AP5724 Datasheet, PDF (11/16 Pages) Diodes Incorporated – WHITE LED STEP-UP CONVERTER

AP5724
WHITE LED STEP-UP CONVERTER
Application Information (Continued)
2. Using a DC Voltage
For some applications, the preferred method of brightness control is a variable DC voltage to adjust the LED
current. The dimming control using a DC voltage is shown in Figure 3. As the DC voltage increases, the
voltage drop on R2 increases and the voltage drop on R_SET decreases. Thus, the LED current decreases. The
selection of R2 and R3 will make the current from the variable DC source much smaller than the LED current
and much larger than the FB pin bias current. For V_DC range from 0V to 2V, the selection of resistors in Figure
3 gives dimming control of LED current from 0mA to 20mA.
3. Using a Filtered PWM Signal
The 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.
AP5724
FB
R3
R2
100k 5k
V_DC
RSET
5
Figure 3. Dimming Control Using a DC Voltage
4. Using a Logic Signal
For applications that need to adjust the LED current in discrete steps, a logic signal can be used as shown in
Figure 4. R_SET sets the minimum LED current (when the NMOS is off). R_SET sets how much the LED current
increases when the NMOS is turned on.
AP5724
Logic
Signal
FB
RINC
RSET
Figure 4. Dimming Control Using a Logic Signal
AP5724 Rev. 1
11 of 16
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JUNE 2009
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