SP6683 Datasheet, PDF (6/10 Pages) Sipex Corporation – High Power LED Driver for Parallel Configuration

PROGRAMMING THE OPERATING MODE
SP6683 can automatically change from X1 mode
to X1.5 mode for highest efficiency. To use this
feature, divider resistors should be chosen ac-
cording to the specific application, as shown in
figure 13.
APPLICATION INFORMATION
Which can be expressed as R_1:
R₁ = V_TH/1.25 _- 1) • R₂
V_IN
3
V_IN
R1
4
SP6683
V_MODE
R2
C2
GND
8
Figure 13. Programming the Vmode Resistors
The guideline for divider resistor selections is as
follows. For high input voltage, the SP6683 will
work in X1 mode. When the input voltage drops
to Vth threshold voltage, it will switch to X1.5
mode automatically. The Vth threshold voltage
for mode change can be calculated by:
V_TH = (V_F + 0.306 + 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 R₁ and R₂
with V_MODE = 1.25V is:
For the typical SP6683 application, using V_F =
3.6V, m = 8, I_LED = 15mA, R_OUT = 6Ω, V_TH will
be 4.63V, Select R₂ = 100kΩ, then R₁ = 270kΩ
Capacitor Selection
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
trade-off 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, smaller fly capaci-
tor leads to larger output resistance, thus de-
creasing the output current capability and the
circuit efficiency. Place all the capacitors as
close to the SP6683 as possible for layout.
Increasing the value of the input and otput
capicitors could further reduce the input and
output ripple.
Refer to table 1 for some suggested low ESR
capicitors.
V_TH = 1.25V • (1+R₁/R₂)
Table: 1
SUGGESTED LOW ESR CAPACITORS
MANUFACTURERS/
TELEPHONE#
TDK/847-803-6100
PART NUMBER
C2012X5R1A225K
CAPACITANCE/
VOLTAGE
2.2µF/10V
CAPACITOR/
SIZE/TYPE
0805/X5R
ESR
AT 100kHz
0.030Ω
TDK/847-803-6100
C2012X5R0J475K
4.7µF/6.3V
0805/X5R
0.020Ω
MURATA/770-436-1300 GRM188R60J225KE01D 2.2µF/6.3V
0603/X5R
0.030Ω
MURATA/770-436-1300 GRM219R60J475KE01D 4.7µF/6.3V
0805/X5R
0.020Ω
Date: 07/07/04
SP6683 White LEDs Driver for Parallel Configuration
6
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