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S P 6 6 8 2
H i g h E f f i c i e n c y W h i t e L E D C h a r g e P u m p R e g u l a t o r
V TH = 1 . 25 × ? 1 +
?
use this feature, divider resistors should be
chosen according to the specific applica-tion,
as shown in Figure 16.
?
?
R 1 ?
R 2 ?
R 1 = ? TH ? 1 ? × R 2
which can be expressed as R1:
? V ?
? 1 . 25 ?
For the typical SP6682 application, using
V F =3.6V, M=4, I LED =15mA, R OUT =16 ? , the V TH
will be 3.24V. Select R2=100k ? , then
R1=158k ? .
C APACITOR S ELECTION
( V F + 0 . 306 + M × I LED × R OUT )
V TH =
Fig. 16: Programming the V MODE Resistors
The guideline for divider resistor selections is
as follows: For high input voltage, the SP6682
will work in X1.5 mode. When the input
voltage drops to the V TH threshold voltage, it
will switch to X2 mode automati-cally. The V TH
threshold voltage for mode change can be
calculated by:
1 . 5
Where V F and M are the forward voltage and
number of the white LEDs, R OUT is the output
resistance of the SP6682. The equation for the
voltage divider R1 and R2 with V MODE = 1.25V
is:
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
capacitors lead to larger output resistance,
thus decreasing the out-put current capability
and the circuit effi-ciency. Place all the
capacitors as close to the SP6682 as possible
for layout. Increasing the value of the input
and output capaci-tors could further reduce
the input and out-put ripple.
Refer to Table 1 for some suggested low ESR
capacitors.
Manufacturer
TDK
TDK
Murata
Murata
Part Number
C2012X5R1A225K
C2012X5R0J475K
GRM188R60J225KE01D
GRM219R60J475KE01D
Capacitance
Voltage
2.2μF/10V
4.7μF/6.3V
2.2μF/6.3V
4.7μF/6.3V
Capacitor
Size/Type
0805/X5R
0805/X5R
0805/X5R
0805/X5R
ESR @ 100KHz
0.030 ?
0.020 ?
0.030 ?
0.020 ?
Table 1: Suggested Low ESR Capacitors
B RIGHTNESS C ONTROL U SING PWM
Dimming control can be achieved by applying
a PWM control signal to the EN/PWM pin. The
brightness of the white LEDs is con-trolled by
increasing and decreasing the duty cycle of
the PWM signal. While the operating frequency
range of the PWM control is from 60Hz to
700Hz, the recommended maxim-um
brightness frequency range of the PWM signal
is from 60Hz to 200Hz. A repetition rate of at
least 60Hz is required to prevent flicker.
? 2009 Exar Corporation
8/13
Rev. 2.0.0
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