Datasheet

PWM Dimming Using Injected Voltage on FB Data
2-12
2.11 PWM Dimming Using Injected Voltage on FB Data
For the PWM Dimming Using Injected Voltage on FB method, a 0-V to 3.3-V
control voltage is used to program a maximum LED current of 20 mA. Use the
equations found in the Analog Dimming Using Analog Voltage section to
calculate the resistor values. The following component values are used:
R2 = 249 kΩ, R3 = 147 kΩ, R4 = 100 Ω. R2 and C2 are unpopulated (left open)
for this configuration. The control voltage is injected into DIM1 on J3.
Figure 2−9 shows the control voltage and the output current for a 5-kHz, 50%
duty cycle control voltage waveform. The top oscilloscope trace is the output
current and the bottom trace is the control voltage. Figure 2−10 shows the
output current versus duty cycle for different PWM frequencies. The increased
current at higher frequencies results from the fact that the discharge time of
the output capacitor takes a larger percentage of the overall off time.
Figure 2−9. Control Voltage and Output Current
50 µs
0.5 V
50 µs
12.4 mV
0.2 V
0.1 V
50 mV
2
3
4
1
BWL
DC x10
DC x10
DC
DC
2 DC 1.64 V
10 MS/s
AUTO
1
50 µs
2 V
2