Data Sheet
WHITE LED STEP-UP CONVERTER
AP3019A
Application Information (Continued)
(2) Changing the Effective Feedback Voltage
There are three methods to change the effective
feedback voltage.
Third, using a logic signal to change the feedback
voltage. For example, the FB pin is connected to the
GND through a MOSFET and a resistor. And this
MOSFET is controlled a logic signal. The luminous
intensity of LEDs will be changed when the
MOSFET turns on or off.
First, adding a constant DC voltage through a resistor
divider to FB pin can control the dimming. Changing
the DC voltage or resistor between the FB Pin and the
DC voltage can get appropriate luminous intensity.
Comparing with all kinds of PWM signal control, this
method features a stable output voltage and LEDs
current. Please refer Figure 21.
Figure 23. Dimming Control Using Logic Signal
Figure 21. Dimming Control Using DC Voltage
Second, using a filtered PWM signal can do it. The
filtered PWM signal can be considered as a varying
and adjustable DC voltage.
Figure 22. Dimming Control Using a Filtered PWM
Voltage
Mar. 2010 Rev. 1.7
BCD Semiconductor Manufacturing Limited
11
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