APW7008_12 datasheet(15/22 Pages) ANPEC

15 / 22 page

Rev. A.4 - Jun., 2009

APW7008

www.anpec.com.tw

Application Information

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

CTRL2

CTRL1

CTRL0

VIN

APW7008

OFF

PWM

90 100

Dimming Duty (%)

f=40kHz

f=100Hz

LED=20mA

VIN=2.7V, 2x mode

f=50kHz

f=10kHz

Figure 3. PWM Dimming Frequency vs. LED Current

Figure 2. PWM Dimming Application Circuit

1. PWM dimming using CTRL0, CTRL1, CTRL2

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

signal into the CTRL0, CTRL1, and CTRL2 pins. Figure 2

shows the application circuit. The average LED current is

proportional to the PWM signal duty cycle. Note that the

frequency of PWM signal will affect the minimum dim-

ming duty. Figure 3 shows the LED current vs. dimming

frequency and dimming duty, the recommend dimming

frequency is below 10kHz. The average LED current is

calculated by the following equation:

Brightness Control

Capacitor Selection

For lower input and output voltage ripples, both input and

output capacitors should be larger values and lower

ESR capacitors. However, the larger output capacitor

values will increase the soft-start time. The lower charge

pump flying capacitors values and ESR improve the

efficiency, but lower capacitor values may limit the

LED’s currents at low input voltage.

It is recommended that the low ESR and low variation over

temperature, such as the ceramic capacitors with X7R

or X5R and the value is 1

µF for the input capacitor, out-

put capacitor, and the charge pump flying capacitors.

2. Analog dimming with analog voltage

The second method for dimming the LEDs is to apply a

voltage through a resistor into the ISET pin. The variation

of LED current is proportional to the variation of the ana-

log voltage. If the resistor values are chosen correctly, the

analog control voltage varies the output current from 0mA

to full LED current. Figure4 shows the application circuit.

See the table2 and choose the required maximum LED

current and the corresponsive R

SET value, using the be-

low equation to calculate the values of R1 and R2, note

that the V

ADJ will need to be greater than 1.2V.

Where:

ISET = 1.2V

ADJ = the analog voltage for dimming the LEDs

SET = the equivalent RSET resistance (see table 2).

ISET

APW7008

VADJ

Figure 4. Analog Voltage Dimming Application Circuit

toff

ton

toff

(max)

LED

)

avg

(

LED

SET

ADJ

ISET

ADJ

)

(

−

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