SP6682 Datasheet, PDF (8/12 Pages) Sipex Corporation – High Efficiency Charge Pump Regulator for White LEDs

APPLICATION INFORMATION
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 SP6682 as possible for layout.
Increasing the value of the input and output
capacitors could further reduce the input and
output ripple.
Refer to Table 1 for some suggested low ESR
capacitors.
Table: 1
SUGGESTED LOW ESR CAPACITORS
MANUFACTURERS/
TELEPHONE#
PART NUMBER
CAPACITANCE/
VOLTAGE
CAPACITOR/ ESR
SIZE/TYPE
AT 100kHz
TDK/847-803-6100
C2012X5R1A225K
2.2µF/10V
0805/X5R
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
Brightness Control Using PWM
Dimming control can be achieved by applying a
PWM control signal to the EN/PWM pin. The
brightness of the white LEDs is controlled 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 maximum brightness
frequency range of the PWM signal is from
60Hz to 200Hz. A repetition rate of at least 60Hz
is required to prevent flicker.
tional to the operating current, for better bright-
ness matching, a higher output voltage could be
used. This could be done by using larger resis-
tor, as shown in figure 14. Rb2 is used to bias the
operating current of the white LED, Rb1 is use
to increase the output voltage. Better brightness
matching was achived at the cost of the power
wasted on the bias resistor.
Brightness Matching
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
always match due to normal manufacturing tol-
erance, thus causing uneven brightness of the
white LEDs.
In figure 14, assume high-precision bias resis-
tors were used, the operating current ratio of two
different branches can be easily derived as shown
by:
I_{1 = VOUT - VF1}
I₂ V_OUT - V_F2
where I₁ I₂ are the operating current of the white
LEDs,V_F1,V_F2 are the forward voltage of the
white LEDs.
Since the brightness of the white LED is propor-
V_{OUT 1} I₁
SP6682
5
V_FB
GND
8
I₂
D1
VF1
Rb1
Rb2
Rb
D2
VF2
Rb
I_n
Dn
VFn
Rb
Figure 14. Increasing brightness matching
Power Efficiency
The efficiency of driving the white LEDs can be
calculated by:
η ₌ ^{VF • IF} ₌
V_F • I_F
≈ ^VF
V_i • I_i V_i • (n • I_F + I_Q) V_i • n
Date: 5/5/04
SP6682 High Efficiency Charge Pump Regulator for White LEDs
8
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