SP6683 Datasheet, PDF (7/10 Pages) Sipex Corporation – High Power LED Driver for Parallel Configuration

Brightness Control Using PWM
Dimming control can be achieved by applying
a PWM contol signal to the EN/PWM pin. The
brightness of the white LEDs is controlled by
increasing and decreasing the duty cycle of the
PWM signal. While operating frequency range
is from 60Hz to 700Hz, the recomended
maxium brightness frequncy range is from
60Hz to 200Hz. A repition rate of at least 60Hz
is required to prevent flicker.
Brightness Matching
For white LEDs, the forward voltage drop is a
function of the operating current. However,
for a given current, the forward voltage drops
do not always match due to normal manufac-
turing tolerance, thus causing uneven bright-
ness of the white LEDs.
In figure 14, assume high-precision bias resis-
tors were used, the operating current ratio of
two different branches can be easily derived as
shown by:
APPLICATION INFORMATION: Continued
Since the brightness of the white LED is propor-
tional to the operating current, for better bright-
ness matching, a higher output voltage could be
used. This could be done by using larger resistor,
as shown in figure 14. Rb2 is used to bias the
operating current of the white LED, Rb1 is use to
increase the output voltage. Better brightness
matching was achived at the cost of the power
wasted on the bias resistor.
Power Efficiency
The efficiency of driving the white LEDs can be
calculated by
η ₌ ^{VF • IF} ₌
V_F • I_F
≈ ^VF
V_i • I_i V_i • (n • I_F + I_Q) V_i • n
Where V_i, I_i are input voltage and current V_F, I_F
are the forward voltage and operating current of
White LEDs I_Q is quiescent current, which is
considered small compared with I_F.
I_{1 = VOUT - VF1}
I₂ V_OUT - V_F2
where I₁ I₂ are the operating current of the
white
LEDs,V_F1,V_F2 are the forward voltage of the
white LEDs.
V_{OUT 1} I₁
SP6683
5
V_FB
GND
8
I₂
D1
VF1
Rb1
Rb2
Rb
D2
VF2
Rb
I_n
Dn
VFn
Rb
Figure 14. Increasing brightness matching
Date: 07/07/04
SP6683 White LEDs Driver for Parallel Configuration
7
© Copyright 2004 Sipex Corporation