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APPLICATION INFORMATION
Component Selection
The guideline for selecting divider resistors is as
follows. For high input voltage, the SP6683 will
work in X1 mode. When the input voltage drops
to V TH threshold voltage, it will switch to X1.5
mode automatically. The V TH threshold voltage
for mode change can be calculated by:
V TH = (V F + 0.315 + m ? I LED ? R OUT )
Where V F and m are the forward voltage and
number of the white LEDs, Rout is the output
resistance of the SP6683.
The equation for the voltage divider R 1 and R 2
with V MODE = 1.25V is:
V TH = 1.25V ? (1+R 1 /R 2 )
Which can be expressed as R 1:
R 1 = [ (V TH /1.25) - 1 ] R 2
Ceramic capacitors are recommended for their
inherently low ESR, which will help produce
low peak-to-peak output ripple, and reduce high
frequency spikes.
The fly capacitor controls the strength of the
charge pump. Selection of the fly capacitor is a
tradeoff between the output voltage ripple and
the output current capability. Decreasing the fly
capacitor will reduce the output voltage ripple
because less charge will be delivered to the
output capacitor. However, a smaller fly capaci-
tor leads to larger output resistance, thus de-
creasing the output current capability and the
efficiency of the circuit. Increasing the value of
the input and output capacitors could further
reduce the input and output ripple. For back-
lighting applications up to 80mA output current,
C IN = C OUT = 1 μ F is typically used. For appli-
cations with more than 80mA output current, for
example in Flash applications where Iout can be
as much as 200mA, C IN = C OUT = 2.2 μ F is
recommended. For peak performance, place all
capacitors as close to the SP6683 as possible.
For the typical SP6683 application, using V F =
3.6V, m = 8, I LED = 15mA, R OUT = 6 ? , V TH will
be 4.63V, Select R 2 = 100k ? , then R 1 = 270k ?
Refer to table 1 below for some suggested low
ESR capacitors.
SUGGESTED LOW ESR CAPACITORS
Manufacturers/ Website
TDK/www.tdk.com
TDK/www.tdk.com
Murata/www.murata.com
Murata/www.murata.com
Part Number
C1005X5R0J105M
C1608X5R0J225K
GRM155R60J105KE19B
GRM188R61A225KE34B
Capacitance/
Voltage
1uF/6.3V
2.2uF/6.3V
1uF/6.3V
2.2uF/6.3V
Capacitor
Size/Type/Thickness
0402/X5R/0.5mm
0603/X5R/0.9mm
0402/X5R/0.55mm
0603/X5R/0.9mm
ESR at
100KHz
0.03
0.02
0.03
0.02
Date: 3/31/06 RevG
SP6683 1X/1.5X Charge Pump LED Driver for Flash or Backlight
10
? Copyright 2006 Sipex Corporation
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