FIXED FREQUENCY WHITE LED DRIVER
Figure 4 shows the dependence on current limit versus duty cycle.
FSP3301
Figure 4 Current Limit vs Duty Cycle
Setting the LED Current
The LED current is controlled by the feedback resistor, R1, in Figure 5. The current through the LEDs is given by the
equation 104mV/R1. Table 2 shows the selection of resistors for a given LED current.
IL E D (mA)
1
5
10
15
20
R1 (Ω)
104
20.8
10.4
6.93
5.2
Figure 5 Dimming Control Using a DC Voltage
Table 1 LED vs R
Analog and Digital Dimming
There are three different ways to control dimming for the FSP3301 during normal operation. The first way uses a DC
voltage to control the feedback voltage. This can be seen in Figure 5. As the DC voltage increases, current starts
flowing down R1, R2 and R3. The loop will continue to regulate the feedback voltage to 104mV. Thus the current has
to decrease through the LEDs by the same amount of current as is being injected from the DC voltage source. With a
VDC from 0V to 2V, the resistor values shown for R2 and R3 can control the LED current from 0mA to 20mA.
Other applications need to use a logic signal to do the dimming. This can be seen in Figure 6. The PWM signal is
applied to the EN pin of the FSP3301. The LEDs will switch between full load to completely shut off. The average
current through the LEDs will increase proportionally to the duty cycle of the PWM signal.
Figure 6 PWM Dimming Control Using a Logic Signal
2007-4-16
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