SP6687 Datasheet, PDF (10/12 Pages) Sipex Corporation – 4 Channel Charge Pump White LED Driver

APPLICATION INFORMATION
VIN
®
SP6687
CTRL2
CTRL1
CTRL0
EN
PWM
LED
LED ON
OFF
Figure 6. Typical application circuit for PWM dimming
when driving 2 LEDs_.
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:
D_MAX =(1-100 x10 ^-6 x F_D)
Where D_MAX is the Maximum Duty of CTRLX and F_D is
the PWM Dimming Frequency.
Dimming using a DC voltage added to I_SET
Using an analog input voltage V_ADJ via a resistor R_ADJ
that connects to the I_SET 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:
[ ] I_LED = 440 X 1.1 x (1/R_SET + 1/R_ADJ) - V_ADJ/R_ADJ
VADJ
®
SP6687 ISET
CTRL2
CTRL1
CTRL0
EN
RADJ
RSET
VIN
Figure 7. Typical application circuit for PWM dimming
using a DC voltage into I_SET.
Dimming
Frequency (Hz)
1K
900
800
700
600
500
400
300
200
CTRLX
Maximum
Duty
0.90
0.91
0.92
0.93
0.94
0.95
0.96
0.97
0.98
ILED
Minimum Duty
0.10
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
Table 4. Dimming frequency relative to Min/Max duty_.
VADJ ^2.5V
1.6V
0.8V
0V
ILED
0mA
13mA ^20mA
6.5mA
Figure 8. SP6687 dimming control application using a
DC voltage into I_SET.
Figure 8 shows the relation between V_ADJ and I_LED of
a typical application example, with V_ADJ from 0 to 2.5V,
RSET = 43kΩ and R_ADJ = 55kΩ.
Date: 11/15/05
SP6687 4Channel Charge Pump White LED Driver
10
© Copyright 2005 Sipex Corporation