SAMHOP Microelectronics Corp.

1.2MHz STEP-UP CONVERTER

APPLICATIONS INFORMATION

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 5. As the DC voltage increases, the voltage

drop on R2 increases and the voltage drop on R1 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

dimming control of LED current from 0mA to 15mA.

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. The circuit is shown in figure 6.

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

Start-up and inrush current

To achieve minimum start-up delay, no internal soft-start circuit is included in SM8237. When first turned on

without an external soft-start circuit, inrush current is about 200mA . If soft-start is desired, the recommended

circuit and the waveforms are shown in figure 6. If both soft-start and dimming are used, a 10kHz PWM signal

on SHDN is not recommended. Use a lower frequency or implement dimming through the FB pin as shown in

figure 3,4 or5 .

SM8237

Figure 4. Dimming control using a filtered PWM control

SM8237

SM8237

Figure 5. Dimming control using a logic signal

Figure 3 Dimming control using a DC voltage

SM8237

SM8237

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Figure 6. Recommended soft-start circuit

SM8237

V.1.3 Aug 28,2007