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
larger than the FB pin bias curent. For V
DC
range from 0V to 2V, the selection of resistors in figure 3 gives
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
figure 5. R1 sets the minimum LED current (when the NMOS is off). R
INC
sets how much the LED current
increases when the NMOS is turned on. The selection of R1 and R
INC
follows formula (1) and Table 1.
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
SM8237
SM8237
Figure 3 Dimming control using a DC voltage
Figure 4. Dimming control using a filtered PWM control
SM8237
SM8237
Figure 5. Dimming control using a logic signal
Figure 6. Recommended soft-start circuit
Page 10
V.1.3 Aug 28,2007
SAMHOP [ SAMHOP MICROELECTRONICS CORP. ]
