SP6682 Datasheet, PDF (7/12 Pages) Sipex Corporation – High Efficiency Charge Pump Regulator for White LEDs

In this application, the bias resistor can be se-
lected by:
R_B = ^VFB
I_{LED (TOTAL)}
where I_LED(TOTAL) is the total operating current
of all the white LEDs.
To use SP6682 as a voltage source for fixed
voltage applications, a voltage divider is need to
program the ouput voltage, as shown in figure 12.
The output voltage is set by the ratio of the two
APPLICATION INFORMATION: Continued
Programming the Operating Mode
SP6682 can automatically change from X1.5
mode to X2 mode for highest efficiency. To use
this feature, divider resistors should be chosen
according to the specific application, as shown
in figure 13.
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 Vth threshold voltage, it will switch to
X2 mode automatically. The Vth threshold volt-
age for mode change can be calculated by:
V_{OUT 1}
SP6682
V_FB
5
GND
8
R5
2.2uF
R6
Anode
2-wire W-LED
module
Cathode
Figure 12. Driving 2-wire white LED module as voltage
source
resistors and the feedback control voltage as
shown by:
V_OUT
=
(
1
+
R₅
R₆
)
•
V_FB
V_TH = (V_F + 0.306 + m • I_LED • R_OUT)/1.5
Where V_F and m are the forward voltage and
number of the white LEDs, Rout is the output
resistance of the SP6682.
The equation for the voltage divider R₁ and R₂
with V_MODE = 1.25V is:
V_TH = 1.25V • (1+R₁/R₂)
which can be expressed as R1:
R₁ = (V_TH / 1.25 -1) • R₂
For the typical SP6682 application, Using
V_F=3.6V, m=4, I_LED=15mA, R_OUT=16Ω, the
V_TH will be 3.24V. Select R₂=100kΩ, then
R₁=158kΩ.
V_IN
3
V_IN
R1
4
SP6682
V_MODE
R2
C2
GND
8
Figure 13. Programming the Vmode Resistors
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-
Date: 5/5/04
SP6682 High Efficiency Charge Pump Regulator for White LEDs
7
© Copyright 2004 Sipex Corporation