SP6690 Datasheet, PDF (10/16 Pages) Sipex Corporation – Micro Power Boost Regulator Series White LED Driver

APPLICATION INFORMAMTION: Continued
precision resistors are recommended. Table 3
below shows the R_b selection for different white
LED currents. For example, to set the operating
current to be 20mA, R_b is selected as 60.4 Ω, as
shown in the schematic.
Table 3. Bias Resistor Selection
I_F (mA)
5
10
12
15
20
R_b (Ω)
243
121
102
80.6
60.4
Output Voltage Program
The SP6690 can be programmed as either a
voltage source or a current source. To program
the SP6690 as voltage source, the SP6690 re-
quires 2 feedback resistors R₁ & R₂ to control
the output voltage. As shown in Figure 22.
VIN
L1
D1
C2
C1
U1
5
1
V_{I N}
SW
SP6690
4
3
SHDN
FB
G ND
1.22V
2
VOUT
R1
R2
Figure 22. Using SP6690 as Voltage Source
The formula and table for the resistor selection
are shown below:
Table 4. Divider Resistor Selection
V_OUT (V)
12
15
18
21
30
R₁ (Ω)
1M
1M
1M
1M
1M
R₂ (Ω)
113K
88.7K
73.2K
61.9K
42.2K
Brightness Control
Dimming control can be achieved by applying a
PWM control signal to the SHDN pin. The
brightness of the white LEDs is controlled by
increasing and decreasing the duty cycle of The
PWM signal. A 0% duty cycle corresponds to
zero LED current and a 100% duty cycle corre-
sponds to full load current. 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. The
magnitude of the PWM signal should be higher
than the minimum SHDN voltage high.
Open Circuit Protection
When any white LED inside the white LED
module fails or the LED module is disconnected
from the circuit, the output and the feedback
control will be open, thus resulting in a high
output voltage, which may cause the SW pin
voltage to exceed it maximum rating. In this
case, a zener diode can be used at the output to
limit the voltage on the SW pin and protect the
part. The zener voltage should be larger than the
maximum forward voltage of the White LED
module.
R₁ =( ^VOUT - 1 ) • R₂
1.22
Date: 05/25/04
SP6690 Micro Power Boost Regualtor, Series White LED Driver
10
© Copyright 2004 Sipex Corporation