SP6682 Datasheet, PDF (9/13 Pages) Sipex Corporation – High Efficiency Charge Pump Regulator for White LEDs

SP6682
High Efficiency White LED Charge Pump Regulator
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 man-
ufacturing tolerances, thus causing uneven
brightness of the white LEDs.In Figure 15,
assume high-precision bias resistors were
used, the operating current ratio of two
different branches can be easily derived as
shown by:
I_{1 =} V_OUT −V_F1
I₂ V_OUT − V_{F 2}
where I₁, I₂ are the operating current of the
white LEDs andV_F1,V_F2 are the forward volt-age
of the white LEDs.
Since the brightness of the white LED is
proportional to the operating current, for
better brightness matching, a higher output
voltage could be used. This could be done by
using a larger resistor, as shown in Figure 17.
R_B2 is used to bias the operating current of the
white LED, R_B1 is used to increase the output
voltage. Better bright-ness matching was
achieved at the cost of the power wasted on
the bias resistor.
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; η
is the boost ratio (x1.5 or x2).
SP6682 HIGH VOLTAGE WHITE LED
DRIVER
The SP6682 can also be configured as a high
voltage boost converter to drive more than 10
white LEDs. Figures 18 and 19 show the
schematic of this application as well as actual
data showing efficiency of > 85%. By using an
external inductor, MOSFET and diode, high
output voltages can be generated to drive 12
white LEDs (2 branches, each branch has 6
white LEDs in series). The current through the
white LEDs is determined by:
I _LED
=
V_FB
R₁
Fig. 18: Using SP6682 as a High Voltage White LED Driver
Fig. 17: Increasing Brightness Matching
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 × η × I _F + I_Q V_I ×η
© 2009 Exar Corporation
9/13
Fig. 19: Efficiency of SP6882
as a High Voltage White LED Driver
Rev. 2.0.0