SM8120A

NIPPON PRECISION CIRCUITS INC.—7

BRIGHTNESS ADJUSTMENT

Brightness Adjustment using FB Pin

The LED brightness can be adjusted using an input DC control voltage connected through resistor R3 to the FB

pin. Alternatively, the brightness can be controlled by a PWM signal by adding a low-pass filter comprising

resistor R4 and capacitor C1. The PWM signal frequency range is determined by the low-pass filter coeffi-

cients. For example, the recommended values for resistor R4 (50k

Ω) and capacitor C1 (0.1µF) provide a PWM

signal frequency range of 1kHz to 1MHz.

Brightness adjustment using FB pin (DC voltage input)

given by equation 1.

If the values R1 = 30

If the values R1 = 30

Taking the above diagram as an example, inserting the values R1 = 30

Brightness adjustment circuit using FB pin

(DC voltage input)

SW

VSS

FB

R1

30

Ω

SBD

DC Voltage

0 to 3V

R3

100k

Ω

R2

20k

Ω

VDD

CE

1.0

µF

L

22

µH

4.7

µF

3.6V

LED

DC voltage vs. LED current

0

5

10

15

20

0.0

0.5

1.0

1.5

2.0

2.5

3.0

DC voltage [V]

R1

R2

R3

−

... (1)

20,000

× (0 − 0.5)

100,000

30

30

=

0.5

−

0.6

20mA

... (2)

20,000

× (3 − 0.5)

100,000

30

30

=

0.5

−

0

0mA

... (3)