Diodes AP5725 Manuel d'utilisateur télécharger pdf (Page 11)

AP5725

WHITE LED STEP-UP CONVERTER

AP5725

Document number: DS31844 Rev. 3 - 2

11 of 17

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July 2010

Applications 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

range from 0V to 2V, the selection of resistors in Figure 3 gives

dimming control of LED current from 0mA to 20mA.

AP5725

RSET

100k

Figure 3. Dimming Control Using a DC Voltage

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.

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.

AP5725

RSET

RINC

Logic

Signal

Figure 4. Dimming Control Using a Logic Signal

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Diodes AP5725 Manuel d'utilisateur Page 11