APW7008 查看數據表（PDF） - Anpec Electronics

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APW7008

1X/1.5X/2x Charge Pump White LED Driver

Anpec Electronics 

APW7008

Application Information

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 1µF for the input capacitor, out-

put capacitor, and the charge pump flying capacitors.

Brightness Control

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:

ILED(avg)

=

toff × ILED(max)

ton + toff

Where:

I

is programmed LED current by ISET pin

LED(max)

toff is the off time of the PWM signal

ton is the on time of the PWM signal

VIN

20

18

LED=20mA

VIN=2.7V, 2x mode

16

14

f=100Hz

12

10

8 f=10kHz

6

4

f=40kHz

2

f=50kHz

0

0 10 20 30 40 50 60 70 80 90 100

Dimming Duty (%)

Figure 3. PWM Dimming Frequency vs. LED Current

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 RSET value, using the be-

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

that the VADJ will need to be greater than 1.2V.

VADJ = (VADJ − VISET ) + VADJ

R1

R2

RSET

Where:

V = 1.2V

ISET

VADJ = the analog voltage for dimming the LEDs

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

OFF

ON

PWM

APW7008

1 EN

2 CTRL0

CTRL1

3

CTRL2

4

VADJ

APW7008

ISET

R2 5

R1

Figure 4. Analog Voltage Dimming Application Circuit

Figure 2. PWM Dimming Application Circuit

Copyright © ANPEC Electronics Corp.

15

Rev. A.3 - Feb., 2009

www.anpec.com.tw

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