SP6690 datasheet(10/16 Pages) SIPEX

10 / 16 page

Date: 05/25/04

SP6690 Micro Power Boost Regualtor, Series White LED Driver

precision resistors are recommended. Table 3

below shows the Rb selection for different white

LED currents. For example, to set the operating

current to be 20mA, Rb is selected as 60.4 Ω, as

shown in the schematic.

Table 3. Bias Resistor Selection

IF (mA)

Rb (

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.

Table 4. Divider Resistor Selection

VOUT (V)

R1 (

Ω)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.

APPLICATION INFORMAMTION: Continued

1.22V

VOUT

SP6690

G ND

SHDN

I N

VIN

Figure 22. Using SP6690 as Voltage Source

The formula and table for the resistor selection

are shown below:

R1 =

(

VOUT

- 1

) • R2

1.22

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