AIC1648
Dimming Control
There are three different ways of dimming control
circuits as follows:
1.
Using a PWM signal
PWM brightness control provides the widest
dimming range by pulsing the LEDs on and off at
full and zero current, respectively. The change of
average LED current depends on the duty cycle of
the PWM signal. Typically, a 0.1kHz to 1kHz
PWM signal is used. Two applications of PWM
dimming with AIC1648 are shown in Figure 16
and Figure 17. One, as Figure 16, uses PWM
signal to drive
SHDN
pin directly for dimming
control. The other, as Figure 17, employs PWM
signal going through a resistor to drive FB pin. If
the
SHDN
pin is used, the increase of duty cycle
results in LED brightness enhancement. If the FB
pin is used, on the contrary, the increase of duty
D1
C2
1µF
cycle will decrease its brightness. In this
application, LEDs are dimmed by FB pin and
turned off completely by
SHDN
.
2.
Using a DC Voltage
For some applications, the preferred method of a
dimming control uses a variable DC voltage to
adjust LED current. The dimming control using a
DC voltage is shown in Figure 18. With a VDC
ranging from 0V to 5V, the selection of resistors in
Figure 18 results in dimming control of LED
current from 20mA to 0mA, respectively.
3.
Using a Filtered PWM Signal
Filtered PWM signal can be considered as an
adjustable DC voltage. It can be used to replace
the variable DC voltage source in dimming
control. The circuit is shown in Figure 19.
L
VIN
C1
1µF
PWM
10µH
VIN
SW
RB521S-30
SHDN OVP
GND
FB
R
FB
4.7Ω
AIC1648
Fig. 16 Dimming Control with a PWM Signal
L
V
IN
C1
1µF
10µH
VIN
SW
RB521S-30
D1
C2
1µF
SHDN OVP
GND
FB
PWM
R2
51K
AIC1648
R1
1K
R
FB
4.7Ω
Fig. 17 Dimming Control Using a PWM Signal
8
AIC [ ANALOG INTERGRATIONS CORPORATION ]
