SP6682EB Exar Corporation, SP6682EB Datasheet - Page 9

• Manufacturer Part Number: SP6682EB
• Description: EVAL BOARD FOR SP6682
• Manufacturer: Exar Corporation

Specifications of SP6682EB

• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Product: Display Modules
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

B
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
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:
where I
white LEDs andV
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
white LED, R
voltage.
achieved at the cost of the power wasted on
the bias resistor.
P
The efficiency of driving the white LEDs can be
calculated by:
© 2009 Exar Corporation
B2
RIGHTNESS
OWER
is used to bias the operating current of the
η
always
=
Fig. 17: Increasing Brightness Matching
1
V
E
V
, I
F
FFICIENCY
I
Better
×
×
2
I
I
B1
are the operating current of the
F
I
M
match
=
is used to increase the output
ATCHING
I
I
F1
1
2
V
,V
bright-ness
=
I
×
F2
V
V
(
OUT
η
OUT
V
due
are the forward volt-age
F
×
×
(
−
−
I
I
V
V
F
F
to
F
F
+
1
2
I
matching
Q
normal
H
H
)
)
i
i
=
g
g
V
h
h
V
I
×
E
E
F
η
f
f
man-
f
f
was
i
i
c
c
i
i
e
e
9/13
n
n
c
c
y
y
Where V
I
current of White LEDs; I
which is considered small compared with I
is the boost ratio (x1.5 or x2).
SP6682 H
D
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:
Fig. 18: Using SP6682 as a High Voltage White LED Driver
W
W
F
RIVER
h
h
are the forward voltage and operating
i
i
t
t
e
e
I
L
, I
L
as a High Voltage White LED Driver
E
E
I
IGH
Fig. 19: Efficiency of SP6882
D
D
are input voltage and current V
C
C
V
h
h
OLTAGE
I
a
a
LED
r
r
g
g
=
e
e
Q
V
P
P
R
FB
is quiescent current,
W
1
u
u
m
HITE
m
p
p
R
R
LED
S
S
e
e
P
P
g
g
Rev. 2.0.0
u
6
u
6
l
l
6
6
a
a
8
t
8
t
F
; η
o
o
2
2
F
r
r
,