Global Mixed-mode Technology Inc.
Dimming control by DC voltage
LED dimming control is also possible by using the DC
voltage to FB pin. LED current is adjustable by DC
voltage and resisters, R1 and R2 in the following fig-
ure.
I
LED
= (V
DC
−
0.2) / R2
−
0.2 / R1
G5126
Duty=0% to 100% PWM signal duty cycle can be used
up to the maximum LED current and minimum LED
current as in the next formulas.
I
LEDMIN
=(0.2
−
R2
×
(V
H
−
0.2) / (R3 + R4)) / R1
I
LEDMAX
=(0.2
−
R2
×
(V
L
−
0.2) / (R3 + R4)) / R1
G5126
R2
FB
R1
V
DC
For example, supposed that the PWM signal level is
set as 3.3V/0V, to adjust the LED current range from
0mA to 20mA by the duty cycle, our recommendation
external components values are, R1=10Ω, R2=12kΩ,
R3=39kΩ, R4=150kΩ or around.
C3 should be set large enough to regard the PWM
signal as adjustable DC voltage by the filter. In this
method, higher frequency control than the frequency
against the EN pin can be used for dimming control.
Dimming control by DC voltage
Dimming control by feedback voltage and filtered
PWM signal
LED dimming control is also possible by using the
feedback voltage and filtered PWM signal. LED
current is adjustable according to the
“H”
level (V
H
)
and
“L”
level (VL) of PWM signal and resisters.
G5126
R2
FB
R3
R4
C3
R1
V
H
V
L
Dimming control by DC voltage
Ver: 0.5
Jan 11, 2008
TEL: 886-3-5788833
http://www.gmt.com.tw
7
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
