to change the

equivalent resistance for RSET with the external NMOS

transistors. The equivalent resistance is the parallel

combinations of the R1, R2, R3, and R4. R4 is always

connected and selected for the minimum LED current.

Figure 5 shows the application circuit.

Figure 5. Digital Dimming Application Circuit

Brightness Control (Cont.)

ISET

APW7008

4. PWM dimming with EN pin

Another method for dimming the LEDs is to apply a

PWM signal into the EN pin. The average LED current

is proportional to the PWM signal duty cycle. Note

that the frequency of PWM signal will affect the mini-

mum dimming duty. The recommend dimming fre-

quency is between 100Hz and 1kHz. The average LED

current is calculated by the following equation:

Where:

LED(max) is programmed LED current by ISET pin

toff is the off time of the PWM signal

ton is the on time of the PWM signal

Layout Consideration

The APW7008 is a high frequency charge pump for white

LED driver and requires some care when laying out the

printed circuit board. The metal GND pad of the bottom

of the package must be soldered to the PCB and con-

nected to the GND plane on the backside through sev-

eral thermal vias. Place the CIN, COUT, C1, and C2 as

close to IC as possible for reducing the switching noise.

toff

ton

(max)

LED

)

avg

(

LED

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