SP6683 Datasheet, PDF (11/14 Pages) Sipex Corporation – High Power LED Driver for Parallel Configuration

APPLICATION INFORMATION
Brightness Matching
For white LEDs, the forward voltage drop is a
function of the operating current. However, for
a given current, the forward voltage drop across
each LED does not always match exactly. This
can cause uneven brightness in the white LEDs.
In the figure below, assuming high-precision
bias resistors were used, the operating current
ratio of two different branches can be easily
derived as shown by:
I_{1 = VOUT - VF1}
I₂ V_OUT - V_F2
where I_{1 &} I₂ are the operating current of the
white LEDs, where V_{F1 &}V_F2 are the forward
voltage of the white LEDs.
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 a larger
resistor, as shown in the figure below. Rb2 is
used to bias the operating current of the white
LED, Rb1 is use to increase the output voltage.
Better brightness matching was achieved 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 the White LED;
I_Q is quiescent current, which is considered
small compared with I_F.
V_{OUT 1} I₁
SP6683
5
V_FB
GND
8
I₂
D1
VF1
Rb1
Rb2
Rb
D2
VF2
Rb
I_n
Dn
VFn
Rb
Improving brightness matching
Date: 3/31/06 RevG
SP6683 1X/1.5X Charge Pump LED Driver for Flash or Backlight
11
© Copyright 2006 Sipex Corporation