SP6687 datasheet(10/12 Pages) SIPEX

10 / 12 page

Date: 11/15/05

SP6687 4Channel Charge Pump White LED Driver

)

(

SP6687

CTRL2

CTRL1

CTRL0

PWM

LED ON

LED

OFF

VIN

Due to the 100µs delay time between mode transfers,

the duty cycle of the dimming frequency should not

exceed the maximum duty cycle on the CTRLx pins.

For best performance it is recommended to keep the

dimming frequency between 200Hz and 1kHz. When

the duty cycle is exceeded, the SP6687 cannot trans-

fer modes properly. The following equation shows the

relation between maximum duty of the CTRLx pins

and the PWM dimming frequency:

Using an analog input voltage VADJ via a resistor RADJ

that connects to the ISET pin is another method for

dimming control of LEDs. Figure 7 shows the applica-

tion circuit. For this application the LED current can be

derived from the following equation:

APPLICATION INFORMATION

Figure 6. Typical application circuit for PWM dimming

when driving 2 LEDs.

Table 4. Dimming frequency relative to Min/Max duty.

SP6687

CTRL2

CTRL1

CTRL0

VIN

ISET

VADJ

RADJ

RSET

Figure 7. Typical application circuit for PWM dimming

using a DC voltage into ISET.

VADJ

2.5V

1.6V

0.8V

0mA

6.5mA

13mA

20mA

ILED

Figure 8. SP6687 dimming control application using a

DC voltage into ISET.

Dimming using a DC voltage added to I

SET

Figure 8 shows the relation between VADJ and ILED of

a typical application example, with VADJ from 0 to 2.5V,

RSET = 43kΩ and RADJ = 55kΩ.

DMAX =(1-100 x10 -6 x FD)

Where DMAX is the Maximum Duty of CTRLX and FD is

the PWM Dimming Frequency.

ILED = 440 X

[1.1 x (1/R

SET + 1/RADJ) - VADJ/RADJ

]

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